Question

Which has higher electron affinity in a period, fluorine or neon?

Answer 1

The amount of energy released when a neutral atom or molecule in the gaseous state accepts an electron to form a negative ion is known as electron affinity. As the energy is released for almost all non-noble gas atoms, the change in energy has a negative sign and reaction is exothermic.

Complete answer:
In the modern periodic table, electron affinity increases on moving left to right in a period while it decreases on moving down the group. The trend in electron affinity can easily be explained on the basis of atomic size of elements as follows:
In a period, the atomic size of the elements decreases as we move from left to right in a period due to increase in the effective nuclear charge. As the size decreases, the outermost orbit becomes closer to the nucleus and thus, has a greater tendency to accept an electron. Conversely, the atomic size increases on moving top to bottom in a group, which means the tendency to accept electrons decreases down the group.
Electron affinity generally follows the following trend:
Halogens $>$ Oxygen family $>$ Carbon family $>$ Nitrogen family $>$ Alkali metals and metals of group 13 $>$ Alkaline earth metals.
Now, on comparing electron affinity of halogens and noble gases, the halogens have the highest electron affinity in a period because they require only one electron to reach the noble gas configuration. On the other hand, the noble gases are chemically inert due to complete octet and have a zero tendency to accept electrons. Thus, noble gases have the least electron affinity in a period.
Hence, we can conclude that in a period, fluorine (halogen) has higher electron affinity than neon (noble gas).

Note:
It is important to note that electron affinity is not the same as the electron gain enthalpy which is a change in enthalpy when a neutral atom gains an electron from outside. In simple words, electron affinity is a tendency to accept electrons whereas electron gain enthalpy is the energy released when an atom gains an extra electron.