<p>So, the question is…
Which atom, selenium (Se) or Arsenic (As), has the higher electron affinity?</p>
<ol>
<li>Selenium, because it has a higher effective nuclear charge.</li>
<li>Selenium, because it does not have a stable, half-filled p subshell and adding an electron does not decrease its stability.</li>
</ol>
<p>Which is right??? I put 2. but…i’m dubious…</p>
<p>2 is correct. :)</p>
<p>Take a look at the Electron Affinity table in Wikipedia.</p>
<p>[Electron</a> affinity - Wikipedia, the free encyclopedia](<a href=“http://en.wikipedia.org/wiki/Electron_affinity]Electron”>Electron affinity - Wikipedia)</p>
<p>Effective nuclear charge continually increases as you move across the 3p block (containing As and Se), but EA goes up and down: 41, 119, 79, 195, 324. Therefore answer #1 is not going to be the best explanation – something else is having a greater impact than effective nuclear charge is.</p>
<p>Germanium’s electron affinity is unusually high (119) because the Ge- ion has one electron in each p orbital, and Arsenic’s electron affinity is unusually low (79) because a neutral As atom has one electron in each p orbital. That island of stability explains why the EA for Ge is high and for As is low. So although effective nuclear charge does play a role in electron affinity, the effect of the half-filled p subshell seems to have a greater effect.</p>