What is Electron Affinity and what are the trends on the periodic table?

Electron affinity is the energy given when an electron is gained by a neutrally charged atom, in the gasous phase, becoming a negatively charged ion.

 

The trend increases across a group, and up a period, just like with the first ionisation energy.

so it's kinda like the desire of atoms to have a complete set of electrons or 8 electrons in their outer energy levels?

 

if memory serves (I don't have a P table on me and I haven't yet cared to memorize it) potassium and lithium are two alkali metals, would they havbe the sam electron affinity, or would potassium have a greater affinity due to its greater mass?

Potassium and lithium have relatively weak electron affinities, when compared to elements such as fluorine and chlorine. This is electron affinity (EA) is, as you sad, the "desire" of atoms to have a complete set of electrons. Elements like potassium and lithium would much rather give up that extra electron than to gain 7 more; atoms like fluorine would much rather gain 1 more electron to have a set of 8 rather than lose 7 electrons.

 

In general, EA increases from left to right (high electron affinity at the right hand side), and decreases from top to bottom (high electron affinity at the top of the periodic table).

 

Left to right sees an increase in electron affinity due to the atoms gaining closer to 8 electrons in their outer shell, while top to bottom sees a decrease because the electron orbitals are further away from the positive nucleus.

k thanks that makes sense, would it be fair to say that groups 1-13 have a negative electron affinity, since they want to get rid of electrons?