301. Electron gain enthalpy is the enthalpy change when
ⓐ. an electron is removed from an isolated gaseous atom to form a gaseous cation
ⓑ. an electron is added to an isolated gaseous atom to form a gaseous anion
ⓒ. a solid atom is converted into a liquid atom
ⓓ. a neutron is added to the nucleus of an atom
Correct Answer: an electron is added to an isolated gaseous atom to form a gaseous anion
Explanation: Electron gain enthalpy refers to the process in which an isolated gaseous atom accepts an electron. The atom becomes a gaseous anion after electron gain. The gaseous state is specified so that the energy change is not mixed with lattice, hydration, or bonding effects. Electron removal is described by ionization enthalpy, not electron gain enthalpy. The concept measures how energetically favourable or unfavourable electron addition is for an atom.
302. The process that represents first electron gain enthalpy of an element \(X\) is
ⓐ. \(\mathrm{X(g)+e^- \rightarrow X^-(g)}\)
ⓑ. \(\mathrm{X(g) \rightarrow X^+(g)+e^-}\)
ⓒ. \(\mathrm{X^-(g) \rightarrow X(g)+e^-}\)
ⓓ. \(\mathrm{X(s) \rightarrow X(g)}\)
Correct Answer: \(\mathrm{X(g)+e^- \rightarrow X^-(g)}\)
Explanation: Electron gain enthalpy involves adding an electron to a neutral gaseous atom. The correct equation must show \(\mathrm{X(g)}\) accepting \(\mathrm{e^-}\) and forming \(\mathrm{X^-(g)}\). Ionization removes an electron, reverse electron gain removes the added electron from the anion, and conversion from solid to gas is a physical change. The sign and meaning of the enthalpy change depend on the electron being added, not removed.
303. For many non-metal atoms, the first electron gain enthalpy is negative because
ⓐ. energy is released when the incoming electron is attracted by the nucleus
ⓑ. energy must always be supplied to force the electron into the nucleus
ⓒ. the atom loses an electron before gaining one
ⓓ. the number of protons becomes zero after electron gain
Correct Answer: energy is released when the incoming electron is attracted by the nucleus
Explanation: When an electron approaches a neutral atom, it may be attracted by the positive nucleus. If the attraction is strong enough, energy is released when the electron is added. In such cases, electron gain enthalpy is negative. Many non-metals have a strong tendency to accept electrons because they are close to completing an octet. The negative sign shows energy release, not absence of energy change.
304. The second electron gain enthalpy for an atom is usually positive because the second electron is added to
ⓐ. a neutral atom with no electron-electron repulsion
ⓑ. an already negatively charged ion, causing strong repulsion
ⓒ. a bare nucleus without any electrons
ⓓ. a noble gas that has lost its shell
Correct Answer: an already negatively charged ion, causing strong repulsion
Explanation: The first electron is added to a neutral gaseous atom. After this addition, the species becomes a negatively charged ion. Adding a second electron means bringing another negative charge toward an already negative ion. Electron-electron repulsion must be overcome, so energy has to be supplied. This is why second electron gain enthalpy values are usually positive, even when the first electron gain enthalpy is negative.
305. Halogens have highly negative first electron gain enthalpy mainly because halogen atoms
ⓐ. have completely filled \(d\)-subshells as the only active feature
ⓑ. are very large metals with one valence electron
ⓒ. need one electron to complete a stable octet
ⓓ. already have no attraction for electrons
Correct Answer: need one electron to complete a stable octet
Explanation: Halogens have the general valence-shell configuration \(ns^2np^5\). They need only one more electron to complete the octet. Because of this, electron addition is energetically favourable for them. The incoming electron is strongly attracted by the nucleus and leads to a more stable valence-shell arrangement. This gives halogens very negative first electron gain enthalpy values.
306. Across a period from left to right, electron gain enthalpy generally becomes more negative because
ⓐ. atomic number decreases steadily
ⓑ. a new shell is added at every step
ⓒ. metallic character increases continuously
ⓓ. effective nuclear charge increases and atoms more strongly attract an added electron
Correct Answer: effective nuclear charge increases and atoms more strongly attract an added electron
Explanation: Across a period, nuclear charge increases while added electrons enter the same main shell. The effective nuclear charge felt by valence electrons generally increases. This stronger attraction also makes the atom more capable of attracting an extra electron. Therefore, electron gain enthalpy generally becomes more negative from left to right, especially toward the halogens. The trend is not perfectly smooth because stable subshell arrangements and electron-electron repulsion can create exceptions.
307. Down a group, electron gain enthalpy generally becomes less negative because
ⓐ. the added electron enters a larger, more shielded shell farther from the nucleus
ⓑ. atomic size always decreases sharply
ⓒ. nuclear charge becomes zero
ⓓ. the added electron enters the nucleus
Correct Answer: the added electron enters a larger, more shielded shell farther from the nucleus
Explanation: Down a group, the atomic size increases because new shells are added. The incoming electron is added farther from the nucleus. Inner electrons also shield the added electron from the full nuclear charge. As a result, the attraction for an added electron usually becomes weaker. This makes electron gain enthalpy generally less negative down a group.
308. A graph of first electron gain enthalpy across a period is described below.
The x-axis shows increasing atomic number across a period. The y-axis shows electron gain enthalpy. The values generally move downward into more negative values as the halogen region is approached, but the noble gas value rises sharply to a positive region.
The sharp rise for the noble gas is best explained by
ⓐ. noble gases have only one valence electron
ⓑ. noble gases have the lowest nuclear charge in every period
ⓒ. addition of an electron to a stable closed shell is unfavourable
ⓓ. noble gases are placed in group \(1\)
Correct Answer: addition of an electron to a stable closed shell is unfavourable
Explanation: Noble gases have closed valence-shell configurations such as \(ns^2np^6\), except helium with \(1s^2\). Adding an extra electron would require entry into a new higher-energy shell or disturb a very stable arrangement. This process is not energetically favourable, so the electron gain enthalpy is positive or much less negative. The graph therefore rises sharply at the noble gas. The sharp change reflects closed-shell stability, not weak nuclear identity.
309. Chlorine has a more negative first electron gain enthalpy than fluorine because
ⓐ. fluorine has no attraction for electrons
ⓑ. the incoming electron in fluorine enters a very small \(2p\) shell with strong electron-electron repulsion
ⓒ. chlorine has fewer occupied shells than fluorine
ⓓ. chlorine is a noble gas with a complete octet
Correct Answer: the incoming electron in fluorine enters a very small \(2p\) shell with strong electron-electron repulsion
Explanation: Fluorine is smaller than chlorine and has a very compact \(2p\) subshell. When an extra electron enters this small region, electron-electron repulsion is relatively strong. Chlorine has a larger \(3p\) subshell, so the added electron experiences less crowding. Although fluorine has strong nuclear attraction, the repulsion in its small shell reduces the energy released. Therefore, chlorine has a more negative first electron gain enthalpy than fluorine.
310. A correct comparison of first electron gain enthalpy is
ⓐ. \(\mathrm{Ne}\) is more negative than \(\mathrm{F}\)
ⓑ. \(\mathrm{Na}\) is more negative than \(\mathrm{Cl}\)
ⓒ. \(\mathrm{Ar}\) is more negative than \(\mathrm{Cl}\)
ⓓ. \(\mathrm{Cl}\) is more negative than \(\mathrm{F}\)
Correct Answer: \(\mathrm{Cl}\) is more negative than \(\mathrm{F}\)
Explanation: Halogens generally have highly negative electron gain enthalpies because they need one electron to complete an octet. However, fluorine is exceptionally small, and the added electron enters a compact \(2p\) subshell with strong repulsion. Chlorine has a larger \(3p\) subshell where the incoming electron is accommodated with less repulsion. Thus chlorine releases more energy on electron gain than fluorine. Noble gases such as \(\mathrm{Ne}\) and \(\mathrm{Ar}\) resist electron gain because of closed-shell configurations.
311. The table gives qualitative observations for four elements.
| Element type | Valence-shell clue | Expected first electron gain enthalpy |
|---|
| P | \(ns^1\) | Not highly negative |
| Q | \(ns^2np^5\) | Highly negative |
| R | \(ns^2np^6\) | Positive or much less favourable |
| S | Closed shell | Highly negative due to easy octet completion |
The row that needs correction is
ⓐ. Row P
ⓑ. Row S
ⓒ. Row Q
ⓓ. Row R
Correct Answer: Row S
Explanation: Row S needs correction because a closed shell does not favour easy electron gain. A closed-shell atom is already stable, and adding another electron requires entry into a higher-energy level or causes strong unfavourable effects. Row Q is correct because \(ns^2np^5\) elements need one electron to complete an octet and usually have highly negative electron gain enthalpy. Row R is also reasonable because \(ns^2np^6\) noble gases resist electron addition. Electron gain is most favourable when the added electron completes a stable arrangement, not when it must go beyond one.
312. Consider the statements about electron gain enthalpy.
I. It is defined for adding an electron to an isolated gaseous atom.
II. It is often negative for halogens.
III. The second electron gain enthalpy is usually negative because an anion attracts another electron strongly.
IV. Noble gases usually have positive or very unfavourable electron gain enthalpy.
The supported statements are
ⓐ. I and III only
ⓑ. II and III only
ⓒ. I, II, III, and IV
ⓓ. I, II, and IV only
Correct Answer: I, II, and IV only
Explanation: Statement I is correct because electron gain enthalpy is defined for gaseous atoms. Statement II is correct because halogens have one-electron-short-of-octet configurations and release energy on electron gain. Statement IV is also correct because noble gases have closed shells, so electron addition is unfavourable. Statement III is wrong because the second electron is added to an already negative ion. The repulsion makes the second electron gain enthalpy positive in usual cases.
313. Match the factor in Column I with its effect on first electron gain enthalpy in Column II.
| Column I | Column II |
|---|
| P. Higher \(Z_{\text{eff}}\) | 1. Usually makes electron gain more favourable |
| Q. Very small compact shell | 2. Can increase electron-electron repulsion for the added electron |
| R. Closed-shell configuration | 3. Makes electron gain unfavourable |
| S. Larger size down a group | 4. Usually weakens attraction for the added electron |
ⓐ. P-1, Q-2, R-3, S-4
ⓑ. P-2, Q-1, R-3, S-4
ⓒ. P-1, Q-3, R-2, S-4
ⓓ. P-4, Q-2, R-1, S-3
Correct Answer: P-1, Q-2, R-3, S-4
Explanation: Higher \(Z_{\text{eff}}\) generally increases attraction for an added electron, making electron gain more favourable. A very small compact shell can create strong electron-electron repulsion, as seen in fluorine. A closed-shell configuration resists electron addition because it is already stable. Larger size down a group places the added electron farther from the nucleus and usually weakens attraction. These factors explain why electron gain enthalpy trends are regular but not perfectly smooth.
314. Assertion: The first electron gain enthalpy of chlorine is more negative than that of fluorine.
Reason: The added electron experiences less electron-electron repulsion in the larger \(3p\) subshell of chlorine than in the compact \(2p\) subshell of fluorine.
ⓐ. Both Assertion and Reason are true, but Reason does not explain Assertion
ⓑ. Assertion is true, but Reason is false
ⓒ. Both Assertion and Reason are true, and Reason explains Assertion
ⓓ. Assertion is false, but Reason is true
Correct Answer: Both Assertion and Reason are true, and Reason explains Assertion
Explanation: The Assertion is true because chlorine has a more negative first electron gain enthalpy than fluorine. The Reason is also true because the incoming electron in fluorine enters a very small \(2p\) region where repulsion is high. In chlorine, the \(3p\) subshell is larger, so electron-electron repulsion is less severe. This allows more energy to be released when chlorine gains an electron. The Reason directly explains the exception to the simple down-group expectation.
315. A neutral atom releases \(349\,\text{kJ mol}^{-1}\) when one mole of gaseous atoms gains one mole of electrons. The electron gain enthalpy should be reported as
ⓐ. \(+349\,\text{kJ mol}^{-1}\)
ⓑ. \(0\,\text{kJ mol}^{-1}\)
ⓒ. \(349\,\text{pm}\)
ⓓ. \(-349\,\text{kJ mol}^{-1}\)
Correct Answer: \(-349\,\text{kJ mol}^{-1}\)
Explanation: \( \textbf{Given information:} \) Energy is released when electrons are added.
\( \textbf{Process type:} \) Electron gain enthalpy describes electron addition to gaseous atoms.
\( \textbf{Sign rule:} \) Energy release corresponds to a negative enthalpy change.
\( \textbf{Magnitude given:} \) \(349\,\text{kJ mol}^{-1}\).
\[
\Delta_{\text{eg}}H=-349\,\text{kJ mol}^{-1}
\]
\( \textbf{Unit check:} \) The unit is energy per mole, not a length unit such as \(\text{pm}\).
\( \textbf{Final answer:} \) The value is \(-349\,\text{kJ mol}^{-1}\). The negative sign carries the information that the process is exothermic.
316. An element \(M\) has first electron gain enthalpy \(-60\,\text{kJ mol}^{-1}\), while element \(N\) has \(-350\,\text{kJ mol}^{-1}\). The stronger tendency to accept an electron is shown by
ⓐ. \(N\), because its electron gain enthalpy is more negative
ⓑ. \(M\), because its value is closer to zero
ⓒ. both equally, because both values have a negative sign
ⓓ. neither, because electron gain enthalpy cannot be compared
Correct Answer: \(N\), because its electron gain enthalpy is more negative
Explanation: \( \textbf{Values given:} \) \(M=-60\,\text{kJ mol}^{-1}\), \(N=-350\,\text{kJ mol}^{-1}\).
\( \textbf{Meaning of negative sign:} \) Energy is released during electron gain.
\( \textbf{Comparison:} \) A more negative value means more energy is released.
\( \textbf{For \(M\):} \) Only \(60\,\text{kJ mol}^{-1}\) is released.
\( \textbf{For \(N\):} \) \(350\,\text{kJ mol}^{-1}\) is released.
\( \textbf{Chemical interpretation:} \) \(N\) has the more favourable electron-gain process.
\( \textbf{Final answer:} \) \(N\) shows the stronger tendency to accept an electron. Negative values must be compared by their sign and magnitude together.
317. Nitrogen has a much less negative first electron gain enthalpy than expected for its position because nitrogen has
ⓐ. a completely empty valence shell
ⓑ. no attraction between nucleus and electrons
ⓒ. a stable half-filled \(2p^3\) arrangement
ⓓ. a filled \(3d^{10}\) subshell
Correct Answer: a stable half-filled \(2p^3\) arrangement
Explanation: Nitrogen has the valence-shell configuration \(2s^2\,2p^3\). The three \(2p\) electrons occupy separate \(p\) orbitals, giving a relatively stable half-filled arrangement. Adding another electron would disturb this stability and introduce pairing in one \(p\) orbital. Therefore, electron gain is less favourable than a simple left-to-right trend might suggest. This is an electronic-configuration exception, not a failure of periodicity.
318. For noble gases, first electron gain enthalpy is positive or unfavourable mainly because the incoming electron must enter
ⓐ. a half-filled \(p\) subshell that needs one electron
ⓑ. the same orbital as a proton
ⓒ. a metallic lattice site
ⓓ. a new higher-energy shell beyond a closed valence shell
Correct Answer: a new higher-energy shell beyond a closed valence shell
Explanation: Noble gases have complete valence-shell configurations. Adding an extra electron cannot simply complete the same valence shell because it is already filled. The incoming electron would have to enter a higher-energy shell, which is unfavourable. This is why noble gases have positive or very small electron gain tendencies compared with halogens. The closed shell explains their low tendency to form anions.
319. A claim says, “The element with the highest first ionization enthalpy in a period must also have the most negative electron gain enthalpy.” The best evaluation is that the claim is
ⓐ. correct because both properties always increase together without exceptions
ⓑ. correct only because noble gases form \(-1\) ions most easily
ⓒ. incorrect because noble gases have very high ionization enthalpy but unfavourable electron gain enthalpy
ⓓ. incorrect because ionization enthalpy and electron gain enthalpy are both unrelated to electronic configuration
Correct Answer: incorrect because noble gases have very high ionization enthalpy but unfavourable electron gain enthalpy
Explanation: Ionization enthalpy and electron gain enthalpy both involve electrons, but they measure different processes. Noble gases have very high ionization enthalpies because their electrons are strongly held in closed-shell configurations. However, the same closed-shell stability makes adding another electron unfavourable. Halogens usually have the most negative electron gain enthalpies because one electron completes their octet. A high resistance to losing electrons does not automatically mean a strong tendency to gain electrons.
320. The best combined statement about electron gain enthalpy is that it
ⓐ. depends only on atomic mass and never on electronic configuration
ⓑ. becomes more favourable with strong attraction for an added electron, but exceptions arise from small size, repulsion, and stable configurations
ⓒ. is always positive for halogens and always negative for noble gases
ⓓ. has the same meaning as first ionization enthalpy
Correct Answer: becomes more favourable with strong attraction for an added electron, but exceptions arise from small size, repulsion, and stable configurations
Explanation: Electron gain enthalpy depends on how strongly an atom can attract an added electron. Higher effective nuclear charge and a near-octet configuration often make electron gain favourable. However, small compact shells can create strong electron-electron repulsion, as in the comparison of \(\mathrm{F}\) and \(\mathrm{Cl}\). Stable configurations such as half-filled or completely filled subshells can also make electron addition less favourable. The trend must therefore be explained through attraction, repulsion, size, and electronic arrangement together.
