Define Electron Affinity in Chemistry
Electron affinity (EA) or electron gain enthalpy or simply affinity in the periodic table define as the amount of energy released or liberated when an electron is added to an isolated neutral gaseous atom at its lowest energy level (ground state) to produce a uni-negative ion or anion. In ionization energy, energy supplied to remove one, two, and more electrons from an atom or cation but in electron affinity, the energy released with the addition of one or more electrons in an atom or anion. Electron affinity is an exothermic reaction with the negative sign according to the usual thermodynamics convention in chemistry but the measurement of affinities always the positive value.
Measurement of Electron Affinities
Electron affinity is difficult to obtain but measure from the indirect measurement of Born-Haber energy cycles in which one step is electron particle capture. Affinities also measure 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 radiation spectrum. This gives the standard free energy for the equilibrium reaction. The free energy is calculated from the temperature dependence of equilibrium constant.
Question: Calculate the electron affinity of chlorine from the Born – Haber cycle data. The crystal lattice energy of sodium chloride = – 774 kJ mol-1, the ionization energy of sodium = 495 kJ mol-1, the specific heat of sublimation of sodium = 108 kJ mol-1, bond energy of chlorine = 240 kJ mol-1, and the heat of formation of sodium chloride = 410 kJ mol-1.
Answer: Born – Haber Cycle equation for the formation of sodium chloride crystal
– UNaCl – IENa + EACl – SNa – ½DCl – ΔHf = 0
or, ECl = UNaCl + IENa + SNa +½DCl + ΔHf
= – 774 + 495 + 108 + 120 + 410
= 359 kJ mol-1
Affecting Trends of electron Affinities
The magnitude of electron affinity influenced by the atomic radious, shielding effect, and electronic structure or configuration of an atom or an ion.
Atomic Radius and Affinity of Atoms
Larger the atomic size lesser the tendency of atoms to attract the additional electrons towards itself. 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.
Shielding effect and Affinity
Higher the magnitude of effective nuclear charge (Zeff) greater the tendency to attract the additional electrons towards itself. Therefore, the greater force of attraction exerted by on 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 increasing effective nuclear charge of an atom.
Electronic Structure and Affinity
The magnitude of electron affinity depends on the electronic structure of atoms. Therefore, the elements having, ns2, np6 valence shell configuration posses the 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 electron affinity (73 kJ mol-1) and form stable alkali hydride. The polarization of hydride ion very high.
Question: Account for the large decrease in electron affinity between lithium and beryllium.
Answer: The atomic number and electronic configuration lithium and beryllium are 1s2 2s1 (3) and 1s2 2s2 respectively. Therefore, lithium has an incompletely filled 2s subshell while beryllium has filled subshell. Hence lithium can affinity to receive electrons in 2s sub-shell but for beryllium, a still higher energy 2p level. Hence beryllium resists to gaining extra electrons in higher energy level or 2p orbitals.
Question: Why the electron affinity of nitrogen is less than phosphorus?
Answer: 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 added to the stable half-filled 2p subshell some amount of energy 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.
Electron Affinity Trends in Periodic Table
When we moving down a group in the periodic table the size of atoms generally increases with increasing atomic number. Hence the magnitude of electron affinity generally decreases in the same direction.
The elements of the second period are relatively smaller in sizes 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.
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. Therefore, the affinities trends for halogen atoms are F < Cl > Br > I.
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. Therefore, 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.
Oxidizing Properties and Electron Affinities
The halogen possesses 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. Therefore, the electron affinity of chlorine greater than the fluorine atom. This indicates that chlorine should be the strongest oxidizing agent. In fact, fluorine has been found to be the strongest oxidizing agent among all environmental elements. Therefore, oxidizing trends of halogen, F > Cl > Br > I but affinities trends, F < Cl > Br > I. The oxidizing power of halogen atoms explains by the oxidation potential of redox reactions and bond dissociation energy of halogen atoms.
- As the vales of chemical potential (E0) increases, the oxidizing power also increases. Values of E0 for halogen molecule like F2 = -186.6 kcal/mol, Cl2 = -147.5 kcal/mole, Br2 = -136.5 kcal/mole, I2 = 122.6 kcal/mole. These value clerly shows that E0 values of flurine molecule is highest, thus flurine is strongest oxidizing agent.
- The strongest oxidizing property also explains by small value of chemical bond dissociation energy of fluorine molecule. 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 (ns2np6) of inert gases are completely filled by the electrons. Therefore, the incoming electron must go into the next higher energy level or principal quantum number and affinity values of inert gases equal to zero. Also, the nuclear energy of noble gases not high enough to hold an electron in new quantum energy levels, and affinity data in learning chemistry of noble gas molecules are unavailable.