What determines how favorable (exothermic) the electron affinity of a certain element is? (Favorable means how exothermic a reaction is, more favorable= more exothermic)

There are some abnormalities in the trends of how favorable the electron affinity is.

From Al to Cl, the 
Electronegativity is increasing, but the energy releasing from the electron affinity of P is smaller than that of Si. Why? Is it because of how the electrons are arranged?

There are many determinants of how favorable electron affinity is. For example, oxygen and sulfur. Generally, we will think oxygen to have a more favorable electron affinity, due to its stronger electronegativity. Both have the same electrons config, except for the number of shells. But sulfur appears to have a more favorable electron affinity because of its larger size. The smaller size of oxygen atom produces more electron-electron repulsion when one or more electrons are added. 

All I ask for is a list of determinants. I tried to find one but couldn't.
The electron affinity (EA) of an atom (A) is defined as the minimal energy that is required to free an electron (e$^-$) from the associated anion (A$^-$)

$$
\text{EA: }\qquad\text{A}^-\text{(g)}\rightarrow \text{A(g)}+\text{e}^-,
$$
in other words, the electron affinity is the ionization energy of the associated anion. 

A large positive EA means that the anion A$^-$ is stable, while a negative EA indicates that the anion ${\text{A}^-}$ is unstable (such as He$^-$). Halogens have the largest electronegativity as the associated anion obtains a completely filled shell. 

In general, if an atom A obtains a full/half full shell/sub shell by addition of an additional electron, then A will have a large EA, while A will have a small EA if it already has a full/half full shell/sub shell. 

In addition there are some competing effects along a group in the periodic table. Consider the EA of the halogens for example:

Atom:      F       Cl      Br      I
EA /eV     3.40    3.61    3.36    3.06


(i) The smaller the principle quantum number $n$ of the highest sub shell, the larger is the attraction between the nucleus and electrons and the larger is EA. This would predict the trend to be EA(F) > EA(Cl) > EA(Br) > EA(I).

(ii) The smaller the principle quantum number $n$ of the highest sub shell, the larger is the electron repulsion the smaller is EA. The expected trend is EA(F) &lt; EA(Cl) &lt; EA(Br) &lt; EA(I).

As these effects are in competition with each other, there is a maximum in the observed trend for the halogens around chlorine. Where the maximum occurs, is unfortunately difficult to predict.