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Electron affinity (EA) is the amount of energy that is released when an electron is grouped with a neutral atom or molecule which is in a gaseous state to create an anion. It’s an atomic property that tells us how likely an element is to take an electron and create an anion. EA is measured in eV/atom, Kcal/mole, and KJ/mole. It can be calculated through the Born–Haber cycle.
X(g) + e− → X−(g) + energy
EA ∝ 1/size of an atom ∝ effective nuclear charge
EA ∝ 1/screening effect
∝ reactivity of non-metals
∝ the oxidising power of elements
Oil Drop Experiment
The oil drop experiment by Millikan was used to determine the charge of an electron. Prior to this experiment, the idea of subatomic particles was not universally accepted.
An electric field was created between a parallel pair of metal plates held apart by an insulating material in Millikan’s apparatus. Electrically charged oil droplets entered the electric field after it was altered, and they were balanced between two plates.
The gravitational and electric forces on it were equal when the charged drops fell at a constant rate. As a result, the charge on the oil drop was determined using the formula
Q = m g.E
Millikan discovered that a single electron has a 1.6 x 10-19 C charge.
Sign Convention
Electron affinity is all about the sign. Therefore, you must be aware of whether the sign of the atom is correct. The change ΔE in total energy occurs for any reaction which releases energy with a negative value. This reaction is referred to as an exothermic process. A majority of non-noble gas atoms release energy as they capture electrons, making them exothermic. The positive values listed in the electron affinity table are magnitudes. The word ‘released’ used in defining electron affinity is the one presenting the negative sign to ΔE.
Confusion arises when EA is confused with a change in energy, ΔE, in which case the positive numbers stated in tables correspond to an endothermic rather than exothermic process.
EA = -ΔE is the relationship they share.
If EA is given a negative value, the negative sign denotes a direction reversal, and energy is necessary in attaching an electron. This case makes it an endothermic process, but the relationship between EA = −ΔE(attach) is still valid. The capturing of the second electron is the reason why negative values arise.
The expression to calculate EA when an electron is attached (exothermic):
EA = (E initial − E final)attach = −ΔE(attach)
The expression to calculate EA when an electron is detached (endothermic):
EA = (E final − E initial)detach = ΔE(detach) = −ΔE(attach).
Types of electron affinity
First electron affinity: This is the energy that is produced when 1 mole of gaseous atoms each obtain an electron in order to become 1 mole of gaseous -1 ions. Their value is always negative.
Second electron affinity: When an electron is added to a uni-negative ion (anion with a negative sign) to overcome the repulsive forces, the energy that gets absorbed is called second electron affinity. EA 2nd has a positive sign.
Factors Affecting Electron Affinity
Electron affinity depends on three main factors:
Atomic Size
As the size of the atom grows, the distance between the nucleus and the arriving electron grows. Hence, the force of attraction the incoming electron experiences towards the nucleus of an atom would be less, and the value of electron affinity will be smaller. In other words, atomic size is inversely proportional to electron affinity.
Nuclear Charge
It is directly proportional to electron affinity. The more electron affinity increases, the more the attraction to the incoming electrons increases.
Electronic Configuration
Atoms with a stable electronic configuration have a lower tendency for accepting electrons. As a result, their electron affinity values are lower.
General Trend
Electron affinity increases from left to right. Along with a group, it increases till the third period. Due to the high electron density, the electron affinity of the p block’s second and third periods is lesser.
Exceptions to Electron Affinity
A fully-filled and half-filled orbital with low values or energy is needed rather than getting released.
Due to the repulsion between electrons, the second period has a lower value than the third.
Conclusion
Electron affinity is the amount of energy that is released when an electron is grouped coupled with an atom in a gaseous state to form an anion. The sign convention is essential and needs to be taken into consideration. There are two types of electron affinity: first and second. The first is always negative, and the second is positive. The factors affecting electron affinity are atomic size, nuclear charge, and electronic configuration.
