Electronegativity Trend

[Claudia Luong 4K]

I know that electronegativity increases as you go right across a period and go up along a group, but between going up a group and going across a period, which causes a larger increase in electronegativity? For example, without being given the exact electronegativity values, can you tell if S and Cl has a greater/lesser electronegativity difference than that of S and O?

[Chem_Mod]

As you go across the period, the valence electrons of the element is closer to achieving a full octect if it steals a few more electrons. On other end of the period, the elements only have a few valence electrons on the outer most layer so it is easier to lose those electrons to gain a full octet. Thus, those on the right tend to steal electrons (thus higher En) and those on the left tend to lose electrons (low En). As you go down a column, the radius is getting larger and larger, so even though the nucleus would very much like to steal electrons to achieve a full octet cannot easily do so.This is because the existing electrons would "shield" the electrostatic attraction of the nucleus for the incoming electron. As a result, elements with larger radius will have a smaller En than their smaller counterparts.

If you are asked to compare the En difference between two elements, the question should not be too ambiguous for you to figure out without memorizing the exact En values. In answer to your question, we can tell that Cl has a larger En than does O because the former is on the right of O, which means that it has a larger tendency to steal an electron. If you have problem figuring out the trend of En, I suggest googling En trend and look for trends and patterns to convince yourself. Hope this helps!