Final answer:
Electron affinity is the energy change when an electron is added to a gaseous atom, forming an anion, and can be negative, positive, or zero. The first electron affinity of noble gases is typically not negative, as they already have a complete valence shell and do not form stable anions easily. Moving across the periodic table, electron affinity values typically become more negative, except for noble gases and certain other exceptions.
Step-by-step explanation:
Electron affinity (EA) is defined as the energy change that occurs when an electron is added to a gaseous atom, forming an anion. It is a measure of how much energy is released or absorbed during this process, and is typically reported in kilojoules per mole (kJ/mol).
While the first electron affinity can be negative (energy is released), zero (no energy change), or positive (energy is added), the second electron affinity is always positive due to increased electron-electron repulsion.
Adding an electron to a noble gas typically does not release energy because noble gases already have a full valence shell and do not tend to form stable anions. Therefore, their electron affinities are often positive or zero.
The trends in electron affinity across the periodic table generally show that values become more negative as we move from left to right—indicating increased energy release upon gaining an electron—and less negative from top to bottom within a group.
Specifically, halogens like chlorine have particularly high (negative) electron affinities because they readily gain an electron to complete their valence shell, releasing a significant amount of energy in the process.
Elements that do not form stable anions, such as noble gases and elements like beryllium (Be) and nitrogen (N), have electron affinities that are greater than or equal to zero.
This is contrasted with electronegativity, which is a calculated dimensionless quantity describing an atom's ability to attract electrons in a bond, while electron affinity is a measurable physical quantity.