Electron Affinity in Periodic Table
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 is supplied to remove one, two, and more electrons from an atom or cation but in electron affinity, the energy is 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 spectrum radiation. This gives the standard free energy for the equilibrium reaction. The free energy is calculated from the temperature dependence of the 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 crystalline solid, sodium chloride, – UNaCl – IENa + EACl – SNa – ½DCl – ΔHf = 0; or, ECl = 359 kJ mol-1.
Affecting Trends of electron Affinities
The magnitude of electron affinity is influenced by the atomic radius, shielding effect, and electronic structure or configuration of an atom or an ion.
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.
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 electron affinity depends on the electronic structure of atoms. The elements having, ns2, np6 valence shell configuration 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 electron affinity (73 kJ mol-1) and forms stable alkali hydride. The electric 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 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.
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 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.
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. 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.
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.
Oxidizing Properties and Electron Affinities
Halogen like fluorine, chlorine, bromine, and iodine has 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 values clearly show that E0 values of fluorine molecule is highest, thus fluorine 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 of noble or inert gases like helium, neon, argon, krypton, xenon, and radon is ns2np6 which is completely filled by the electrons. The incoming electron must go into the next higher energy level or principal quantum number and electron affinity values of inert gases equal to zero. In learning chemistry, the nuclear energy of noble gas not high enough to hold an electron in new quantum energy levels, and electron affinity data of noble gas are unavailable.