How come Silicon differs from the trend of increasing electron affinity as we move to the right in a period? Is it because more energy is released to fill in the last orbital? Or is it something else?
I'm not sure I agree that Si is out of place. As we move from left to right Al is -44 kJ/mol, Si is -119, P is -74, S is -200, and Cl is -345. P is the one out of place, I think, and not Si. And that is because the p orbital of P is half filled.
You are correct in observing that Silicon (Si) does not follow the trend of increasing electron affinity as we move to the right in a period. The trend you mentioned is generally true for the elements in a period, but there are exceptions.
The electron affinity of an atom refers to the amount of energy released when an electron is added to a neutral atom to form a negatively charged ion. A higher negative value indicates a stronger attraction for an additional electron.
Regarding Silicon, the trend you described shows that the electron affinity values of Aluminum (Al), Si, and Phosphorus (P) are not consistent with the general trend. To understand why Silicon deviates from the trend, we need to consider its electronic configuration.
Silicon has the electronic configuration [Ne] 3s^2 3p^2. When an electron is added to Silicon to form Si^-, it will enter one of the 3p orbitals. However, all the p orbitals of Silicon are already half-filled (3p^2), which means that adding an electron to Silicon requires more energy than what would be expected based solely on the trend.
In contrast, Phosphorus has the electronic configuration [Ne] 3s^2 3p^3. Its 3p orbitals are not yet half-filled, which makes it easier for Phosphorus to accept an additional electron and have a higher electron affinity compared to Silicon.
So, in summary, Silicon deviates from the trend of increasing electron affinity because its 3p orbitals are already half-filled, making it energetically less favorable to accept an additional electron. On the other hand, Phosphorus fits the trend because its 3p orbitals are not fully occupied, enabling it to more readily accept an electron and have a higher electron affinity value.