Electronegativity refers to the ability of an atom to attract electrons toward itself. A more electronegative atom has higher ability to attract electrons towards itself.
Electronegativity depends on two factors
- Effective Nuclear Charge (ENC)
- Distance from nucleus
- ↑ Effective Nuclear Charge (ENC) = ↑ Electronegativity
- ↑ Distance from nucleus = ↓ Electronegativity
Effective Nuclear Charge ≈ No. of protons – No. of inner shell shielding electrons
This is an intuitive formula. The protons in the atom are trying to attract electrons (unlike charges attract) while the inner shell electrons are trying to repel away electrons that are being added to the valence shell (like charges repel). Thus, ENC measures the net attraction that an atom has on the valence electrons.
For example, Fluorine (electronic arrangement: 2,7) is more electronegative than Oxygen (2,6).
ENC of F = 9 – 2 = 7
ENC of O = 8 – 2 =6
Therefore, it can be concluded from the above example that electronegativity increases across a period, from Group I to Group VII. Group 0 is excluded as it has stable electronic arrangement, hence will not form bonds.
How about elements from the same group? Don’t they have the same ENC?
Yes, you are right! Elements of the same group have the same ENC. For instance Fluorine (2,7) and Chlorine (2,8,7)
ENC of F = 9 – 2 = 7
ENC of Cl = 17 – 10 = 7
To compare elements in the same group, we have to use distance from the nucleus. Chlorine has 3 electron shells as compared to 2 for Fluorine. Thus, the valence shell of Chlorine is further away from the positive nucleus as compared to Fluorine.
The greater distance between the positive nucleus and the valence shell makes it more difficult for Chlorine to attract electrons as compared to Fluorine. Hence, Chlorine is less electronegative as compared to Fluorine.
Therefore, it can be concluded from the above example that electronegativity decreases down the group!
Putting these two factors together, you should be able to conclude for yourself that Fluorine is the most electronegative atom while Francium is the least electronegative! (For A level students, I am basing this on the Pauling scale.)