Question

What is the value of electron affinity of $N{{a}^{+}}$ if $I{{E}_{1}}$ of Na = 5.1 eV?
a.) 0 eV
b.) -3.52 eV
c.) -5.1 eV
d.) -2.84 eV

Answer 1

Hint: This is a definition based question. To solve this question first of all, understand the ionization energy and electron affinity and after that find out how the ionization energy of Na atom and electron affinity of $N{{a}^{+}}$ is related to each-other. Here energy conservation principle will help us to solve this question.

Complete step by step answer:
- Ionization energy: It is the minimum amount of energy required to remove the most loosely bound electron of an isolated gaseous atom or ion.
- Electron affinity: electron affinity of an atom or specie is defined as the amount of energy released or absorbed when an electron is attached to the atom or ion
When we have to take out an electron from Na atom, we have to give it energy and according to the question statement that energy is equal to 5.1eV.
 \[Na+5.1eV\to N{{a}^{+}}+{{e}^{-}}\]
- Now when we have to add the electron into $N{{a}^{+}}$ ion to convert it into the Na atom again, the same amount of energy will be released that was absorbed when the electron was removed from the sodium atom.
\[N{{a}^{+}}+{{e}^{-}}\to Na+5.1eV\]
- So the magnitude of energy will be the same but the sign will change as in the former case energy was getting absorbed while in latter it is getting released. Electron affinity of sodium ion = $-5.1eV$
The correct answer is option “C” .

Additional Information :
- In a period when we move from left to right Ionization energy increases because metals, those are on the left side of the periodic table, easily lose the electrons in comparison to the non-metals, those are on the right most side of the periodic table.
- Going down in the group Ionization energy decreases because atom size increases, so it becomes comparatively easy to remove electrons from the bigger size atom.
- In a period when we move from left to right electron affinity increases as non-metals are always ready to gain electrons but here remember that noble gases are exceptions.
Going down in the group electron affinity decreases as bigger atom size decreases the tendency to gain electrons.

Note: Energy conservation is fundamental principle of chemistry; energy can’t be destroyed or created. So whatever the energy is absorbed or released during the forward process, the same amount of energy will be released or absorbed during the backwards process. Magnitude of the energy will be the same, only the signs will be different.