The electron affinities of the elements from aluminum to chlorine are -44, -120, -74, -200.4, and -384.7 kJ/mol, respectively. Rationalize the trend in these values.
To rationalize the trend in electron affinities from aluminum to chlorine, we need to consider the characteristics of atoms and their electron configurations.
Electron affinity is the energy released when an atom gains an electron to form a negative ion. It is influenced by several factors, including atomic size, effective nuclear charge, and electron configuration.
When moving from aluminum to chlorine in the periodic table, we move across the period from left to right. Let's analyze the trend based on these factors:
1. Atomic Size:
The atomic size decreases from left to right across a period due to the increasing nuclear charge, which attracts electrons more strongly and causes the atomic radius to decrease. Smaller atomic size leads to a higher electron affinity because the added electron is closer to the nucleus, experiencing a stronger attraction.
2. Effective Nuclear Charge:
As we move across the period, the effective nuclear charge experienced by the valence electrons increases. Effective nuclear charge is the positive charge felt by the outermost electrons, which is determined by the number of protons in the nucleus and the shielding effect of inner electron shells. With higher effective nuclear charge, the attractive force on incoming electrons increases, resulting in higher electron affinities.
3. Electron Configuration:
When moving across the period, the electron configuration gradually fills up the 2p orbital with additional electrons. This leads to increased electron-electron repulsions, making it harder for an additional electron to be added. As a result, the electron affinity decreases.
Based on these factors, we can explain the trend in electron affinities from aluminum to chlorine:
- Aluminum (Al):
Aluminum has a relatively low electron affinity (-44 kJ/mol) because it has a larger atomic size compared to the elements on its right. The added electron is farther away from the nucleus, experiencing weaker attraction.
- Silicon (Si):
The electron affinity of silicon has an intermediate value (-120 kJ/mol) because it has a smaller atomic size and higher effective nuclear charge compared to aluminum. The added electron experiences a stronger attraction but still faces some electron-electron repulsion.
- Phosphorus (P):
Phosphorus has a lower electron affinity (-74 kJ/mol) compared to silicon. While the atomic size continues to decrease, the additional electron experiences slightly higher repulsion due to the filling up of the 2p orbital.
- Sulfur (S):
The electron affinity of sulfur is significantly higher (-200.4 kJ/mol) due to its smaller atomic size and higher effective nuclear charge. The added electron experiences a strong attraction and relatively low electron-electron repulsion.
- Chlorine (Cl):
Chlorine has the highest electron affinity (-384.7 kJ/mol) among these elements. Its smaller atomic size, high effective nuclear charge, and the filling of the 2p orbital make it extremely favorable for an additional electron to be added, resulting in a high electron affinity.
In summary, the trend in electron affinities from aluminum to chlorine can be rationalized by considering the decreasing atomic size, increasing effective nuclear charge, and the electron configuration filling the 2p orbital.