What is electron affinity?
Electron affinity (EA) or electron gain enthalpy or simply affinity in the periodic table defines the amount of energy released when an electron is added to an isolated neutral gaseous atom in its lowest energy level (ground state) to produce an anion. In ionization energy, energy is supplied to remove one or more electrons from an atom or a cation. In electron affinity, the energy is released with the addition of one or more electrons in an atom or anion.
A (g) + electron → A– (g) + EA
The above electron affinity equation is an exothermic reaction with the negative sign according to the usual thermodynamics convention in chemistry but the measurement of affinities is always the positive value. Electron affinity value measured by unit eV per atom or kJ mol-1. The periodic table trend of EA is affected by the atomic size, shielding electron, and electron configuration or structure of atom or ion.
How to calculate electron affinity?
Indirectly, electron affinity can be calculated from Born – Haber cycle data.
|Lattice energy of sodium chloride||– 757.3 kJ mol-1|
|Ionization energy of sodium||495.4 kJ mol-1|
|Heat of sublimation of sodium||108.4 kJ mol-1|
|Bond dissociation energy of chlorine||241.8 kJ mol-1|
|Heat of formation of sodium chloride||-381.2 kJ mol-1|
Born – Haber Cycle equation for the formation of crystalline solid, sodium chloride, ΔHf = UNaCl + IENa + EACl + SNa + ½DCl. Putting the values from above table, the calculated value of ECl = -348.6 kJ mol-1.
Electron affinity measurement
It is difficult to obtain but measure from the indirect measurement of Born-Haber energy cycles in which one step is electron capture reaction. Affinities are also measured by direct study of electron capture from heated filaments. The second method determined the number of neutral atoms, ions, and electrons with the mass spectrometer in the electromagnetic spectrum radiation. This gives the standard free energy for the equilibrium reaction. The free energy is calculated from the temperature dependence of the chemical equilibrium constant.
Electron affinity trend in periodic table
The electron affinity trend in the periodic table is influenced by the following factors,
- Atomic radius
- Effective nuclear charge
- Electronic structure or configuration
Larger the atomic size lesser the tendency of atoms to attract the additional electrons towards themselves. Which decreases the force of attraction exerted by the nucleus of an atom. Therefore, the electron affinities decrease with increasing the size or radius of an atom.
Effective nuclear charge
Higher the magnitude of effective nuclear charge (Zeff) greater the tendency to attract the additional electrons towards itself. The greater force of attraction is exerted by the nucleus of an atom. As a result, higher energy is released when extra electrons are added to an atom. Hence the magnitude of the electron affinity of periodic elements increases with the increasing effective nuclear charge of an atom.
Electronic structure or configuration
The magnitude of EA depends on the electronic structure or configuration of an atom. The noble gases have ns2, np6 valence shell configurations that possess a very low value of affinity due to stable valence shell configuration. For example, hydrogen atom when gaining one electron to form H– ion (1s2) has very low EA (73 kJ mol-1) due to the formation of stable alkali hydride. The electric polarization of hydride ions is very high.
Electron affinity in Periodic Table
Electron affinity group trend
When we move down a group in the periodic table the size of atoms generally increases with increasing atomic number. The magnitude of electron affinity generally decreases in the same direction. The elements of the second period are relatively smaller in size than the third-period elements. But the electron affinities values of the second-period elements are smaller than the third-period elements. These unexpected behavior explained by charge densities for the respective negative ions. Because of a high value of electron density opposed by the interelectronic repulsion forces.
Electron affinity periodic trend
EA values generally increase on moving left to right in a period of the periodic table. There are many exceptions to general periodic trends. The EA trend for period-1 and period-2 elements of the periodic table are given below the picture,
Electron affinity exceptions
Electron affinity of lithium and beryllium
The atomic number and electronic configuration lithium and beryllium are 1s2 2s1 and 1s2 2s2 respectively. Therefore, lithium has an incompletely filled 2s subshell while beryllium has a filled subshell. Lithium can affinity to receive electrons in 2s sub-shell but for beryllium, a still higher energy 2p level. Hence beryllium resists gaining extra electrons in higher energy levels or 2p orbitals.
Electron affinity of nitrogen and phosphorus
Electron configuration of nitrogen and phosphorus 1s2 2s2 2p3 and 1s2 2s2 2p6 3s2 3p3. Due to the smaller size of the nitrogen atom when an extra electron is added to the stable half-filled 2p subshell some amount of energy is required. Hence the electron affinity of nitrogen is negative. On the other hand, due to the bigger size of a phosphorus comparison to nitrogen small amount of energy is released when an electron is added to the stable half-filled 3p subshell.
Question: Why the electron affinity of fluorine is lower than the chlorine atom?
Answer: The lower values of the electron affinity of the fluorine atom due to electronic repulsion in compact 2p-orbital. The affinities trends for halogen atoms are fluorine < chlorine > bromine > iodine.
Question: Why the electron affinity of beryllium and magnesium is almost zero?
Answer: Beryllium and magnesium have completely filled s-subshell with electronic configuration, 1s2 2s2 and 1s2 2s2 2p6 3s2. The additional electrons will be entering in 2p-subshell of beryllium and 3p-subshell in the case of magnesium. This resists the capture of electrons in a new higher quantum energy level.
Electron affinities of halogens
|Halogens||Electron affinity (kJ/mol)|
Halogens like fluorine, chlorine, bromine, and iodine have a large affinity indicating the strong tendency to pick up an electron or act as powerful oxidizing agents. The charge density of fluorine is greater than the chlorine atom due to the small size of the fluorine atom. The electron affinity of chlorine is greater than the fluorine atom. Therefore, the electron affinities trends of a halogen atom, F < Cl > Br > I.
Oxidizing power of halogens
The above fact indicates that chlorine should be the strongest oxidizing agent. In fact, fluorine has been found to be the strongest oxidizing agent among all periodic table elements. Therefore, oxidizing trends of halogen, F > Cl > Br > I but electron affinities trends of a halogen atom, F < Cl > Br > I.
The oxidizing power of halogen molecules explains by the oxidation potential of redox reactions and bond dissociation energy of halogen atoms. As the vales of chemical potential (E0) increase, the oxidizing power also increases.
|Iodine||– 122.6 kcal/mol|
These values clearly show that E0 values of fluorine molecules are highest, thus fluorine is the strongest oxidizing agent. The strongest oxidizing property is also explained by the small value of the chemical bond dissociation energy of fluorine molecules. Dissociation energies of non-polar halogens molecule, F2 = 1.64 eV/mole, Cl2 = 2.48 eV/mole, Br2 = 2.00 eV/mole, I2 = 1.56 eV/mole.
Electron affinity of noble gases
The valence shell electronic configuration of noble or inert gases like helium, neon, argon, krypton, xenon, and radon is ns2 np6 which is completely filled by the electrons. The incoming electron must go into the next higher energy level or principal quantum number. Therefore, the electron affinity values of noble gases equal to be zero. In learning chemistry, the nuclear energy of noble gas is not high enough to hold an electron in new quantum energy levels, and electron affinity data of noble gas are unavailable.