Electron affinity is defined as the quantitative measurement of the energy change that results from adding a new electron to a neutral atom or molecule in the gaseous state. The more negative the electron affinity value, the higher an atom’s affinity for electrons.
The energy of an atom is stated when an atom loses or gains energy through chemical reactions that cause the loss or gain of electrons. The reaction that releases energy is called an ‘exothermic’ reaction and the reaction in which energy is absorbed is called an ‘endothermic’ reaction. Talking about energies, energy from an endothermic reaction is positive. Hence, given a positive sign whereas energy from an exothermic reaction is negative. It is given a negative sign.
The energy is released when an electron is being added to a neutral atom. Thus first electron affinities are always negative whereas second electron affinity ( electron to negative ion ) is positive. The electron affinity is further discussed below:
First Electron Affinity: Negative energy because energy is released.
X(g) + e- X- (g)
Second Electron Affinity: Positive energy because the energy needed is more than gained.
X-(g) + e- X2- (g)
The Energies are always concerned by the formation of positive ions. The first electron affinity is the energy released when 1 mole of gaseous atoms acquire an electron to form 1 mole of gaseous -1 ions.
Example: The first electron affinity of chlorine is -349 kJ mol-1
The energy is needed to gain the electron when an electron is added to a metal element. However, metals are less likely to gain electrons as it is easier to lose their valence electrons and form cations. The reason behind losing their valence electrons is that metals’ nuclei do not have a strong pull on their valence electrons. Therefore, metals are said to have lower electron affinities.
Lithium (Li): -60 KJ mol-1
Sodium (Na): -53 KJ mol-1
Potassium (K): -48 KJ mol-1
Rubidium (Rb): -47 KJ mol-1
Cesium (Cs): -46 KJ mol-1
When non-metals gain electrons, the energy change noted is negative because they give off energy. Non-metals have greater electron affinity because of their atomic structures. There are two reasons associated with why non-metals have greater electron affinity.
Non-metals have more valence electrons than metals have, this makes non-metals easy to gain electrons to fulfill a stable octet.
The valence electron shell is closer to the nucleus, this makes it harder to remove an electron. It is rather easy to attract an electron from another element.
Example: Non-metals like the elements in the Halogen series in Group 17 have higher electron affinity. The electron affinity trend is stated below:
Fluorine (F) : -328 KJ mol-1
Chlorine (Cl) : -349 KJ mol-1
Bromine (Br) : -324 KJ mol-1
Iodine (I) : -295 KJ mol-1
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Second electron affinity is only met concerning the group 16 elements oxygen and sulfur which both form -2 ions. It is the energy required to add an electron to each ion in 1 mole of gaseous 1- ions to produce 1 mole of gaseous 2- ions.
The greater the nuclear charge, the greater will be the attraction of the incoming electron. This will result in a larger value of electron affinity.
The larger the size of an atom, the larger will be the distance between the nucleus and electron. This will result in a smaller force of attraction by electrons. Therefore, the value of electron affinity will be small. In general, too, the electronic affinity increases by going down the group and decreases from left to right across the periods.
Stable the configuration of an atom, its tendency will be less to accept the electron. Therefore, it will face a lower value of its electron affinity. Electron affinity is almost zero or low in elements having a stable electronic configuration. This is due to the small tendency to accept another electron.
Electron affinities of inert gases are zero. This is due to their atoms have stable ns2 np6 configuration in their shell. Electron affinity of Beryllium, and calcium is practically zero. If the atom has fully or half-filled orbits, its electron affinity will be less.
Example: Electron affinity of Be and N is almost 0 because they are having filled electrons in their valence shells. Full filled orbits are all stable due to symmetry. Therefore, these elements will be having the least tendency to accept any electron.
In general, the electron affinity follows the below trend:
Halogens > Oxygen family > Carbon family > Nitrogen family > Metals of group 1 and 13 > Metals of group 2.
1. As we move down the group of the periodic table, electron affinity increase or decrease? If so, why?
As we move down the group on the periodic table, electron affinity tends to decrease. There are three reasons associated with why it tends to decrease by moving down. Here are the reasons:
Electrons are subtly being placed in energy levels, away from the nucleus. This result in electrons not having a strong attraction to the nucleus
The atom does not gain electrons as there is a minimum charge on the outer energy level from the nucleus.
There is repulsion between the electrons due to the shielding effect. Hence, they move further away from each other and the nucleus itself.
2. Why the Halogens show high electron affinity?
Electron affinity reflects the ability of an atom to accept an electron. The halogens show high electron affinity due to their small size. It has a high nuclear charge and an almost full outer shell of electrons. The smaller the electron size, the greater the affinity. Halogens prove this factor to be true. High energy is released when an electron is added to halogen. This makes it have a high electron affinity.