Question #16078

1 Answer
Ernest Z. · Stefan V. · Michael
Jan 28, 2015

#"Cl"# has a higher electron affinity than #"F"# because electronic repulsions are greater in the smaller #"F"# atom.

Explanation:

Electron affinity is the enthalpy change when 1 mol of gaseous atoms each gain an electron to form 1 mol of gaseous ions.

It is the enthalpy change for:

#X_((g)) + e^(-) rarr X_((g))^-#

For fluorine the electron affinity = #"-328 kJ·mol"^"-1"#

For chlorine the electron affinity= #"-349 kJ.mol"^"-1"#

So, we can see that the value for chlorine is larger and negative compared with fluorine.

Both #"Cl"# and #"F"# are quite happy to accept an electron to complete their octet and get to a lower energy level.

But #"F"# is a smaller atom, so the electrons are crowded together more closely.

The electron repulsions are greater in the #"F"# atom, so there is not as much energy available to be released when the electron is added.

https://www.xtremepapers.com/revision/a-level/chemistry/inorganic/group7/properties.php

Fluorine, since it's such a small atom, has a very high electron density, which means that the repulsion the incoming electron feels will diminish the attraction coming from the nucleus and thus reduce its electron affinity.

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