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301. The spin quantum number ($m_s$) describes:
ⓐ. Size of orbital
ⓑ. Orientation of orbital
ⓒ. Intrinsic angular momentum of electron
ⓓ. Energy of nucleus
Correct Answer: Intrinsic angular momentum of electron
Explanation: The spin quantum number represents the electron’s intrinsic spin, a fundamental property independent of orbital motion. It gives rise to magnetic behavior of electrons.
302. The possible values of the spin quantum number ($m_s$) are:
ⓐ. 0 and 1
ⓑ. $-1$ and $+1$
ⓒ. $-\tfrac{1}{2}$ and $+\tfrac{1}{2}$
ⓓ. Any integer from $-n$ to $+n$
Correct Answer: $-\tfrac{1}{2}$ and $+\tfrac{1}{2}$
Explanation: An electron has only two possible spin states, often called “spin-up” ($+\tfrac{1}{2}$) and “spin-down” ($-\tfrac{1}{2}$).
303. What is the maximum number of electrons that can occupy a single orbital?
ⓐ. 1
ⓑ. 1.1
ⓒ. 3
ⓓ. 2
Correct Answer: 2
Explanation: Each orbital can hold two electrons, provided their spins are opposite. This follows Pauli’s exclusion principle, which uses the spin quantum number to distinguish them.
304. Which principle is directly related to the spin quantum number?
ⓐ. Hund’s rule of maximum multiplicity
ⓑ. Pauli’s exclusion principle
ⓒ. Aufbau principle
ⓓ. Heisenberg’s uncertainty principle
Correct Answer: Pauli’s exclusion principle
Explanation: Pauli’s principle states that no two electrons in an atom can have all four quantum numbers the same. The spin quantum number provides the 4th identifier for electrons in the same orbital.
305. The two values of spin quantum number correspond to:
ⓐ. Two different orbital shapes
ⓑ. Two orientations of electron spin (up and down)
ⓒ. Two different shells
ⓓ. Two values of angular momentum quantum number
Correct Answer: Two orientations of electron spin (up and down)
Explanation: Electrons behave like tiny magnets. The two possible spin states correspond to clockwise and anticlockwise orientation of intrinsic spin angular momentum.
306. Which of the following is TRUE about spin quantum number?
ⓐ. It is dependent on the value of $n$.
ⓑ. It is dependent on the value of $l$.
ⓒ. It is independent of $n, l, m_l$.
ⓓ. It changes with orbital size.
Correct Answer: It is independent of $n, l, m_l$.
Explanation: Spin quantum number is a fundamental property of the electron itself. It does not depend on the orbital quantum numbers.
307. The spin of an electron generates:
ⓐ. Gravitational field
ⓑ. Nuclear field
ⓒ. Electric charge
ⓓ. Magnetic moment
Correct Answer: Magnetic moment
Explanation: Electron spin produces a tiny magnetic dipole. This is why spin is crucial in explaining magnetism in atoms and materials (e.g., ferromagnetism).
308. Which of the following spectroscopic phenomena is explained using electron spin?
ⓐ. Splitting of spectral lines into fine structure
ⓑ. Reflection of light
ⓒ. Refraction of light
ⓓ. Polarization of light
Correct Answer: Splitting of spectral lines into fine structure
Explanation: Fine structure in atomic spectra arises from spin–orbit coupling, where interaction between electron spin and orbital motion slightly shifts energy levels.
309. What is the significance of the spin quantum number in electronic configuration?
ⓐ. It limits the number of orbitals in a shell
ⓑ. It ensures each orbital can accommodate two electrons with opposite spins
ⓒ. It determines energy level spacing in hydrogen
ⓓ. It defines orientation of orbital in space
Correct Answer: It ensures each orbital can accommodate two electrons with opposite spins
Explanation: Spin allows pairing of electrons in the same orbital. Without spin, only one electron could occupy an orbital.
310. Which experiment first gave evidence of electron spin?
ⓐ. Millikan’s oil drop experiment
ⓑ. Davisson–Germer experiment
ⓒ. Stern–Gerlach experiment
ⓓ. Rutherford’s gold foil experiment
Correct Answer: Stern–Gerlach experiment
Explanation: In 1922, the Stern–Gerlach experiment showed the quantization of electron spin when a beam of silver atoms split into two distinct parts in a magnetic field.
311. Quantum numbers are used to:
ⓐ. Locate protons inside the nucleus
ⓑ. Describe the state of an electron in an atom uniquely
ⓒ. Determine the mass of an electron
ⓓ. Describe only the spin of electron
Correct Answer: Describe the state of an electron in an atom uniquely
Explanation: Each electron in an atom is described by a unique set of four quantum numbers ($n, l, m_l, m_s$). Together they specify its energy, subshell, orientation, and spin.
312. The principal quantum number ($n$) signifies:
ⓐ. The spin of an electron
ⓑ. The size and energy of the orbital
ⓒ. The shape of the orbital
ⓓ. The orientation of the orbital
Correct Answer: The size and energy of the orbital
Explanation: $n$ determines the main shell, orbital size, and energy level. Larger $n$ values mean electrons are farther from the nucleus and have higher energy.
313. The azimuthal quantum number ($l$) signifies:
ⓐ. The orientation of orbital in space
ⓑ. The probability density of orbital
ⓒ. The shape of the orbital
ⓓ. The spin orientation of electron
Correct Answer: The shape of the orbital
Explanation: The azimuthal number defines subshells (s, p, d, f) and thus the orbital’s shape. $l=0$ → spherical (s), $l=1$ → dumbbell (p), $l=2$ → cloverleaf (d).
314. The magnetic quantum number ($m_l$) signifies:
ⓐ. The direction of electron spin
ⓑ. The orientation of orbital in space
ⓒ. The energy level of orbital
ⓓ. The total number of electrons in a shell
Correct Answer: The orientation of orbital in space
Explanation: $m_l$ defines the orientation of orbitals relative to the coordinate axes. For $l=1$, three values of $m_l$ give $p_x, p_y, p_z$.
315. The spin quantum number ($m_s$) signifies:
ⓐ. The orientation of electron’s intrinsic spin
ⓑ. The orbital shape
ⓒ. The orbital size
ⓓ. The shell number
Correct Answer: The orientation of electron’s intrinsic spin
Explanation: $m_s = +\tfrac{1}{2}$ (spin-up) or $-\tfrac{1}{2}$ (spin-down). It distinguishes two electrons in the same orbital.
316. How many quantum numbers are required to specify an electron completely in an atom?
ⓐ. 1
ⓑ. 2
ⓒ. 3
ⓓ. 4
Correct Answer: 4
Explanation: Four quantum numbers ($n, l, m_l, m_s$) are required. Together, they uniquely describe each electron in an atom according to Pauli’s exclusion principle.
317. Which combination of quantum numbers is not possible?
ⓐ. $n=3, l=0, m_l=0, m_s=+1/2$
ⓑ. $n=2, l=1, m_l=-1, m_s=-1/2$
ⓒ. $n=1, l=1, m_l=0, m_s=+1/2$
ⓓ. $n=4, l=3, m_l=+2, m_s=-1/2$
Correct Answer: $n=1, l=1, m_l=0, m_s=+1/2$
Explanation: For $n=1$, $l$ can only be 0. So $l=1$ is not possible. This makes the combination invalid.
318. The maximum number of electrons that can be accommodated in a shell with principal quantum number $n$ is:
ⓐ. $2n$
ⓑ. $n^2$
ⓒ. $4n^2$
ⓓ. $2n^2$
Correct Answer: $2n^2$
Explanation: Each orbital holds 2 electrons, and number of orbitals in shell = $n^2$. Thus, maximum electrons = $2n^2$.
319. Which rule is based directly on quantum numbers to fill atomic orbitals?
ⓐ. Newton’s rule
ⓑ. Aufbau principle
ⓒ. Coulomb’s law
ⓓ. Ohm’s law
Correct Answer: Aufbau principle
Explanation: Aufbau principle states that electrons occupy orbitals in increasing order of energy, guided by quantum numbers. This principle determines electron configurations.
320. The significance of quantum numbers is that they:
ⓐ. Predict the color of atoms
ⓑ. Completely define the position, energy, and properties of electrons in atoms
ⓒ. Show that electrons follow fixed orbits
ⓓ. Prove that protons move in orbitals
Correct Answer: Completely define the position, energy, and properties of electrons in atoms
Explanation: Quantum numbers collectively give complete information about electrons — shell ($n$), subshell ($l$), orbital orientation ($m_l$), and spin ($m_s$). They are fundamental to quantum mechanics and atomic structure.
321. What is the shape of an s-orbital?
ⓐ. Dumbbell-shaped
ⓑ. Cloverleaf-shaped
ⓒ. Complex multi-lobed
ⓓ. Spherical
Correct Answer: Spherical
Explanation: All s-orbitals ($l=0$) are spherically symmetrical around the nucleus. Their electron density depends only on the distance from the nucleus, not the direction.
322. The number of s-orbitals in any shell is:
ⓐ. 2
ⓑ. 3
ⓒ. 4
ⓓ. 1
Correct Answer: 1
Explanation: For any principal quantum number $n$, $l=0$ corresponds to only one s-orbital (e.g., 1s, 2s, 3s). Each orbital can hold up to 2 electrons.
323. The probability distribution of electrons in an s-orbital depends on:
ⓐ. Direction only
ⓑ. Distance from the nucleus only
ⓒ. Both distance and direction
ⓓ. Spin of electron only
Correct Answer: Distance from the nucleus only
Explanation: s-orbitals are spherical; thus the probability density depends only on radial distance, not on direction.
324. Which of the following statements about s-orbitals is correct?
ⓐ. They have angular nodes
ⓑ. They are non-spherical in shape
ⓒ. They can be oriented along x, y, and z axes
ⓓ. They are spherically symmetrical around the nucleus
Correct Answer: They are spherically symmetrical around the nucleus
Explanation: s-orbitals are spherical and hence identical in all directions. They do not have angular nodes but may have radial nodes depending on $n$.
325. How many radial nodes does the 3s orbital have?
ⓐ. 0
ⓑ. 4
ⓒ. 3
ⓓ. 2
Correct Answer: 2
Explanation: Radial nodes = $n-l-1$. For 3s, $n=3$, $l=0$, so nodes = $3-0-1=2$. These nodes appear as spherical shells where probability density is zero.
326. The electron density in an s-orbital is maximum at:
ⓐ. At a distance equal to Bohr’s radius
ⓑ. At infinite distance
ⓒ. Near the nucleus (at the center)
ⓓ. In nodal planes
Correct Answer: Near the nucleus (at the center)
Explanation: For s-orbitals, electron density is maximum at the nucleus (r=0) and decreases gradually with distance.
327. Which orbital is spherical in shape and smallest in size?
ⓐ. 4p
ⓑ. 3p
ⓒ. 2s
ⓓ. 1s
Correct Answer: 1s
Explanation: The 1s orbital is spherical and closest to the nucleus. It is the smallest orbital since it belongs to the first energy level ($n=1$).
328. The difference between 1s and 2s orbitals is:
ⓐ. 1s is spherical, 2s is dumbbell-shaped
ⓑ. 1s has 1 radial node, 2s has none
ⓒ. 1s is smaller with no node, 2s is larger with 1 radial node
ⓓ. Both have equal size but different energies
Correct Answer: 1s is smaller with no node, 2s is larger with 1 radial node
Explanation: The 1s orbital has no nodes and is closer to the nucleus. The 2s orbital is larger and has 1 radial node where probability is zero.
329. Which of the following orbitals has a spherical shape but different size compared to 1s?
ⓐ. 2p
ⓑ. 2s
ⓒ. 3d
ⓓ. 3p
Correct Answer: 2s
Explanation: 2s orbital is spherical like 1s but larger in size and contains one radial node. Higher s-orbitals (3s, 4s) are also spherical with more nodes.
330. The radial probability distribution curve of a 2s orbital shows:
ⓐ. No radial nodes
ⓑ. One radial node
ⓒ. Two radial nodes
ⓓ. Three radial nodes
Correct Answer: One radial node
Explanation: Radial nodes = $n-l-1$. For 2s, $n=2$, $l=0$, so nodes = $2-0-1=1$. The curve has one point (other than r=0 and ∞) where probability density is zero.
331. What is the general shape of p-orbitals?
ⓐ. Spherical
ⓑ. Cloverleaf
ⓒ. Dumbbell-shaped
ⓓ. Multi-lobed complex shape
Correct Answer: Dumbbell-shaped
Explanation: p-orbitals ($l=1$) have a dumbbell shape with two lobes on opposite sides of the nucleus. The nucleus lies at the nodal plane between the lobes.
332. How many p-orbitals exist in each shell where $l=1$?
ⓐ. 1
ⓑ. 5
ⓒ. 4
ⓓ. 3
Correct Answer: 3
Explanation: For $l=1$, the magnetic quantum number $m_l$ has 3 values ($-1,0,+1$), corresponding to three orbitals: $p_x, p_y, p_z$.
333. The orientation of p-orbitals in space is given by:
ⓐ. Principal quantum number ($n$)
ⓑ. Azimuthal quantum number ($l$)
ⓒ. Magnetic quantum number ($m_l$)
ⓓ. Spin quantum number ($m_s$)
Correct Answer: Magnetic quantum number ($m_l$)
Explanation: Orientation is determined by $m_l$. The three values of $m_l$ for $l=1$ correspond to the three perpendicular axes in space.
334. Which statement is true about the nodal plane in a p-orbital?
ⓐ. There is no nodal plane
ⓑ. There is one nodal plane passing through the nucleus
ⓒ. There are two nodal planes passing through the nucleus
ⓓ. The nodal plane is spherical
Correct Answer: There is one nodal plane passing through the nucleus
Explanation: Each p-orbital has a nodal plane at the nucleus where the probability of finding an electron is zero. For example, $p_x$ has a nodal plane in the yz-plane.
335. The maximum number of electrons that can occupy all p-orbitals in a given shell is:
ⓐ. 12
ⓑ. 10.4
ⓒ. 8
ⓓ. 6
Correct Answer: 6
Explanation: A p-subshell has 3 orbitals, each holding 2 electrons. Therefore, maximum = $3 \times 2 = 6$.
336. Which set of axes represents the orientations of the three p-orbitals?
ⓐ. x, y, z
ⓑ. x, y only
ⓒ. x-axis only
ⓓ. xy-plane only
Correct Answer: x, y, z
Explanation: The three p-orbitals are oriented along the x, y, and z axes, hence denoted as $p_x, p_y, p_z$.
337. The probability distribution of a p-orbital is:
ⓐ. Uniform in all directions
ⓑ. Concentrated in two opposite lobes along an axis
ⓒ. Found only near the nucleus
ⓓ. Found only in spherical shells
Correct Answer: Concentrated in two opposite lobes along an axis
Explanation: p-orbitals have two lobes of electron density located on opposite sides of the nucleus, giving the dumbbell appearance.
338. Which of the following statements about p-orbitals is correct?
ⓐ. They are spherical in shape
ⓑ. They have one angular node
ⓒ. They can hold a maximum of 10 electrons
ⓓ. They do not exist for $n=2$
Correct Answer: They have one angular node
Explanation: p-orbitals have one nodal plane (angular node). They exist from $n=2$ onwards and can hold 6 electrons in total.
339. At what minimum value of $n$ do p-orbitals first appear?
ⓐ. 5
ⓑ. 4
ⓒ. 3
ⓓ. 2
Correct Answer: 2
Explanation: For $n=1$, only the 1s orbital exists. p-orbitals ($l=1$) first appear in the second shell ($n=2$).
340. Which of the following is NOT correct about p-orbitals?
ⓐ. They are dumbbell-shaped
ⓑ. They have one nodal plane
ⓒ. They exist only from the second shell onwards
ⓓ. They are always circular in cross-section
Correct Answer: They are always circular in cross-section
Explanation: p-orbitals are dumbbell-shaped, not circular. Their cross-sections vary depending on orientation. The other statements are correct.
341. What is the general shape of most d-orbitals?
ⓐ. Spherical
ⓑ. Dumbbell-shaped
ⓒ. Cloverleaf-shaped
ⓓ. Linear
Correct Answer: Cloverleaf-shaped
Explanation: The majority of d-orbitals ($d_{xy}, d_{yz}, d_{xz}, d_{x^2-y^2}$) have four lobes arranged in a cloverleaf pattern around the nucleus. Each lobe represents a region of high electron probability. This complex shape arises because the azimuthal quantum number $l=2$ produces orbitals with two angular nodes.
342. How many d-orbitals exist in a given shell?
ⓐ. 10
ⓑ. 8
ⓒ. 7
ⓓ. 5
Correct Answer: 5
Explanation: The number of orbitals in a subshell is given by $2l+1$. For $l=2$, we get 5 values of $m_l$: $-2, -1, 0, +1, +2$. These correspond to five d-orbitals: $d_{xy}, d_{yz}, d_{xz}, d_{x^2-y^2}, d_{z^2}$. Each orbital has a unique orientation in space, and together they can accommodate up to 10 electrons.
343. Which of the following d-orbitals has a different shape compared to the others?
ⓐ. $d_{xy}$
ⓑ. $d_{xz}$
ⓒ. $d_{z^2}$
ⓓ. $d_{yz}$
Correct Answer: $d_{z^2}$
Explanation: Four of the d-orbitals ($d_{xy}, d_{xz}, d_{yz}, d_{x^2-y^2}$) have cloverleaf shapes with four lobes. In contrast, the $d_{z^2}$ orbital has a unique shape consisting of two lobes along the z-axis with a donut-shaped ring of electron density around the middle. This distinct geometry plays a special role in crystal field splitting.
344. The maximum number of electrons that can occupy all five d-orbitals is:
ⓐ. 6
ⓑ. 9
ⓒ. 7
ⓓ. 10
Correct Answer: 10
Explanation: Each orbital can hold two electrons with opposite spins according to Pauli’s exclusion principle. Since there are 5 d-orbitals, the maximum capacity is $5 \times 2 = 10$. This is why the d-block of the periodic table has 10 elements in each period, corresponding to the gradual filling of the d-orbitals.
345. The orientation of d-orbitals in space is determined by which quantum number?
ⓐ. Principal quantum number ($n$)
ⓑ. Azimuthal quantum number ($l$)
ⓒ. Magnetic quantum number ($m_l$)
ⓓ. Spin quantum number ($m_s$)
Correct Answer: Magnetic quantum number ($m_l$)
Explanation: The magnetic quantum number decides the orientation of orbitals in 3D space. For d-orbitals ($l=2$), the possible $m_l$ values are $-2, -1, 0, +1, +2$, giving rise to 5 distinct orientations. Each orientation corresponds to a specific d-orbital, such as $d_{xy}$ (lobes between x and y axes) or $d_{x^2-y^2}$ (lobes along x and y axes).
346. Which d-orbital lies along the x and y axes?
ⓐ. $d_{xz}$
ⓑ. $d_{z^2}$
ⓒ. $d_{x^2-y^2}$
ⓓ. $d_{xy}$
Correct Answer: $d_{x^2-y^2}$
Explanation: The $d_{x^2-y^2}$ orbital has its four lobes oriented directly along the x and y axes. This orientation is crucial in coordination chemistry, where ligands approaching along axes strongly interact with this orbital, influencing crystal field splitting patterns in transition metal complexes.
347. Which d-orbitals have lobes oriented between the axes (at 45° angles)?
ⓐ. $d_{x^2-y^2}$ and $d_{z^2}$
ⓑ. $d_{xy}, d_{xz}, d_{yz}$
ⓒ. $d_{xy}, d_{x^2-y^2}$
ⓓ. $d_{xz}, d_{yz}, d_{z^2}$
Correct Answer: $d_{xy}, d_{xz}, d_{yz}$
Explanation: In the $d_{xy}, d_{xz}, d_{yz}$ orbitals, the lobes are oriented between the coordinate axes, not directly along them. For example, $d_{xy}$ has lobes between the x and y axes. This orientation reduces overlap with ligands aligned along the axes in an octahedral field, affecting their relative energies.
348. In transition metal complexes, splitting of d-orbitals into $t_{2g}$ and $e_g$ sets is due to:
ⓐ. Pauli’s exclusion principle
ⓑ. Crystal field created by surrounding ligands
ⓒ. Hund’s rule of maximum multiplicity
ⓓ. Uncertainty principle
Correct Answer: Crystal field created by surrounding ligands
Explanation: In an octahedral complex, ligands approach along the axes. Orbitals oriented along axes ($d_{x^2-y^2}, d_{z^2}$) experience strong repulsion and form the higher-energy $e_g$ set. Orbitals oriented between axes ($d_{xy}, d_{xz}, d_{yz}$) experience less repulsion, forming the lower-energy $t_{2g}$ set. This splitting is fundamental to crystal field theory.
349. The number of angular nodes in any d-orbital is:
ⓐ. 2
ⓑ. 3
ⓒ. 5
ⓓ. 8
Correct Answer: 2
Explanation: The number of angular nodes is equal to the azimuthal quantum number $l$. For d-orbitals, $l=2$, so they have 2 angular nodes. These nodes correspond to planes where the probability of finding an electron is zero, shaping the cloverleaf pattern of d-orbitals.
350. Which of the following statements about d-orbitals is CORRECT?
ⓐ. All five d-orbitals are identical in shape and orientation.
ⓑ. Four d-orbitals are cloverleaf-shaped, while one has a different shape.
ⓒ. d-orbitals can hold a maximum of 14 electrons.
ⓓ. d-orbitals exist only in shells with $n \geq 4$.
Correct Answer: Four d-orbitals are cloverleaf-shaped, while one has a different shape.
Explanation: The d-subshell contains five orbitals. Four ($d_{xy}, d_{xz}, d_{yz}, d_{x^2-y^2}$) have cloverleaf shapes, while the $d_{z^2}$ orbital is unique with a dumbbell and donut shape. d-orbitals can hold up to 10 electrons, and they appear first in the $n=3$ shell (3d orbitals), not only at $n=4$.
351. The orientation of an orbital in space is determined by:
ⓐ. Principal quantum number ($n$)
ⓑ. Spin quantum number ($m_s$)
ⓒ. Magnetic quantum number ($m_l$)
ⓓ. Azimuthal quantum number ($l$)
Correct Answer: Magnetic quantum number ($m_l$)
Explanation: The magnetic quantum number specifies how orbitals of the same subshell are oriented in 3D space. For example, for $l=1$ (p-orbitals), $m_l = -1, 0, +1$, which correspond to $p_x, p_y, p_z$.
352. How many orientations are possible for p-orbitals?
ⓐ. 3
ⓑ. 5
ⓒ. 7
ⓓ. 9
Correct Answer: 3
Explanation: For p-orbitals ($l=1$), $m_l$ has 3 values ($-1, 0, +1$). These correspond to three orientations: along x, y, and z axes.
353. The three p-orbitals are oriented:
ⓐ. Randomly in space
ⓑ. Along the diagonal directions
ⓒ. Along mutually perpendicular axes
ⓓ. In the same plane
Correct Answer: Along mutually perpendicular axes
Explanation: $p_x, p_y, p_z$ are oriented along x, y, and z axes. Their orthogonal orientation minimizes electron repulsion and explains the geometry of molecules such as methane.
354. The five d-orbitals have orientations described by:
ⓐ. $m_l$ values from –2 to +2
ⓑ. $m_l$ values from –1 to +1
ⓒ. $n$ values only
ⓓ. Spin quantum numbers only
Correct Answer: $m_l$ values from –2 to +2
Explanation: For $l=2$, $m_l$ = –2, –1, 0, +1, +2, giving five orientations: $d_{xy}, d_{yz}, d_{xz}, d_{x^2-y^2}, d_{z^2}$.
355. Which orbital has no directional orientation in space?
ⓐ. p-orbital
ⓑ. d-orbital
ⓒ. s-orbital
ⓓ. f-orbital
Correct Answer: s-orbital
Explanation: s-orbitals are spherical and symmetrical in all directions, so they do not have specific orientation in space. In contrast, p, d, and f orbitals have directional character.
356. The orientation of $d_{x^2-y^2}$ orbital is:
ⓐ. Between the x and y axes
ⓑ. Along the x and y axes
ⓒ. Along the z-axis
ⓓ. Between x, y, and z axes
Correct Answer: Along the x and y axes
Explanation: The $d_{x^2-y^2}$ orbital has lobes directly along the x and y axes. This makes it strongly interact with ligands approaching along axes in crystal field theory.
357. The orientation of $d_{xy}$ orbital is:
ⓐ. Along x and y axes
ⓑ. Along z-axis
ⓒ. Between x and y axes
ⓓ. Along diagonal of cube
Correct Answer: Between x and y axes
Explanation: The $d_{xy}$ orbital has lobes lying in between the x and y axes (at 45° angles). Similarly, $d_{xz}$ and $d_{yz}$ lie between respective axes.
358. How many orientations are possible for f-orbitals?
ⓐ. 1
ⓑ. 4
ⓒ. 8
ⓓ. 7
Correct Answer: 7
Explanation: For $l=3$, the number of orbitals is $2l+1=7$. Hence, f-orbitals have 7 orientations with very complex multi-lobed shapes.
359. Which orbital is oriented along the z-axis with a unique donut-shaped ring?
ⓐ. $d_{xy}$
ⓑ. $d_{x^2-y^2}$
ⓒ. $d_{z^2}$
ⓓ. $p_z$
Correct Answer: $d_{z^2}$
Explanation: The $d_{z^2}$ orbital has two lobes along the z-axis and a toroidal ring of electron density around the nucleus. This gives it a different geometry from other d-orbitals.
360. In an octahedral field, orbitals pointing directly along the axes experience:
ⓐ. Least repulsion
ⓑ. Maximum repulsion
ⓒ. No interaction with ligands
ⓓ. Equal interaction as those between axes
Correct Answer: Maximum repulsion
Explanation: Orbitals oriented along the axes ($d_{x^2-y^2}$ and $d_{z^2}$) directly overlap with ligand electron clouds, leading to maximum repulsion and higher energy ($e_g$ set). Orbitals between axes form the $t_{2g}$ set with lower energy.
361. Aufbau principle states that:
ⓐ. Electrons occupy lower-energy orbitals before higher-energy ones
ⓑ. Electrons fill orbitals in random fashion
ⓒ. Electrons first pair up in each orbital
ⓓ. Orbitals are filled only after ionization
Correct Answer: Electrons occupy lower-energy orbitals before higher-energy ones
Explanation: The Aufbau (building-up) principle gives ground-state configurations by filling from lowest to highest energy. This is systematized by the $n+l$ rule and the tie-breaker on $n$. It works well for most atoms but has notable exceptions (e.g., Cr, Cu) due to exchange/relativistic effects.
362. According to the $n+l$ rule, which orbital fills before $3d$?
ⓐ. $4p$
ⓑ. $5s$
ⓒ. $4d$
ⓓ. $4s$
Correct Answer: $4s$
Explanation: Compute $n+l$: for $4s$, $4+0=4$; for $3d$, $3+2=5$. The orbital with the lower $n+l$ value fills first, so $4s$ precedes $3d$. This matches observed sequences and explains why Ca ends with $4s^2$ before any $3d$ occupation.
363. After $4s$ is filled, which orbital is filled next in most atoms?
ⓐ. $4p$
ⓑ. $3d$
ⓒ. $5s$
ⓓ. $3p$
Correct Answer: $3d$
Explanation: Although $3d$ belongs to $n=3$, its $n+l=5$ makes it higher in energy than $4s$ ($n+l=4$). Thus the sequence goes $…3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$. Subshell energies reflect shielding and penetration, not just principal quantum number.
364. Which subshell fills earlier on the basis of the $n+l$ rule (use tie-breaker if needed): $4d$ or $5p$?
ⓐ. $5p$
ⓑ. $5s$
ⓒ. $4d$
ⓓ. $5d$
Correct Answer: $4d$
Explanation: $4d:\, n+l=4+2=6$; $5p:\, n+l=5+1=6$. On a tie, the subshell with the smaller $n$ fills first, so $4d$ precedes $5p$. This illustrates the second part of the $n+l$ rule beyond simply comparing sums.
365. When two subshells have the same $n+l$ value, the one that fills first is the one with:
ⓐ. Lower $l$ only
ⓑ. Higher $l$ only
ⓒ. Larger $n$
ⓓ. Smaller $n$
Correct Answer: Smaller $n$
Explanation: The tie-breaker of the $n+l$ rule favors lower $n$ because, on average, those orbitals are more penetrating and slightly lower in energy. Hence $4d$ fills before $5p$, and $3p$ before $4s$ if their $n+l$ matched (hypothetical illustration).
366. The ground-state configuration of chromium (Z = 24) is:
ⓐ. $[Ar]\,3d^5\,4s^1$
ⓑ. $[Ar]\,3d^4\,4s^2$
ⓒ. $[Ar]\,3d^6\,4s^0$
ⓓ. $[Ar]\,3d^5\,4s^2$
Correct Answer: $[Ar]\,3d^5\,4s^1$
Explanation: Chromium is an Aufbau exception. Half-filled $3d^5$ plus $4s^1$ gains extra stability (exchange energy + symmetry) compared with the naive $[Ar]\,3d^4\,4s^2$. Small energy differences and correlation effects drive this rearrangement.
367. The ground-state configuration of copper (Z = 29) is:
ⓐ. $[Ar]\,3d^9\,4s^2$
ⓑ. $[Ar]\,3d^{10}\,4s^2$
ⓒ. $[Ar]\,3d^{10}\,4s^1$
ⓓ. $[Ar]\,3d^8\,4s^3$
Correct Answer: $[Ar]\,3d^{10}\,4s^1$
Explanation: Copper favors a completely filled $3d^{10}$ subshell, stabilizing the atom despite promoting an electron from $4s$. This again reflects exchange/relativistic effects and subtle energy balances that occasionally override the simple Aufbau order.
368. Upon forming cations for first-row transition metals, electrons are removed first from:
ⓐ. $3d$ and $4s$ simultaneously
ⓑ. $4s$ before $3d$
ⓒ. $3d$ before $4s$
ⓓ. The subshell with lower $l$ regardless of $n$
Correct Answer: $4s$ before $3d$
Explanation: Although $4s$ fills before $3d$, in the ion the effective energy ordering reverses due to reduced shielding and radial contraction of $3d$. Hence $4s$ electrons are lost first (e.g., Fe: $[Ar]\,3d^6\,4s^2 \rightarrow Fe^{2+}:[Ar]\,3d^6$).
369. Which subshell is filled immediately after $6s$ in the neutral-atom filling order?
ⓐ. $6p$
ⓑ. $5d$
ⓒ. $6d$
ⓓ. $4f$
Correct Answer: $4f$
Explanation: The lanthanoid insertion occurs after $6s$. The sequence goes $…\,6s \rightarrow 4f \rightarrow 5d \rightarrow 6p$. This reflects the delicate balance of shielding/penetration that places $4f$ slightly below $5d$ for neutral atoms.
370. Which sequence correctly lists the filling order up to $4p$?
ⓐ. $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$
ⓑ. $1s \rightarrow 2s \rightarrow 3s \rightarrow 2p \rightarrow 4s \rightarrow 3d \rightarrow 4p$
ⓒ. $1s \rightarrow 2p \rightarrow 2s \rightarrow 3s \rightarrow 3p \rightarrow 3d \rightarrow 4s$
ⓓ. $1s \rightarrow 2s \rightarrow 2p \rightarrow 3p \rightarrow 3s \rightarrow 4s \rightarrow 3d$
Correct Answer: $1s \rightarrow 2s \rightarrow 2p \rightarrow 3s \rightarrow 3p \rightarrow 4s \rightarrow 3d \rightarrow 4p$
Explanation: This order follows $n+l$ with the tie-breaker on $n$: after $3p$ comes $4s$ (lower $n+l$), then $3d$, then $4p$. Options B–D scramble subshells (e.g., placing $3s$ after $2p$, or moving $3p$ before $3s$), which violates the Aufbau pattern.
371. Pauli’s exclusion principle states that:
ⓐ. Two electrons in the same atom must have identical quantum numbers
ⓑ. Two electrons in the same orbital must have opposite spins
ⓒ. An orbital can hold unlimited electrons if spins are paired
ⓓ. Electrons fill orbitals in random order
Correct Answer: Two electrons in the same orbital must have opposite spins
Explanation: Pauli’s principle says no two electrons in an atom can have the same four quantum numbers. In practice, this means an orbital (same $n, l, m_l$) can hold only two electrons, and they must differ in spin ($m_s = +\tfrac{1}{2}$ and $-\tfrac{1}{2}$).
372. How many maximum electrons can occupy a single orbital?
ⓐ. 1
ⓑ. Unlimited
ⓒ. 4
ⓓ. 2
Correct Answer: 2
Explanation: Each orbital is defined by a unique set of $n, l, m_l$. According to Pauli’s principle, only two electrons can fit into one orbital, and they must have opposite spins to keep their quantum numbers unique.
373. Which of the following sets of quantum numbers is NOT allowed?
ⓐ. $n=2, l=1, m_l=0, m_s=+1/2$
ⓑ. $n=2, l=1, m_l=0, m_s=-1/2$
ⓒ. $n=2, l=1, m_l=0, m_s=+1/2$ (second electron)
ⓓ. $n=2, l=0, m_l=0, m_s=+1/2$
Correct Answer: $n=2, l=1, m_l=0, m_s=+1/2$ (second electron)
Explanation: If two electrons in the same orbital both had identical spin, their four quantum numbers would be identical, violating Pauli’s principle. Instead, the second electron must have opposite spin.
374. Why does the 1s orbital of hydrogen contain only one electron in the ground state?
ⓐ. Because of Hund’s rule
ⓑ. Because the atom has only one electron
ⓒ. Because Pauli’s exclusion principle forbids any orbital from having two electrons
ⓓ. Because s-orbitals cannot pair electrons
Correct Answer: Because the atom has only one electron
Explanation: Hydrogen has a single electron, so 1s is singly occupied. Pauli’s principle would allow up to two electrons in 1s, but hydrogen simply does not have a second electron.
375. The maximum number of electrons in the second shell ($n=2$) is:
ⓐ. 2
ⓑ. 4
ⓒ. 6
ⓓ. 8
Correct Answer: 8
Explanation: For $n=2$, there are four orbitals (one 2s and three 2p). Each orbital can accommodate 2 electrons with opposite spins, giving a total of 8 electrons.
376. Pauli’s exclusion principle explains which property of matter?
ⓐ. Stability of atomic nuclei
ⓑ. Periodic table structure
ⓒ. Mass of protons
ⓓ. Existence of isotopes
Correct Answer: Periodic table structure
Explanation: Because electrons are restricted by Pauli’s rule, shells and subshells fill in a definite order. This gives rise to the structure of the periodic table, where elements repeat properties after specific electron configurations.
377. What would happen if Pauli’s exclusion principle did not exist?
ⓐ. Atoms would remain unchanged
ⓑ. Nuclei would collapse
ⓒ. Electrons would stop having spin
ⓓ. All electrons would occupy the lowest orbital
Correct Answer: All electrons would occupy the lowest orbital
Explanation: Without Pauli’s rule, no restriction would prevent electrons from crowding into the lowest-energy orbital (1s). This would eliminate the periodic table and chemical diversity.
378. Which experiment indirectly supported Pauli’s exclusion principle?
ⓐ. Davisson–Germer experiment
ⓑ. Stern–Gerlach experiment
ⓒ. Rutherford’s gold foil experiment
ⓓ. Millikan’s oil drop experiment
Correct Answer: Stern–Gerlach experiment
Explanation: The Stern–Gerlach experiment demonstrated the existence of two spin orientations for electrons. Pauli’s principle relies on this property to allow only two electrons per orbital with opposite spins.
379. Which statement is correct according to Pauli’s principle?
ⓐ. Two electrons in an orbital must differ in at least one quantum number
ⓑ. Two electrons in an orbital can have identical spins
ⓒ. Four electrons can fit in a single orbital if spins alternate
ⓓ. The principle applies only to s-orbitals
Correct Answer: Two electrons in an orbital must differ in at least one quantum number
Explanation: Since $n, l, m_l$ are the same for both electrons in an orbital, the only way to distinguish them is by opposite $m_s$ values. Hence, all four quantum numbers cannot be the same.
380. Which of the following is a direct consequence of Pauli’s exclusion principle?
ⓐ. Line spectra of hydrogen atom
ⓑ. Quantum mechanical tunneling
ⓒ. Structure of the periodic table and electron configurations
ⓓ. Quantization of angular momentum
Correct Answer: Structure of the periodic table and electron configurations
Explanation: Pauli’s rule forces electrons into different orbitals and shells after filling limits are reached. This explains why elements show periodic properties and why the periodic table is structured in rows and groups.
381. Hund’s rule of maximum multiplicity states that:
ⓐ. Electrons pair up as soon as they enter a subshell
ⓑ. Electrons first occupy different orbitals singly with parallel spins before pairing
ⓒ. Orbitals are filled randomly without regard to spin
ⓓ. Only one electron can exist per orbital
Correct Answer: Electrons first occupy different orbitals singly with parallel spins before pairing
Explanation: Hund’s rule minimizes electron–electron repulsion and stabilizes the atom by maximizing the number of unpaired electrons with parallel spins in degenerate orbitals (orbitals of equal energy).
382. In the p-subshell ($l=1$), how many orbitals are available for filling according to Hund’s rule?
ⓐ. 5.1
ⓑ. 5
ⓒ. 4
ⓓ. 3
Correct Answer: 3
Explanation: A p-subshell has three orbitals ($p_x, p_y, p_z$). According to Hund’s rule, electrons fill each of these singly before pairing up, all with parallel spins.
383. Which element’s ground-state configuration demonstrates Hund’s rule?
ⓐ. Helium (Z=2)
ⓑ. Carbon (Z=6)
ⓒ. Sodium (Z=11)
ⓓ. Neon (Z=10)
Correct Answer: Carbon (Z=6)
Explanation: Carbon has configuration [He] 2s² 2p². The two electrons in 2p occupy different orbitals with parallel spins, showing Hund’s rule in action.
384. Why do electrons prefer to occupy different orbitals of equal energy before pairing?
ⓐ. To decrease nuclear attraction
ⓑ. To minimize spin
ⓒ. To minimize repulsion and maximize stability
ⓓ. To conserve angular momentum
Correct Answer: To minimize repulsion and maximize stability
Explanation: When electrons occupy separate orbitals, they remain farther apart, reducing electron–electron repulsion. Parallel spins add stability due to exchange energy, making this arrangement more favorable.
385. In nitrogen (Z=7), how are the three 2p electrons distributed according to Hund’s rule?
ⓐ. All in one orbital with paired spins
ⓑ. Two in one orbital, one in another
ⓒ. Two in one orbital, one with opposite spin
ⓓ. One in each of the three orbitals with parallel spins
Correct Answer: One in each of the three orbitals with parallel spins
Explanation: Nitrogen’s ground-state configuration is [He] 2s² 2p³. The three 2p electrons are unpaired and occupy $p_x, p_y, p_z$ with parallel spins, maximizing multiplicity.
386. Which concept explains the extra stability of half-filled and fully filled subshells?
ⓐ. Aufbau principle
ⓑ. Pauli’s exclusion principle
ⓒ. Hund’s rule of maximum multiplicity
ⓓ. Heisenberg’s uncertainty principle
Correct Answer: Hund’s rule of maximum multiplicity
Explanation: Half-filled (e.g., 2p³) and fully filled (e.g., 2p⁶) subshells gain extra stability from exchange energy and symmetrical distribution, explained by Hund’s rule.
387. How many unpaired electrons are there in oxygen (Z=8) based on Hund’s rule?
ⓐ. 0
ⓑ. 4
ⓒ. 3
ⓓ. 2
Correct Answer: 2
Explanation: Oxygen’s configuration is [He] 2s² 2p⁴. Following Hund’s rule, the first three p-electrons occupy separately, and the fourth pairs with one, leaving two unpaired electrons.
388. Hund’s rule is especially relevant for:
ⓐ. Degenerate orbitals
ⓑ. s-orbitals only
ⓒ. Orbitals with different energies
ⓓ. Core electrons
Correct Answer: Degenerate orbitals
Explanation: Hund’s rule applies when multiple orbitals of the same subshell have equal energy (degenerate). Electrons prefer to spread out singly before pairing to minimize repulsion.
389. Which of the following orbital filling violates Hund’s rule?
ⓐ. ↑ ↑ ↑
ⓑ. ↑↓ ↑ ↑
ⓒ. ↑ ↑↓
ⓓ. ↑ ↑
Correct Answer: ↑↓ ↑ ↑
Explanation: In this case, pairing occurred in one orbital before the remaining degenerate orbitals were singly filled. This violates Hund’s rule, which requires single occupation first.
390. The term “maximum multiplicity” in Hund’s rule refers to:
ⓐ. Maximum number of orbitals in a shell
ⓑ. Maximum number of paired spins
ⓒ. Maximum number of unpaired electrons with parallel spins
ⓓ. Maximum number of nodes in an orbital
Correct Answer: Maximum number of unpaired electrons with parallel spins
Explanation: Multiplicity is given by $2S+1$, where $S$ is total spin. More unpaired electrons with parallel spins mean higher multiplicity and greater stability for the atom.
391. The electronic configuration of oxygen (Z=8) can be written as:
ⓐ. 1s² 2s² 2p⁴
ⓑ. 1s² 2s² 2p⁶
ⓒ. 1s² 2s² 2p²
ⓓ. 1s² 2s¹ 2p⁵
Correct Answer: 1s² 2s² 2p⁴
Explanation: Oxygen has 8 electrons. The configuration is 1s² (2), 2s² (2), and 2p⁴ (4). This matches the Aufbau principle sequence and leaves two unpaired electrons.
392. The notation “3p⁶” represents:
ⓐ. 3rd subshell, s-orbital, 6 electrons
ⓑ. 3rd orbital, paired spins, 6 electrons
ⓒ. 3rd nucleus, p-shape, 6 protons
ⓓ. 3rd shell, p-subshell, 6 electrons
Correct Answer: 3rd shell, p-subshell, 6 electrons
Explanation: In configuration notation, the number (3) indicates principal quantum number, the letter (p) indicates subshell type, and the superscript (6) shows electron count in that subshell.
393. Which is the correct ground-state configuration of sodium (Z=11)?
ⓐ. 1s² 2s² 2p⁶ 3s¹
ⓑ. 1s² 2s² 2p⁶ 3p¹
ⓒ. 1s² 2s² 2p⁶ 2p³
ⓓ. 1s² 2s² 2p⁵ 3s²
Correct Answer: 1s² 2s² 2p⁶ 3s¹
Explanation: Sodium has 11 electrons. The first 10 fill up to 2p⁶, and the 11th electron enters 3s, giving the configuration [Ne] 3s¹.
394. Which subshell can accommodate a maximum of 14 electrons?
ⓐ. s
ⓑ. p
ⓒ. d
ⓓ. f
Correct Answer: f
Explanation: The number of electrons in a subshell is $2(2l+1)$. For $l=3$ (f-subshell), maximum = 14 electrons.
395. The ground-state configuration of chlorine (Z=17) is:
ⓐ. 1s² 2s² 2p⁶ 3s² 3p⁵
ⓑ. 1s² 2s² 2p⁶ 3s² 3p⁴
ⓒ. 1s² 2s² 2p⁶ 3s² 3p⁶
ⓓ. 1s² 2s² 2p⁶ 3s² 3d¹
Correct Answer: 1s² 2s² 2p⁶ 3s² 3p⁵
Explanation: Chlorine has 17 electrons. Filling orbitals in order gives the configuration [Ne] 3s² 3p⁵, leaving one unpaired electron in the p-subshell.
396. Which of the following notations is INCORRECT?
ⓐ. 1s²
ⓑ. 2p⁸
ⓒ. 3d¹⁰
ⓓ. 4f¹⁴
Correct Answer: 2p⁸
Explanation: The p-subshell can hold a maximum of 6 electrons. So writing 2p⁸ is invalid. Correct filling would be 2p⁶ maximum.
397. Which noble gas configuration is equivalent to neon (Z=10)?
ⓐ. [He] 2s² 2p⁶
ⓑ. [He] 2s² 2p⁵
ⓒ. [Ar] 3s² 3p⁶
ⓓ. [He] 2s¹ 2p⁶
Correct Answer: [He] 2s² 2p⁶
Explanation: Neon has 10 electrons. The shorthand notation uses [He] (2 electrons) + 2s² 2p⁶ (8 electrons), giving a closed-shell configuration.
398. Which is the correct configuration for iron (Z=26)?
ⓐ. [Ar] 3d⁶ 4s²
ⓑ. [Ar] 3d⁸ 4s²
ⓒ. [Ar] 3d⁴ 4s²
ⓓ. [Ar] 3d⁵ 4s¹
Correct Answer: [Ar] 3d⁶ 4s²
Explanation: Iron has 26 electrons. After [Ar] (18), the next 8 electrons fill 3d⁶ 4s². This matches the observed stable configuration of Fe in its ground state.
399. Which orbital fills immediately after 4p according to Aufbau sequence?
ⓐ. 4d
ⓑ. 4f
ⓒ. 3d
ⓓ. 5s
Correct Answer: 5s
Explanation: The order of filling is …4p → 5s → 4d → 5p. Thus, after 4p, electrons enter the 5s orbital.
400. The electronic configuration 1s² 2s² 2p⁶ 3s² 3p⁶ corresponds to which element?
ⓐ. Argon (Z=18)
ⓑ. Neon (Z=10)
ⓒ. Calcium (Z=20)
ⓓ. Fluorine (Z=9)
Correct Answer: Argon (Z=18)
Explanation: This configuration accounts for 18 electrons, which corresponds to argon. It ends in a filled 3p subshell, indicating a noble gas.
The Structure of Atom is a high-weightage chapter in Class 11 Chemistry (NCERT/CBSE syllabus), and it introduces advanced aspects of atomic theory. Topics such as wave mechanical model of the atom, Schrödinger equation, probability distributions of orbitals, and concepts of orbital energy levels are essential for understanding modern Chemistry. Questions from this section are commonly asked in board exams, NEET Chemistry, and JEE Main & Advanced. With a grand total of 475 MCQs spread over 5 parts, this resource ensures in-depth practice. This page features the fourth set of 100 MCQs with solutions for higher-order thinking and practice. Navigate easily through other parts and chapters using the options above.
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