What is electron affinity? How does it vary along with the period and group?

Hint: We have to know that, the electron proclivity $\left( {{E_{ea}}} \right)$ of an iota or particle is characterized as the measure of energy delivered when an electron is joined to a nonpartisan molecule or atom in the vaporous state to frame a negative ion.
${X_{\left( g \right)}} + {e^ - } \to X_{\left( g \right)}^ - + energy$

Complete answer:
We have to know that, an electron fondness is estimated for particles and atoms in the vaporous state just, since in the strong or fluid expresses their energy levels would be changed by contact with different iotas or atoms. Robert S. Mulliken utilized a rundown of electron affinities to foster an electronegativity scale for molecules by tracking down the normal of the electron fondness and ionization potential. A particle or molecule that has a more certain electron proclivity esteem is frequently called an electron acceptor; one with a more negative electron fondness is called an electron contributor. Together they may go through charge-move responses.
To utilize electron affinities appropriately, it is fundamental to monitor the sign. For any response that discharges energy, the adjustment of energy $\left( {\Delta E} \right)$ has a negative value, and the response is called an exothermic interaction. Electron catch for practically all non-honorable gas iotas includes the arrival of energy and hence is an exothermic interaction.
We have to see that there are general patterns in electron fondness across and down the intermittent table of components. Electron liking for the most part increments across a period in the intermittent table and some of the time diminishes down a gathering.
The substance reasoning for changes in electron fondness across the occasional table is the expanded compelling atomic charge across a period and up a gathering.

Note:
 The electron proclivity of atoms is a muddled capacity of their electronic design. For example the electron fondness for benzene is negative, just like that of naphthalene, while those of anthracene, phenanthrene and pyrene are positive. In silico tests show that the electron liking of hexacyanobenzene outperforms that of fullerene.