Electron Affinity

The quantitative assessment of the energy shift that occurs when a new electron is added to a neutral molecule or atom in the gaseous state is regarded as electron affinity. The higher an atom’s affinity for electrons, the lower its electron affinity value. When an atom gains or loses energy during chemical reactions which result in the loss or gain of electrons, the energy of the atom is expressed.

Electron Affinity

If an electron is added to a neutral gaseous atom to generate a negative ion, the electron affinity is the charge of the potential energy of the atom. As a result, the electron addition process is more favourable when the electron affinity is negative. Not all elements produce stable negative ions, where case the electron affinity is either zero or positive.

When an atom gains or loses energy in chemical reactions which result in the gain or loss of electrons, the atom’s energy is defined. A chemical process which releases energy is referred to as an exothermic reaction, while one that absorbs energy is referred to as an endothermic reaction. 

Electron Affinity Definition

If an electron is added to the neutral atom to produce an anion, a certain amount of energy is released. This is known as electron affinity.

First Electron Affinity

The generation of positive ions is always a concern for ionisation energies. Electron affinities are equivalent to the negative ion, and they’re virtually generally reserved for elements in Periodic Table groups 16 and 17. The energy produced when one mole of gaseous atoms get an electron to become 1 mole of gaseous -1 ions is known as the first electron affinity. When this transformation occurs, it releases energy (per mole of X). The negative values of electron affinities are the first. Chlorine, for example, has a 1st electron affinity of -349 kJ mol-1. The negative symbol, by tradition, denotes a release of energy.

Energy is required to obtain an electron when it is added to a metal element (endothermic reaction). Since it is easier to shed valence electrons and create cations, metals have a lower likelihood of gaining electrons. Because metals’ nuclei do not exert a strong pull on their valence electrons, it is easier for them to shed them. As a result, metals have lower electron affinities.

Because nonmetals emit energy when they gain electrons, the energy change is negative. Since their atomic structures, non-metals have a higher electron affinity. There are 2 reasons why non-metals have a higher electron affinity than metals. Because non-metals have more valence electrons as compared to metals, gaining electrons to complete a stable octet is simple. Because the valence electron shell is closer to nucleus, removing one electron is more difficult. Attracting an electron from another element is rather simple.

Group one Electron Affinity

1. Lithium (Li): -60 KJ mol-¹
2. Sodium (Na): -53 KJ mol-¹
3. Potassium (K): -48 KJ mol-¹ 
4. Rubidium (Rb): -47 KJ mol-¹
5. Cesium (Cs): -46 KJ mol-¹

Group 17 Electron Affinity

Nonmetals, such as those in Group 17’s halogen series, have a high electron affinity than that of metals. The following is a description of this trend. The electron affinity has a negative sign, indicating that energy has been released.

1. Fluorine (F) -328 KJ mol-¹
2. Chlorine (Cl) -349KJ mol-¹
3. Bromine (Br) -324 KJ mol-¹
4. Iodine (I) -295 KJ mol-¹

Second Electron Affinity

Only oxygen and sulphur from the group 16 elements, which both generate -2 ions, are likely to come up against this. The energy needed to add an electron to each ion in one mole of gaseous 1- ions to make 1 mole of gaseous 2- ions is referred to as the second electron affinity. 

X– (g) +e–X-² (g)

Trends of Electron Affinity in Periodic Table

Because the electrons added to energy levels get closer to the nucleus, there is a higher attraction between the nucleus and its electrons. Electron affinity increases higher for the groups and from left to right throughout periods of a periodic table. Remember that the greater the distance between two objects, the less attraction there is; hence, whenever an electron is added to outside orbital, less energy is produced. 

Because the electrons are put in a higher energy level distance from the nucleus, electron affinity diminishes down the groups and from right to left throughout the periods on the periodic table. However, because the quantity of valence electrons increases as the group number decreases, one may assume that the element will be more stable and have a larger electron affinity. The shielding effect is not taken into account. As the period decreases, the shielding effect rises, causing electrons to repel one another. This is why, as one moves down the periodic table, the attraction between electron and nucleus lessens.

Conclusion

The amount of energy required to extract one electron from a negatively charged ion of a molecule or atom is known as electron affinity.

When we move from left to right over time, the atomic size shrinks due to an increase in the nuclear force, and thus the electron gain enthalpy rises. Moving down a group in periodic table causes the atomic size to increase, leading the value of electron gain enthalpy to drop.