ELECTRON AFFINITY

Electron affinity (EA) of an atom is defined as the energy released when an electron is added to the valance shell of a gaseous atom in its ground state.

In other words, electron affinity of an atom is defined as the energy released when a gaseous atom captures an electron.

X(g) + electron → X^-(g) + EA

The process of addition of an electron to an atom is an exothermic process. Electron affinity is also called enthalpy of electron attachment because electron affinity is the energy involved in addition of an electron to an atom. According to our usual thermodynamic convention, since electron affinity is the energy released it should be represented with a negative sign, but unfortunately electron affinity values are normally represented with a positive sign.

However, negative electron affinity values are also known. Such value indicates that the species does not want to have an extra electron. Subsequent addition of electrons to an atom gives successive electron affinity values such as (EA)₁, (EA)₂, (EA)₃ --- etc. Once an electron is added to an atom, subsequent entry of 2 nd, 3 rd--- etc, electron would involve repulsion between the existing electrons and the entering electrons. In such cases energy would be required to push the electron against the interelectronic repulsion. Hence the 2 nd, 3 rd etc electron affinity values are always negative.

Electron affinity values are expressed in ev/atom, Kcal/mole and KJ/mole.

Electron affinity values can not obtain experimentally. Electron affinity values are obtained indirectly from Born-Haber Cycle.

Periodic Trend of Electron Affinity

(1) Variation Along a Period

On moving from left to right along a period that is from alkali metals to halogens along a period, the radius of atom decreases and nuclear charge increases. As a result of these two factors, the attractive force between the nucleus and the added electrons is increase. Therefore, the atom has greater tendency to captures an electron. So, generally electron affinity values gradually increase from left to right along a period, that is from alkali metals to halogens. Since, each period start with an alkali metal, it lies extreme left in periodic table and it has the lowest value of electron affinity. Halogen lie extreme right in the periodic table, it has the highest value of electron affinity.

(2) Variation Along a Group

On moving from top to bottom along a group that is downwards along a group, the radius of atom increases and the nuclear charge also increases on the same direction. The increase in atomic radius decreases the electron affinity values and increase nuclear charge increases the electron affinity values. Here the effect of atomic radius overcome the effect of nuclear charge. So, the value of electron affinity decreases on going from top to bottom along a group that is downwards along a group.

Factors Affecting the Magnitude of Electron Affinity

(1) Size of the Atom

The smaller is the size of the atom, the greater is the electron affinity value.

Electron Affinity ∝ Atomic Size

For smaller atom the attraction of the nucleus for the added electron is stronger. So, the electron affinity value is greater for smaller atom.

(2) Nuclear Charge

Electron affinity directly proportional to the nuclear charge.

Electron Affinity ∝ Nuclear Charge

With increase in nuclear charge the attraction of the nucleus for the added electron increases. Therefore, the electron affinity value increases with increase in nuclear charge.

(3) Electron Configuration

Atoms with stable electronic configuration, that is with half filled or full filled electronic configuration have little or no tendency to accept any more electron. So, they have low or negative electron affinity values. Tendency to attain stable electronic configuration is reflected by high electron affinity value.

Electron Affinity of Some Cases

(1) The electron affinity values of the noble gases are negative---

The valance shell electronic configurations of noble gases are ns² np⁶ (He has 1s² configuration). So, noble gas atoms have stable electronic configuration. They have no tendency to captures any extra electron. Therefore, electron affinity values of noble gases are negative. That is energy will be required to push the electron in the valance shell of noble gases.

(2) Halogens possess large positive electron affinity values---

The valance shell electron configuration of halogens is ns² np⁵. So, the halogens atoms have one electron short to fulfill their octet and to obtained stable noble gas electronic configuration. So, to attain stable electronic configuration they have very high tendency to captures an electron. Accordingly, halogens possess large positive electron affinity values.

(3) Electron affinity values of Beryllium (Be) Magnesium (Mg) and Nitrogen (N) are negative---

Valance shell electron configurations of Beryllium (Be) and magnesium (Mg) are ns², so they have filled s shell. Valance shell electron configuration of nitrogen (N) is ns² np³, so nitrogen has half filled p shell. According to Hund’s rule a half filled shell or full filled shell attains extra stability. Hence, the atoms such as Be, Mg and N would be much reluctant to captures extra electron. So, the electron affinity values of Be, Mg and N are negative.

(4) The 2 nd electron affinity value of oxygen (O) and sulfur (S) are negative---

When an atom captures an electron an uninegative ion is formed.

O(g) + e^- → O^-(g) + Energy

S(g) + e^- → S^-(g) + Energy

So, the 1 st electron affinity value is positive for oxygen and sulfur, because they have captured an electron readily. In spite of their high tendency to attain stable electronic configuration, entry of the second electron will be difficult. Because entry of the second electron would involve repulsion between the existing electrons and the entering electron. In such situation energy will be required to push the electron against such repulsion. So, the second electron affinity value of oxygen and sulfur are negative.