Hund’s rule and Ionization potential and Electron affinity and Electronegativity

Hund’s Rule

·      Hund’s rule of maximum multiplicity states that electron pairing takes place only after all the available degenerate orbitals are occupied by one electron each.
Ex: Carbon atom (At.No.: 6)
·      Carbon atom has six electrons
·      The first electron goes into the ‘1s’ orbital of k-shell.
·      The second electron will be paired up with the first in the same ‘1s’ orbital.
·      Similarly the third and fourth electrons occupy the ‘2s’ orbital of the L-shell.
·      The fifth electron goes into one of the three ‘2p’ orbitals of L-shell. (2Px)
·      Since the three ‘p’ orbitals (2Px, 2Py, 2P3 are degenerate) the sixth electron goes into 2Py or 2Pz but not 2Px



IONIZATION POTENTIAL or IONIZATION ENERGY

·      Definition : “Ionization energy or Ionization potential is defined as the minimum energy required to remove an electron from the outermost orbital of an atom in the gaseous state.”
·      Expressed in units of electron volt or K.J.Mole-1 



·      Ionization potential is dependent on
1) Nuclear charge
2) Size of the atom
3) Charge on the ion
4) Half filled and fully filled orbitals
·      In general ionization energy increases across a period, because of decrease in the atomic size
·      In a group ionization energy decreases from top to bottom because atomic radius increases.

ELECTRON AFFINITY

Definition:Electron affinity is defined as the energy released when an electron is added to a neutral gaseous atom in its lowest energy state
·      Expressed in electron volt (eV)
·      Halogens have high values of electron affinity
·      The electron affinity for halogens follows the order of
·      Cl > F> Br > I
·      In a group, the electron affinity decreases from top to bottom as the atom size increases
·      In a period the electron affinity increases from left to right.

ELECTRO NEGATIVITY (E.N) 

Definition: Electro negativity is defined as the tendency of bonded atom in a molecule to attract the electron density of the shared pair of electrons.
·      Small size atoms have more tendency to attract the shared electron density and hence to attract the shared electron density and hence have larger EN values.
·      In a period, the electro negativity increases due to the decrease in size of atom
·      In a group the electro negativity decreases due to increase in atomic size.
·      Fluorine has been assigned a value of 4.0 for electro negativity in the Pauling scale. 



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