The electron affinities of the elements from aluminum to chlorine are \(-44,-120,-74,-200.4,\) and \(-384.7 \mathrm{kJ} / \mathrm{mol}\) respectively. Rationalize the trend in these values.

Electron affinity is the amount of energy released or gained when an electron is added to a neutral atom to form a negatively charged ion. In general, electron affinity increases from left to right across a period and decreases as you move down a group in the periodic table. This is due to effective nuclear charge and shielding effect.

The elements mentioned in the exercise are aluminum (Al), silicon (Si), phosphorus (P), sulfur (S), and chlorine (Cl). These elements belong to the third period of the periodic table, and their atomic numbers are 13, 14, 15, 16, and 17, respectively.

The given electron affinities are: Aluminum: -44 kJ/mol Silicon: -120 kJ/mol Phosphorus: -74 kJ/mol Sulfur: -200.4 kJ/mol Chlorine: -384.7 kJ/mol As we move from aluminum to chlorine, we observe an overall increase in the electron affinity with one exception in the case of phosphorus. This trend correlates with the general trend, which states that electron affinity increases from left to right across a period.

The trend in electron affinities can be attributed to the following factors: 1. Effective Nuclear Charge (Zeff): As we move from left to right across a period, Zeff increases, attracting electrons more strongly and thus making it energetically more favorable to add an electron. This trend explains the general increase in electron affinity as we move across the period from aluminum to chlorine. 2. Shielding Effect: The shielding effect remains relatively constant across the elements in the same period, as they have the same number of electron shells. As a result, the effective nuclear charge predominantly determines the trend in electron affinities within the same period. 3. Electron Configuration: Phosphorus has an exception in the trend due to its electron configuration (3s^2 3p^3). When an electron is added to phosphorus, it experiences a certain amount of electron-electron repulsion as it occupies the same 3p orbital as the other electrons. This repulsion causes its electron affinity to be lower than that of Silicon (less negative). In conclusion, the trend in electron affinities of the elements from aluminum to chlorine can be rationalized by considering the effective nuclear charge, shielding effect, and electron configuration. The general trend shows an increase in electron affinity across the period, with the exception of phosphorus, which can be explained by electron-electron repulsion in its electron configuration.