1. The main purpose of an atomic model is to describe
ⓐ. only the shape of the atom drawn on paper
ⓑ. internal arrangement and observed behaviour
ⓒ. only the chemical colour of an element
ⓓ. how atoms can be made indivisible again
Correct Answer: internal arrangement and observed behaviour
Explanation: An atomic model is a proposed picture of the internal structure of an atom. It tries to explain how electrons, positive charge, and mass are arranged inside the atom. A useful model must also explain observations such as scattering results, stability, and spectra. The model is not just a drawing; it is a physical explanation tested by experiments. Different atomic models are compared by checking whether they can explain observed behaviour of atoms.
2. A neutral atom with atomic number \(Z\) contains
ⓐ. \(Z\) protons and \(Z\) electrons
ⓑ. \(Z\) neutrons and no electrons
ⓒ. \(Z\) electrons and no protons
ⓓ. \(2Z\) protons and \(Z\) electrons
Correct Answer: \(Z\) protons and \(Z\) electrons
Explanation: The atomic number \(Z\) represents the number of protons in the nucleus. A proton has charge \(+e\), while an electron has charge \(-e\). For an atom to be electrically neutral, the total positive charge must equal the total negative charge. Therefore, a neutral atom with \(Z\) protons must have \(Z\) electrons. The word neutral refers to total charge, not to the absence of charged particles.
3. In the usual notation for an atom, \(A\) mainly represents
ⓐ. the number of electrons outside the nucleus
ⓑ. the total charge of all electrons
ⓒ. protons plus neutrons in the nucleus
ⓓ. the radius of the outermost orbit
Correct Answer: protons plus neutrons in the nucleus
Explanation: The mass number \(A\) is the total number of nucleons in the nucleus. Nucleons include protons and neutrons. The atomic number \(Z\) counts only protons, while \(A\) counts protons plus neutrons. Electrons are much lighter and are not counted in the mass number at this level. Confusing \(A\) with electron number hides the fact that almost all atomic mass is associated with the nucleus.
4. The hydrogen atom is often introduced first in atomic-structure calculations because it has
ⓐ. one proton and one electron
ⓑ. many electrons with strong electron-electron repulsion
ⓒ. no positive charge inside its nucleus
ⓓ. a nucleus larger than that of every other atom
Correct Answer: one proton and one electron
Explanation: A neutral hydrogen atom contains one proton and one electron. This makes it the simplest atom for building an atomic model. Since there is only one electron, complications due to electron-electron interactions are absent. Bohr’s model works especially well for hydrogen and hydrogen-like one-electron ions. The simplicity of hydrogen is a physical reason, not merely a historical convenience.
5. A model of the atom must preserve electrical neutrality of ordinary atoms. This means that the model must include
ⓐ. only negative charge spread throughout the atom
ⓑ. no charged particles at all inside the atom
ⓒ. balanced total positive and negative charge
ⓓ. only positive charge concentrated outside the atom
Correct Answer: balanced total positive and negative charge
Explanation: Ordinary atoms are electrically neutral, so their net charge is zero. This does not mean that the atom has no charged particles. It means that positive charge and negative charge balance each other exactly. Any atomic model must explain where the positive charge is and how the electrons are arranged. A model that includes only one kind of charge cannot represent a neutral atom properly.
6. The discovery of the electron showed that
ⓐ. atoms have no internal structure
ⓑ. atomic mass is always zero
ⓒ. all atoms are made only of photons
ⓓ. atoms contain electrons
Correct Answer: atoms contain electrons
Explanation: The electron is a negatively charged particle found inside atoms. Its discovery showed that atoms are not indivisible solid particles. Once electrons were known, an atomic model also had to explain the balancing positive charge. This led to early models such as Thomson’s model and later to scattering experiments. The discovery did not by itself reveal the nucleus, but it made internal atomic structure unavoidable.
7. A simple description says that an atom contains a small central nucleus and electrons around it. In this description, the nucleus contains
ⓐ. electrons and photons
ⓑ. only empty space
ⓒ. de Broglie waves but no particles
ⓓ. protons and neutrons
Correct Answer: protons and neutrons
Explanation: In the nuclear picture of the atom, the nucleus is the small central part of the atom. It contains protons and neutrons. Protons provide positive charge, while neutrons add mass without changing the positive charge. Electrons occupy the region outside the nucleus. This separation of a tiny nucleus and extranuclear electrons becomes important when explaining Rutherford scattering.
8. The link between atomic structure and electrostatics appears because
ⓐ. atomic particles do not have charge
ⓑ. electrons and nuclei exert electric forces
ⓒ. gravity is the strongest force inside atoms
ⓓ. photons always carry negative charge
Correct Answer: electrons and nuclei exert electric forces
Explanation: Electrons are negatively charged and nuclei are positively charged. Therefore, the attraction between an electron and the nucleus is an electrostatic force. In Bohr’s model, this attraction provides the centripetal force needed for circular motion of the electron. Rutherford scattering also involves electrostatic repulsion between an alpha particle and a positive nucleus. Atomic models repeatedly use charge, force, energy, and motion together.
9. In the context of atomic models, an electron revolving around a nucleus is undergoing accelerated motion because
ⓐ. its direction of velocity changes continuously
ⓑ. its speed must always increase
ⓒ. its charge becomes zero during revolution
ⓓ. its mass changes into photon energy at every point
Correct Answer: its direction of velocity changes continuously
Explanation: In circular motion, the direction of velocity keeps changing even when the speed remains constant. A change in velocity means acceleration. Therefore, a revolving electron is an accelerated charged particle in the classical picture. This point later creates a serious difficulty for Rutherford’s model because classical theory predicts radiation from an accelerated charge. The acceleration is due to direction change, not necessarily due to increasing speed.
10. In atomic physics, a photon is best described as
ⓐ. a proton moving outside the nucleus
ⓑ. a particle of radiation carrying energy
ⓒ. a neutron emitted from every atom
ⓓ. an electron whose charge has become positive
Correct Answer: a particle of radiation carrying energy
Explanation: A photon is a quantum of electromagnetic radiation. In atomic transitions, radiation is emitted or absorbed in packets of energy called photons. The energy of a photon depends on its frequency through the relation \(E=h\nu\). Photons are not electrons, protons, or neutrons. This idea is needed later when spectral lines are connected with energy differences between atomic levels.
11. Atomic spectra are useful in studying atoms because they show that atoms
ⓐ. absorb or emit all wavelengths equally
ⓑ. emit or absorb selected wavelengths
ⓒ. have no internal energy structure
ⓓ. are visible only under a microscope
Correct Answer: emit or absorb selected wavelengths
Explanation: Atomic spectra contain discrete lines rather than a continuous spread of all wavelengths. These lines indicate that atoms emit or absorb only certain photon energies. The selected photon energies point toward selected energy differences inside the atom. This evidence cannot be explained by a model that allows all possible energies continuously. Spectral lines therefore become an important clue for quantised energy levels.
12. A student compares kinetic energy and potential energy in atomic models. Kinetic energy is most directly associated with
ⓐ. the motion of the electron
ⓑ. the colour of the atom
ⓒ. the absence of charge
ⓓ. the number of spectral series names
Correct Answer: the motion of the electron
Explanation: Kinetic energy is the energy associated with motion. In atomic models with orbiting electrons, the moving electron has kinetic energy. Potential energy is associated with the interaction between charged particles, such as the attraction between an electron and the nucleus. Both energies appear later in Bohr’s model, where their signs and magnitudes have definite relations. Keeping motion energy separate from interaction energy prevents confusion in later energy-level questions.
13. Use the table below to identify the row that gives the most suitable basic meaning.
| Row | Term | Meaning |
|---|
| P | \(Z\) | Number of protons in the nucleus |
| Q | \(A\) | Number of protons plus neutrons |
| R | Neutral atom | Total positive charge equals total negative charge |
| S | Photon | A proton inside the nucleus |
ⓐ. Row P only
ⓑ. Rows P and Q only
ⓒ. Rows Q, R, and S only
ⓓ. Rows P, Q, and R only
Correct Answer: Rows P, Q, and R only
Explanation: The atomic number \(Z\) is the number of protons in the nucleus. The mass number \(A\) is the total number of protons and neutrons. A neutral atom has equal total positive and negative charges, so its net charge is zero. A photon is a quantum of radiation, not a proton inside the nucleus. The incorrect row confuses a particle of light with a nuclear particle, which would break the later connection between spectra and photon energy.
14. The statement “an atom is neutral, so it contains no charged particles” is flawed because
ⓐ. electrons have no charge inside atoms
ⓑ. protons and electrons are both negatively charged
ⓒ. neutrality means charge balance, not no charge
ⓓ. the nucleus always has zero charge
Correct Answer: neutrality means charge balance, not no charge
Explanation: A neutral atom has zero net charge. This happens because the positive charge of protons balances the negative charge of electrons. The charged particles are still present inside the atom. Removing the charged particles altogether would not describe an atom with a nucleus and electrons. Neutrality is a balance condition, not a statement that charges are absent.
15. The idea of de Broglie wavelength enters atomic structure because an electron can show
ⓐ. only gravitational behaviour
ⓑ. positive charge after entering an orbit
ⓒ. matter-wave behaviour
ⓓ. no relation with motion
Correct Answer: matter-wave behaviour
Explanation: De Broglie associated a wavelength with a moving particle. For an electron in an atom, this wave idea helps explain why only certain orbits are allowed in Bohr’s model. Later, the condition that an integral number of wavelengths fits around a circular orbit leads to angular momentum quantisation. The electron is still treated as a particle with mass and charge, but it also has a matter-wave aspect. The wave idea does not replace the charge of the electron; it adds a condition for stable allowed motion.
16. A neutral atom has \(Z=8\). The number of electrons in it is
ⓐ. \(4\)
ⓑ. \(8\)
ⓒ. \(16\)
ⓓ. \(0\)
Correct Answer: \(8\)
Explanation: \( \textbf{Known quantity:} \) The atomic number is \(Z=8\).
\( \textbf{Meaning of } Z\textbf{:} \) \(Z\) gives the number of protons in the nucleus.
\( \textbf{Neutrality condition:} \) In a neutral atom, total positive charge equals total negative charge.
\( \textbf{Electron count rule:} \) Number of electrons \(=\) number of protons for a neutral atom.
\( \textbf{Substitution:} \) Number of electrons \(=Z=8\).
\( \textbf{Charge check:} \) The \(8\) protons give charge \(+8e\), and \(8\) electrons give charge \(-8e\).
\( \textbf{Net charge:} \) \(+8e-8e=0\).
\( \textbf{Final answer:} \) The neutral atom contains \(8\) electrons.
17. In the symbol language used for atoms, the pair \(Z\) and \(A\) is most useful because it tells us
ⓐ. the colour and brightness of every spectral line
ⓑ. the exact speed of every electron without a model
ⓒ. the wavelength of every photon emitted by any atom
ⓓ. charge balance and nuclear composition information
Correct Answer: charge balance and nuclear composition information
Explanation: The atomic number \(Z\) gives the number of protons in the nucleus. For a neutral atom, it also gives the number of electrons. The mass number \(A\) gives the total number of protons and neutrons in the nucleus. Together, \(Z\) and \(A\) describe basic nuclear composition and help track charge balance. They do not by themselves give electron speeds or spectral wavelengths; those require additional atomic models and energy relations.
18. A first sketch of atomic structure shows a compact positive centre and electrons in the surrounding region. The compact positive centre is called the
ⓐ. nucleus
ⓑ. spectrum
ⓒ. photon
ⓓ. orbit number
Correct Answer: nucleus
Explanation: The nucleus is the small central part of the atom. It contains the positive charge due to protons and most of the atomic mass. Electrons are outside this central region. This picture is different from a model in which positive charge is spread throughout the entire atom. The term nucleus is essential because Rutherford scattering later shows that the atom’s positive charge is concentrated in a very small region.
19. In a comparison of three ideas, select the best set of associations.
| Item | Best association |
|---|
| P. Electron | Negatively charged atomic particle |
| Q. Proton | Positive charge in the nucleus |
| R. Spectrum | Pattern of emitted or absorbed radiation |
ⓐ. P and Q only
ⓑ. Q and R only
ⓒ. P and R only
ⓓ. P, Q, and R
Correct Answer: P, Q, and R
Explanation: An electron is a negatively charged particle found in atoms. A proton is positively charged and is located in the nucleus. A spectrum is a pattern of radiation, usually described by wavelengths or frequencies. All three associations are basic building blocks for atomic-structure reasoning. The first two describe atomic constituents, while the third connects atomic structure with observed radiation.
20. An atomic model that includes charged particles, circular motion, and radiation must be able to deal with all of the following ideas except
ⓐ. electrostatic force between charged particles
ⓑ. energy of a moving electron
ⓒ. chemical taste of a sample
ⓓ. photons emitted or absorbed by atoms
Correct Answer: chemical taste of a sample
Explanation: Atomic models use electrostatic attraction or repulsion between charged particles. They also use kinetic and potential energy when describing electron motion. Photons are needed to connect energy changes with spectra. Chemical taste is not a physical ingredient of the atomic models discussed here. The relevant ideas are charge, force, motion, energy, and radiation, not sensory properties of materials.
