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Ch.8 - Periodic Properties of the Elements
All textbooksTro 4th EditionCh.8 - Periodic Properties of the ElementsProblem 79c,d
Chapter 8, Problem 79c,d

Choose the element with the more negative (more exothermic) electron affinity from each pair. c. C or N d. Li or F

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Step 1: Understand the concept of electron affinity. Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negative ion. It is usually exothermic, meaning energy is released when an electron is added. The more negative the electron affinity, the more energy is released, and the more an atom wants to gain an electron.
Step 2: Look at the periodic table. Electron affinity generally increases from left to right across a period and decreases from top to bottom down a group. This is because as you move from left to right across a period, the number of protons increases, which increases the positive charge of the nucleus and the desire to attract electrons. As you move down a group, the number of energy levels increases, which means the outer electrons are further from the nucleus and less attracted to it.
Step 3: Compare the positions of Carbon (C) and Nitrogen (N) on the periodic table. Both elements are in the same period (Period 2), but Nitrogen is to the right of Carbon.
Step 4: Based on the trend of electron affinity, Nitrogen (N), being to the right of Carbon (C) in the same period, should have a more negative (more exothermic) electron affinity.
Step 5: However, there is an exception to this trend. Half-filled and fully filled subshells are particularly stable arrangements of electrons. Nitrogen, with a p3 configuration, has a half-filled p subshell, which is a particularly stable arrangement. Therefore, it is less inclined to accept an additional electron, making its electron affinity less negative than expected. So, in this case, Carbon (C) has a more negative electron affinity than Nitrogen (N).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Electron Affinity

Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gas phase. A more negative electron affinity indicates that the process is more exothermic, meaning energy is released when the atom gains an electron. This property is crucial for understanding how easily an atom can accept an electron and form an anion.
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Trends in Electron Affinity

Electron affinity varies across the periodic table, generally increasing (becoming more negative) from left to right and decreasing down a group. This trend is influenced by atomic size and effective nuclear charge. For example, elements in the same group exhibit less negative electron affinities as you move down due to increased distance from the nucleus and electron shielding.
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Comparison of Carbon and Nitrogen

When comparing carbon (C) and nitrogen (N), it is essential to consider their positions in the periodic table. Nitrogen has a more negative electron affinity than carbon because it is closer to achieving a stable electron configuration (octet) upon gaining an electron. This makes nitrogen more favorable for electron gain, resulting in a more exothermic reaction.
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Related Practice
Textbook Question

Consider this set of ionization energies. IE1 = 578 kJ/mol IE2 = 1820 kJ/mol IE3 = 2750 kJ/mol IE4 = 11,600 kJ/mol To which third-period element do these ionization values belong?

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Textbook Question

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