When two different atoms in a molecule are bonded together by a covalent bond, the electron pair forming the the covalent bond is not shared equally by both the atoms.
The relative ability of a bonded atom in a molecule to attract the shared electron pair towards itself is termed as its electronegativity.
Factors affecting the magnitude of electronegativity:
- Size of the atom: The smaller atoms have greater electronegativity values than the larger atoms.
(a). Order of Atomic radius: H(0.32 Angstrom) < Li (1.23 Angstrom) < Na (1.54 Angstrom) < K (2.03 Angstrom) < Rb (2.16 Angstrom ) < Cs (2.35 Angstrom)
Order of Electronegativity: H(2.1) > Li (1.0) > Na (0.9)> K (0.8) > Rb (0.8) > Cs (0.7)
(b) Electronegativity values of the elements of 2nd period increases from Li (Z = 3) to F(Z= 9), since the atomic radii of these elements decrease in the same order.
Size: Li ( 1.23 Angstrom)> Be (0.90 Angstrom) > B ( 0.82 Angstrom)> C (0.77 Angstrom) > N ( 0.75 Angstrom)> O (0.73 Angstrom) > F(0.72 Angstrom)
Electronegativity: Li (1.0)<Be (1.5) < B ( 2.0) < C (2.5) < N ( 3.0)< O (3.5 ) < F(4.0)
2. Number of Inner Shells: The atom with greater number of inner shells has less value of electronegativity than the atom with smaller number of inner shells.
For example, the electronegativity values of halogens decrease from F ( Z = 9) to At (Z = 85).
3. Charge of the ion : A cation attracts the electron pair more readily towards itself than the atom from which it has been derives. This is due to the smaller size of the cation as compared to its parent atom. Thus a cation has higher electronegativity than its parent atom.
If an element shows variable oxidation states, the element with a higher positive oxidation state has more value of electronegativity than that in the lower oxidation state. This is due to the fact that the atom in a higher positive oxidation state is smaller in size and hence has a greater attraction for electrons.
Atomic/ Ionic Radius: Fe ( 1.17 Angstrom) > Fe2+ (0.76 Angstrom) > Fe3+ ( 0.64)
Electronegativity: Fe ( 1.80) < Fe2+ (1.83 ) < Fe3+ ( 1.96)
4. Number and nature of atoms to which the atom is bonded: Electronegativity value of P atom in PCl3 molecule is different from that in the PF5 molecule.
5. Ionization energy and electron affinity: The elements of Group VII A, which have the highest ionization energies and electron affinities also have the highest values of electronegativity. Similarly, Group I A elements, which have the lowest ionization energies and electron affinities have the lowest values of electronegativity.
Electronegativity in a period:
- On moving left to right in a period ( in case of s and p-block elements), there is a decrease in the size of the atoms. Smaller atoms have greater tendency to attract the electrons towards themselves i.e. smaller atoms have higher electronegativity values.
Electronegativity in a group:
- As we move down a group, there is an increase in the size of the atoms. With the increase in size of the atoms, ionization potential and electron affinity also decrease. As a result, their electronegativity values also decrease.
From the above discussion, we can able to explain the following:
- In a group electronegativity decreases from top to bottom
- In a period electronegativity increases from left to right
- F has the highest value of electronegativity among all halogen atoms.
Source : Inorganic Chemistry By R.D. Madan