1. In an atom, the nucleus is best described as the region that contains most of the atom's mass and has what kind of charge?
ⓐ. Negative charge because electrons are near the centre
ⓑ. Zero charge because protons and neutrons cancel each other
ⓒ. Positive charge because it contains protons
ⓓ. Alternating charge because atoms are electrically neutral
Correct Answer: Positive charge because it contains protons
Explanation: The nucleus is the tiny central part of an atom. It contains protons and neutrons, and almost all the mass of the atom is concentrated there. Protons carry positive charge, while neutrons have no charge. Therefore, the nuclear charge comes only from the protons. A neutral atom as a whole has zero net charge because electrons outside the nucleus balance the positive nuclear charge. Neutrality of the whole atom does not mean that the nucleus itself is neutral.
2. The particles called nucleons are listed in the table below. Identify the row that gives the correct pair.
| Row | Nucleons |
|---|
| P | Protons and electrons |
| Q | Protons and neutrons |
| R | Neutrons and photons |
| S | Electrons and photons |
ⓐ. Row P
ⓑ. Row R
ⓒ. Row S
ⓓ. Row Q
Correct Answer: Row Q
Explanation: Nucleons are the particles that make up the nucleus. The two nucleons are protons and neutrons. Electrons are part of the atom, but they are outside the nucleus in atomic structure and are not nucleons. Photons are packets of electromagnetic radiation, not matter particles inside the nucleus. This distinction is needed because the mass number \(A\) counts only protons and neutrons. It does not count electrons or photons.
3. A neutral atom has atomic number \(Z=11\) and mass number \(A=23\). What does \(Z=11\) directly tell about its nucleus?
ⓐ. The nucleus has \(11\) protons
ⓑ. The nucleus has \(11\) neutrons
ⓒ. The nucleus has \(23\) protons
ⓓ. The nucleus has \(23\) electrons
Correct Answer: The nucleus has \(11\) protons
Explanation: The atomic number \(Z\) is the number of protons in the nucleus. It also determines the chemical element. The mass number \(A\) gives the total number of nucleons, so it includes both protons and neutrons. Electrons are not inside the nucleus, although a neutral atom has \(Z\) electrons outside the nucleus. For \(Z=11\), the nucleus contains \(11\) protons, while the neutron number would require \(N=A-Z\). Mixing \(A\) and \(Z\) gives the wrong composition of the nucleus.
4. For a nuclide with mass number \(A\) and atomic number \(Z\), the neutron number \(N\) is given by which relation?
ⓐ. \(N=A+Z\)
ⓑ. \(N=Z-A\)
ⓒ. \(N=AZ\)
ⓓ. \(N=A-Z\)
Correct Answer: \(N=A-Z\)
Explanation: The mass number \(A\) is the total number of nucleons in the nucleus. Since nucleons are protons and neutrons, \(A=Z+N\). Rearranging this relation gives \(N=A-Z\). The atomic number \(Z\) counts only protons, not total particles in the atom. The relation does not involve multiplication because \(A\) is a count formed by addition of proton and neutron numbers. The subtraction \(A-Z\) removes the protons from the total nucleon count and leaves the neutrons.
5. A nucleus contains \(8\) protons and \(9\) neutrons. Its mass number is:
ⓐ. \(8\)
ⓑ. \(17\)
ⓒ. \(9\)
ⓓ. \(72\)
Correct Answer: \(17\)
Explanation: The mass number \(A\) is the total number of protons and neutrons in a nucleus. Here the number of protons is \(Z=8\), and the number of neutrons is \(N=9\). Therefore, \(A=Z+N=8+9=17\). The value \(8\) is only the atomic number, not the mass number. The value \(9\) is only the neutron number. Multiplying \(8\) and \(9\) would not represent any nuclear count used in nuclide notation.
6. The nuclear charge of an atom with atomic number \(Z\) is:
ⓐ. \(-Ze\)
ⓑ. \(+Ae\)
ⓒ. \(+Ze\)
ⓓ. \((A-Z)e\)
Correct Answer: \(+Ze\)
Explanation: A nucleus contains \(Z\) protons, and each proton has charge \(+e\). Neutrons do not contribute to nuclear charge because they are electrically neutral. Therefore, the total nuclear charge is \(+Ze\). The mass number \(A\) includes neutrons also, so \(+Ae\) would count neutral particles as if they carried charge. The quantity \((A-Z)\) is the neutron number \(N\), which does not decide nuclear charge. The sign is positive because the charge carriers inside the nucleus are protons.
7. The symbol \(X\) in the notation \({}^{A}_{Z}X\) is decided by:
ⓐ. The neutron number \(N\)
ⓑ. The atomic number \(Z\)
ⓒ. The mass number \(A\)
ⓓ. The number of electrons lost during ionisation
Correct Answer: The atomic number \(Z\)
Explanation: In nuclide notation, \(X\) represents the chemical symbol of the element. The element is determined by the atomic number \(Z\), which is the number of protons in the nucleus. Different elements must have different values of \(Z\). The mass number \(A\) can change among isotopes of the same element without changing the chemical symbol. Ionisation changes the number of electrons, but it does not change the nucleus or the element. The identity of an element is therefore fixed by proton number, not by electron count alone.
8. Two atoms are isotopes of the same element when they have:
ⓐ. Same \(A\) but different \(Z\)
ⓑ. Same \(Z\) but different \(A\)
ⓒ. Same \(N\) but different \(Z\)
ⓓ. Same number of electrons but different number of protons
Correct Answer: Same \(Z\) but different \(A\)
Explanation: Isotopes are atoms of the same element with different mass numbers. Same element means the value of \(Z\) is the same, because \(Z\) fixes the proton number. Different mass number means the number of neutrons is different. For example, hydrogen isotopes have the same \(Z=1\) but different neutron numbers. Same \(A\) with different \(Z\) describes isobars, not isotopes. Same \(N\) with different \(Z\) describes isotones.
9. A neutral atom has \(Z=6\) and \(A=14\). How many electrons does the neutral atom have?
ⓐ. \(8\)
ⓑ. \(14\)
ⓒ. \(20\)
ⓓ. \(6\)
Correct Answer: \(6\)
Explanation: In a neutral atom, the number of electrons equals the number of protons. The atomic number \(Z\) gives the number of protons in the nucleus. Here \(Z=6\), so a neutral atom has \(6\) electrons outside the nucleus. The mass number \(A=14\) counts protons and neutrons inside the nucleus, not electrons. The neutron number would be \(A-Z=14-6=8\), but that is not the electron number. Neutrality connects electrons with \(Z\), not with \(A\).
10. A simple atom model shows a tiny central region and a much larger surrounding region of electrons. The tiny central region should be identified as:
ⓐ. The nucleus, because it contains protons and neutrons
ⓑ. The atomic shell, because it contains all electrons
ⓒ. The photon cloud, because it stores light energy
ⓓ. The neutral zone, because all charges cancel there
Correct Answer: The nucleus, because it contains protons and neutrons
Explanation: The nucleus is the small central region of the atom. It contains protons and neutrons and therefore carries almost all the atomic mass. Electrons occupy the surrounding atomic region and are not part of the nucleus. A photon is not a permanent constituent of the nucleus in this basic composition picture. The nucleus is positively charged, not a place where all charge disappears. The atom may be neutral overall, but its positive and negative charges are located in different parts.
11. For the nuclide \({}^{31}_{15}P\), the number of neutrons is:
ⓐ. \(16\)
ⓑ. \(15\)
ⓒ. \(31\)
ⓓ. \(46\)
Correct Answer: \(16\)
Explanation: \( \textbf{Given notation:} \) The nuclide is \({}^{31}_{15}P\).
\( \textbf{Mass number:} \) \(A=31\).
\( \textbf{Atomic number:} \) \(Z=15\).
\( \textbf{Relation used:} \)
\[
N=A-Z
\]
\( \textbf{Why this relation applies:} \) The mass number counts all nucleons, while \(Z\) counts only protons.
\( \textbf{Substitution:} \)
\[
N=31-15
\]
\( \textbf{Calculation:} \)
\[
N=16
\]
\( \textbf{Final answer:} \) The nucleus contains \(16\) neutrons. The chemical symbol \(P\) is fixed by \(Z=15\), while the neutron count comes from the difference between \(A\) and \(Z\).
12. In the notation \({}^{A}_{Z}X\), the upper number and lower number represent, respectively:
ⓐ. Atomic number and neutron number
ⓑ. Mass number and atomic number
ⓒ. Neutron number and mass number
ⓓ. Nuclear charge and electron number
Correct Answer: Mass number and atomic number
Explanation: In \({}^{A}_{Z}X\), the upper left number \(A\) is the mass number. It gives the total number of protons and neutrons in the nucleus. The lower left number \(Z\) is the atomic number. It gives the number of protons and fixes the element \(X\). The neutron number is not written directly in this notation but is found from \(N=A-Z\). The nuclear charge is related to \(Z\) as \(+Ze\), but it is not the lower number itself.
13. A neutral atom and its nucleus are compared for charge. The nucleus has charge \(+Ze\), while the neutral atom has net charge:
ⓐ. \(+Ze\)
ⓑ. \(-Ze\)
ⓒ. Zero
ⓓ. \(+(A-Z)e\)
Correct Answer: Zero
Explanation: The nucleus contains \(Z\) protons, so its charge is \(+Ze\). In a neutral atom, there are also \(Z\) electrons outside the nucleus. The total electron charge is \(-Ze\). Adding nuclear charge and electron charge gives \(+Ze-Ze=0\). This cancellation applies to the whole atom, not to the nucleus alone. The nucleus remains positively charged even when the atom as a whole is neutral.
14. Match the nuclear terms with their meanings.
| Column I | Column II |
|---|
| P. \(Z\) | 1. Number of neutrons |
| Q. \(A\) | 2. Number of protons |
| R. \(N\) | 3. Total number of nucleons |
| S. \(+Ze\) | 4. Nuclear charge |
ⓐ. P-3, Q-2, R-1, S-4
ⓑ. P-2, Q-1, R-3, S-4
ⓒ. P-4, Q-3, R-1, S-2
ⓓ. P-2, Q-3, R-1, S-4
Correct Answer: P-2, Q-3, R-1, S-4
Explanation: The symbol \(Z\) stands for atomic number, which is the number of protons in the nucleus. The symbol \(A\) stands for mass number, which is the total number of nucleons. The symbol \(N\) stands for neutron number. The expression \(+Ze\) gives the charge of the nucleus because each of the \(Z\) protons contributes \(+e\). These four quantities are connected by \(A=Z+N\), but they do not mean the same thing. Confusing \(A\) with \(Z\) changes both the composition and the charge interpretation.
15. The pair \({}^{12}_{6}C\) and \({}^{14}_{6}C\) represents two nuclei of carbon. Their classification is:
ⓐ. Isobars, because their mass numbers differ
ⓑ. Isotones, because their neutron numbers are the same
ⓒ. Isotopes, because their atomic numbers are the same
ⓓ. Identical nuclides, because both contain carbon
Correct Answer: Isotopes, because their atomic numbers are the same
Explanation: Both nuclides have \(Z=6\), so both are carbon nuclei. Their mass numbers are different: \(A=12\) and \(A=14\). Since \(N=A-Z\), their neutron numbers are also different. Nuclides with the same \(Z\) but different \(A\) are isotopes. They are not identical nuclides because their neutron numbers are not the same. The same chemical symbol does not mean the same nuclear composition.
16. A nuclide has \(Z=19\), \(A=39\), and is neutral as an atom. Choose the complete composition statement.
ⓐ. \(20\) protons, \(19\) neutrons, and \(19\) electrons
ⓑ. \(19\) protons, \(20\) neutrons, and \(19\) electrons
ⓒ. \(19\) protons, \(39\) neutrons, and \(20\) electrons
ⓓ. \(39\) protons, \(19\) neutrons, and \(39\) electrons
Correct Answer: \(19\) protons, \(20\) neutrons, and \(19\) electrons
Explanation: \( \textbf{Given data:} \) \(Z=19\), \(A=39\), and the atom is neutral.
\( \textbf{Protons:} \) Atomic number gives the number of protons, so protons \(=19\).
\( \textbf{Neutrons relation:} \)
\[
N=A-Z
\]
\( \textbf{Substitution:} \)
\[
N=39-19=20
\]
\( \textbf{Electrons in a neutral atom:} \) Electron number equals proton number, so electrons \(=19\).
\( \textbf{Composition:} \) The atom has \(19\) protons, \(20\) neutrons, and \(19\) electrons.
\( \textbf{Final answer:} \) The complete composition is \(19\) protons, \(20\) neutrons, and \(19\) electrons. The neutron number is not equal to \(Z\) unless the nucleus happens to have \(A=2Z\).
17. The unit \(1\,\text{fm}\) is used in nuclear physics mainly because it is suitable for measuring:
ⓐ. Atomic masses
ⓑ. Radioactive activity
ⓒ. Binding energy
ⓓ. Nuclear radii
Correct Answer: Nuclear radii
Explanation: The fermi, written as \(\text{fm}\), is a very small unit of length. It is commonly used for nuclear dimensions because nuclear radii are of the order of a few \(\text{fm}\). Atomic radii are much larger than nuclear radii, so ordinary atomic sizes are not naturally expressed on the same scale. Activity is measured in units such as \(\text{Bq}\), and energy is often measured in \(\text{eV}\) or \(\text{MeV}\). The use of \(\text{fm}\) reminds us that the nucleus is far smaller than the atom.
18. The energy unit most commonly used for nuclear binding energies is:
ⓐ. \(\text{MeV}\)
ⓑ. \(\text{fm}\)
ⓒ. \(\text{kg}\)
ⓓ. \(\text{s}^{-1}\)
Correct Answer: \(\text{MeV}\)
Explanation: Nuclear binding energies are very small on the joule scale for a single nucleus, but they are naturally expressed in electron-volt based units. The mega electron volt, written as \(\text{MeV}\), is especially convenient for nuclear energy values. The unit \(\text{fm}\) is a length unit used for nuclear size. The unit \(\text{kg}\) is a mass unit, not an energy unit. The unit \(\text{s}^{-1}\) is used for quantities such as decay constant \(\lambda\). Using \(\text{MeV}\) connects nuclear energy directly with mass-energy conversion in later calculations.
19. The relation \(1\,u\,c^2\approx931.5\,\text{MeV}\) is useful because it connects:
ⓐ. Nuclear radius with mass number
ⓑ. Half-life with decay constant
ⓒ. Atomic mass unit to energy
ⓓ. Nuclear charge with electron charge
Correct Answer: Atomic mass unit to energy
Explanation: The symbol \(u\) denotes the atomic mass unit. Multiplying a mass by \(c^2\) gives its energy equivalent according to mass-energy equivalence. Thus \(1\,u\,c^2\approx931.5\,\text{MeV}\) tells how much energy corresponds to a mass of \(1\,u\). This relation is later used when mass defect is converted into binding energy or reaction energy. It is not a radius law, because nuclear radius involves \(R=R_0A^{1/3}\). It is also different from radioactive decay relations, where \(\lambda\), \(T_{1/2}\), and \(\tau\) appear.
20. In radioactive decay notation, the decay constant \(\lambda\) has which type of unit?
ⓐ. Inverse-time unit
ⓑ. Unit of length, such as \(\text{fm}\)
ⓒ. Unit of mass, such as \(\text{u}\)
ⓓ. Unit of energy, such as \(\text{MeV}\)
Correct Answer: Inverse-time unit
Explanation: The decay constant \(\lambda\) represents the probability per unit time that a nucleus will decay. Since it is measured per unit time, its unit is inverse time. If time is measured in seconds, the unit of \(\lambda\) is \(\text{s}^{-1}\). This is different from \(T_{1/2}\) and \(\tau\), which are times and therefore have units such as \(\text{s}\). It is also different from \(\text{MeV}\), \(\text{u}\), and \(\text{fm}\), which belong to energy, mass, and length respectively. The inverse-time unit helps keep the exponential factor \(\lambda t\) dimensionless.
