What is Electron Affinity?

When an element undergoes a chemical reaction, it either gains energy or loses energy. This gain or loss of energy is due to the phenomena that occur at atomic level. During reaction, atoms either gain electrons from other atoms or lose electrons to other atoms, and in that process, energy is produced.

Concept

During an endothermic reaction, energy is absorbed by the system while in an exothermic reaction, energy is dissipated by the system. In thermodynamics, energy added to a system is taken as positive, whereas energy released from the system is taken as negative. Thus, it can be concluded from the two that endothermic reaction takes positive sign convention, whereas exothermic reaction takes a negative sign.

Definition: Suppose there is a neutral atom, a neutral atom is one in which exists a number of electrons and protons. When we add an electron from outside to this neutral atom, energy is released, the first electron addition or we can say electron affinity is negative.

Now we know that charges repel each other. The atom was neutral initially, the addition of the first electron made the atom negatively charged, so during the next electron addition there will be additional need of energy to overcome this repulsive force. Hence this energy comes out to be positive.

First Electron Affinity

Similar to electron affinity, we have ionization energies which come into picture when positive ions are added. Simply put, first electron affinity is the energy released when 1 mole of gaseous atom each acquires an electron to make 1 mole of gaseous negative ions.

Energy is used to gain an electron as it is applied to a metal portion (endothermic reaction). Metals have a lower probability of gaining electrons so it is easier for them to lose their valence electrons and form cations. Since metal nuclei do not exert a heavy pressure on their valence electrons, it is simpler for them to lose them. Metals are believed to have lower electron affinities than nonmetals.

In the periodic table, the elements of the group 16 and 17 generally show negative ion, electron affinity.

Second Electron Affinity

This is shown by elements of group 16 of periodic tables and of that oxygen and sulfur shows -2 ions. It can be defined as energy needed to add an electron to each ion in 1 mole of gaseous -1 ion to produce 1 mole of gaseous -2 ions. The overall energy needed to carry out this process is known as Second electron affinity. It comes out to be positive as we have to supply energy externally to add a second electron in a negatively charged atom.

Electron Affinity Trends

Electrons majorly emphasize upward for groups and from left to right through periodic tables because electrons attached to energy ranges get nearer to the nucleus, resulting in a greater bond between both the nucleus and its electrons. Notice that the larger the distance, the less of an attraction exists; hence, as an electron is applied to the outside orbital, fewer energy is emitted. Furthermore, the more valence electrons an entity possesses, the more probable it is to acquire electrons and form a stable octet. The less valence electrons an atom possesses, the less likely it is to gain electrons.

Electron affinity reduces through the groups as well as from right to left around the periodic table when electrons are put at a higher energy level further from the nucleus, resulting in a reduction in its pull. However, one might believe that because the number of valence electrons increases as one moves down the group, the product should be much more stable but will have greater electron affinity. One neglects to take into account the shielding effect. If one decreases the time, the shielding effect increases, resulting in electron repulsion. This is why, when one moves down the periodic table, the attraction between the electron and the nucleus reduces.

First electron affinities decrease as you move down the group (in the sense that less energy is evolved when the negative ions are formed). Fluorine deviates from this trend and must be compensated for independently. The electron affinity measures the attraction between the incoming electron and the nucleus; the greater the attraction, the more energy is emitted. The influences that influence this attraction are the same as those that influence ionization energies - nuclear charge, distance, and screening. Extra screening electrons compensate for the added nuclear charge as you go down the group. Regardless of the product, each outer electron feels a tug of 7+ from the center of the atom.

Ionization Energy

The amount of energy that an isolated, gaseous atom throughout the ground electronic state must consume in order to discharge an electron and form a cation is referred to as ionization energy.That energy is generally represented in kJ/mol, which is the amount of energy required for each atom in a mole to lose one electron.

H(g)  H⁺(g) + e^-

In the case of an originally neutral atom, expelling the first electron requires less energy than expelling the second, which requires less energy than expelling the third, and so on. Each electron that is released necessitates the release of more energy. This is since, when the first electron is lost, the atom's total charge remains positive, and the electron's negative forces are drawn towards the positive charge of the newly formed ion. Therefore, the more electrons that are missing, the more constructive this ion becomes, and the more difficult it will be to remove the electrons from the molecule.

In the case of an originally neutral atom, expelling the first electron requires less energy than expelling the second, which requires less energy than expelling the third, and so on. Each electron that is released necessitates the release of more energy. This seems to be since, when the first electron is lost, the atom's total charge remains positive, and the electron's negative forces are drawn to the positive charge of the newly formed ion. The further electrons lost, the more positive this ion becomes, and the more difficult it is to remove the electrons from the molecule.

Factors Affecting Electron Affinity

Atomic size: The electron affinity shows an inverse relation with the atomic size of the element. When the size is small, the electron affinity will be greater. It can be simply understood as when atom size is small, the overall force of the nucleus will be higher on the electrons, and electrons will be gripped strongly.

Nuclear charge: Electron affinity  increases with increase in nuclear charge, as effective nuclear attraction force increases on valence electrons.

Screening effect: The electron affinity is inversely proportional to screening effect. Screening effect is the electron cloud between the incoming electron and the nucleus which reduces the force of attraction between them.

Oxidizing power: The electron affinity is directly proportional to oxidizing power of the element.

Confusion between electron affinity and electronegativity

People often get confused between the two. Electronegativity is a chemical property that signifies how well an atom of a given element will attract electrons towards itself. On the other hand electron affinity is the energy released when an extra electron is added to the atom.

Context and Application

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for  

• Bachelor of Science     
• Master of Science