6.0 Electron Affinity

  Periodic Table

• The energy released on adding up one mole of electron to one mole of neutral atom (A) in its gaseous state to form an anion $(A^-)$ is called electron affinity of that atom. Since the electron adds up in the outermost orbit, energy is given out. Therefore, electron affinity is associated with an exothermic process.$${A_{(g)}} + {e^ - } \to {A^ - }_{(g)},\Delta H = - {E_n}$$
• When one electron adds up to a neutral atom, it gets converted to negatively charged ion and energy is released. On adding one more electron to mono-negative anion, these is a repulsion between the negatively charged electron and anion. In order to counteract the repulsive forces, energy has to be provided to the system. Therefore, the value of the second electron affinity is positive.$${A^ - }_{(g)} + {e^ - } \to {A^{2 - }}_{(g)},\Delta H = + {E_n}$$

• Factors Affecting Electron Affinity
• Atomic size or atomic radius: When the size or radius of an atom increases, the electron entering the outermost orbit is more weakly attracted by the nucleus and the value of the electron affinity is lower.
• Effective nuclear charge: When the effective nuclear charge is more, then the atomic size is less. So the atom can easily gain an electron and higher electron affinity.
• Stability of fully-filled and half-filled orbitals: The stability of the configuration having fully-filled orbitals $(p^6, d^{10}, f^{14)}$ and half-filled orbital $(p^3, d^5, f^7)$ is relatively higher than that of other configurations. So such types of atoms have less tendency to gain an electron. Therefore their electron affinity values will be very low or zero.

• Trends in Electron Affinity
• In a period: From left to right in a period, the atomic size decreases with increase in effective nuclear charge and hence increase in electron affinity.
• Exceptions

a. On going from $C^6 \:to\: N^7$ in the second period, the value of electron affinity decreases instead of increasing. This is because there are half-filled $(2p^3)$ orbitals in the outermost orbit of $N$, which is more stable.

b. In the 3rd period, the value of electron affinity of $Si$ is greater than that of $P$. This is because electronic configuration of the outermost orbit in $P$ atom is $3p^3$, which is being half-filled, is relatively more stable.

c. The value of electron affinity of inert gases are zero, because their outermost orbit has fully filled $p$ orbital.

d. In a period, the value of electron affinity goes on decreasing on going from group $I A$ to group $II A$. The value of electron affinity of the elements of group $II A$ is zero because $ns$ orbitals are fully-filled and such orbitals have no tendency to accept electrons.

• In a group: The value of electron affinity normally decreases on going from top to bottom in a group because the atomic size increases which decrease the actual force of attraction by the nucleus.
• Exceptions

a. The value of electron affinity of $F$ is lower than that of $Cl$, because the size of $F$ is very small and compact and the charge density is high on the surface. Therefore, the incoming electron experiences more repulsion in comparison to $Cl$. That is why the value of electron affinity of $Cl$ is highest in the periodic table.

c.The value of electron affinity of alkali metals and alkaline earth metals can be regarded as zero, because they do not have tendency to form anions by accepting electron.