Electron affinity meaning in chemistry
Electron affinity of an element defined as the amount of energy released when an electron is added to a neutral gaseous atom in its lowest energy state or ground state to convert it into uni-negative gaseous ion.
Simply, the electron affinity of an atom defined as the energy liberated when a gaseous atom captures an electron.
A(g) + electron → A- (g) + EA
Energy release for electron capture reaction equal in magnitude to the ionization energy of the species formed. The definition of EA as the exothermicity of the electron capture reaction would lead to the negative sign according to our usual thermodynamic convention. But EA is normally described with a positive sign.
Negative electron affinities are known. These negative values indicate that the species concerned does not like to have any more electron.
How is electron affinity measured?
Electron affinities are difficult to measure. EA obtained from indirect measurements, by analysis of Born-Haber energy cycles in which one step is electron capture or by the direct study of electron capture from heated filaments.
Calculate the electron affinity of chlorine from the Born – Haber cycle, given the following data: lattice energy = – 774 kJ mol⁻¹, ionization energy of sodium = 495 kJ mol⁻¹, heat of sublimation of sodium = 108 kJ mol⁻¹, energy for bond dissociation of chlorine = 240 kJ mol⁻¹ and heat of formation of sodium chloride = 410 kJ mol⁻¹.
Born – Haber Cycle for the formation of sodium chloride
∴ – UNaCl – INa + ECl – SNa – ½DCl – ΔHf = 0
or, ECl = UNaCl+INa+ SNa +½DCl + ΔHf
= -774 + 495 + 108 + 120 + 410
= 359 kJ mol⁻¹.
Factors affecting the electron affinity
The magnitude of EA influenced by the atomic size, shielding effect and electronic configuration of an atom or molecules.
Atomic radii trend of electron gain enthalpy
Larger the atomic size lesser the tendency of the atom to attract the additional electron towards itself. Which decreases the force of attraction exerted by the nucleus of an atom on the extra electron being added to the valence shell of the atom.
Thus EA values decrease with increases atomic size.
How the shielding effect affect electron affinity?
Higher the magnitude of effective nuclear charge (Zeff) towards the periphery of an atom greater the tendency of the atom to attract the additional electron towards itself.
Again greater the force of attraction exerted by the nucleus on the extra electron being added to the valence shell of the atom.
As a result, higher energy released when an extra electron added to form an anion. Thus the magnitude of the electron affinity of an atom increases with increasing Zeff value.
The electron configuration of elements
The magnitude of electron affinity depends on the electronic configuration of an atom. The elements having, nS², nP⁶ valence shell configuration posses very low electron affinity value due to having their stable valence shell configuration.
Account for the large decrease in electron affinity between lithium and beryllium.
The atomic number of lithium and beryllium 3 and 4. The electronic distribution of lithium and beryllium
Lithium has an incompletely filled 2S sub-shell while beryllium has the subshell filled. Thus lithium can receive an electron in its 2S sub-shell but for beryllium, a still higher energy 2P level has to be made of.
A filled shell or sub-shell leads to some extra stability. Hence beryllium resists an extra electron more than does lithium.
Why the EA of nitrogen is less than phosphorus?
Electronic configuration of nitrogen and phosphorus
Due to the smaller size of nitrogen atom when an extra electron added to the stable half-filled 2P subshell some amount of energy required and hence the electron affinity of nitrogen is negative.
On the other hand, due to the bigger size of a phosphorus atom in comparison to nitrogen less amount of energy released when the extra electron added to the stable half-filled 3P subshell and thus electron affinity of phosphorus expressed with a positive sign.
Electron affinity trend in the periodic table
In a group when move from top to bottom the size of atom generally increases with increasing atomic number and consequently, the magnitude of electron affinity decreases in the same direction. There are some exceptions to this general rule across the group in the periodic table.
Although the elements of the second period of the periodic table are relatively smaller in size than those of the third-period elements, the electron affinity values of elements of the second period are smaller than the electron affinity values of third-period elements.
This unexpected behavior explained by saying that the much smaller sizes of the second-period elements give a very much higher value of charge densities for the respective negative ions. A high value of electron density is opposed by the interelectronic repulsion forces.
EA of fluorine is lower than that of chlorine. The lower values of EA for Florine due to the electron-electron repulsion in relatively compact 2P – orbital of the fluorine atom.
F < Cl > Br > I
Electron affinity trend across a period
In a period, when we move from left to right Zeff value towards the new coming electron gradually increases with the increasing atomic number and hence the electron affinity gradually increases in the same direction.
Why electron gain enthalpy of Be and Mg are almost zero?
Berrilium and magnesium have their electron affinity values equal to almost zero. Since beryllium and magnesium have completely filled S sublevel.
1S² 2S² 2P⁶ 3S²
The additional electron will be entering the 2P – subshell in the case of beryllium atom and 3P – subshell in the case of magnesium atom which is of considerably higher energy than the 2S – subshell.
Why chlorine has a higher electron affinity than fluorine?
The halogen possesses large EA values indicating their strong tendency to form anions. This can easily understandable because their electronic configurations are only one electron short of the next noble gas element.
The EA of chlorine greater than that of fluorine. Due to the very small size of fluorine atom which results in a very high charge density and consequently strong repulsion between existing valence shell electrons and the entering electron or new coming electron.
On the other hand, chlorine is a bigger size, charge density small and thus such repulsion not strong enough. Hence the EA of chlorine greater than that of fluorine.
Noble gases electron gain enthalpy
Inert gases in which the nS and nP orbitals are completely filled or nS² nP⁶ electronic configuration. The incoming electron must go into an electron shall have the larger values for the principal quantum number, n. Thus inert gas has its electron affinity values equal to zero.