Electronegativity increases across a period due to an increase in effective nuclear charge.

The charge of protons is known as nuclear charge and the net effect of that charge on a shared pair of electrons present in a bond, considering the shielding effect of the inner electrons, is known as an effective nuclear charge.

Zeff   =   Z – σ

In the above equation,

  • Zeff is the effective nuclear charge.
  • Z is the nuclear charge.
  • σ is the shielding effect of inner shell electrons

When we move left to right across a period in the periodic table, the number of shells remains constant but one electron adds to every next element. For example, in third period,  sodium (Na) atom has 11 electrons and 11 protons while the Mg atom has 12 electrons and 12 protons. It is like a tug of war between protons and electrons.

So, magnesium (Mg) atom has a smaller atomic radius as compared to the sodium (Na) atom. The greater the participants, the greater would be the nuclear attraction. Thus, from left to right across a period, the effective nuclear charge increases so does the electronegativity.

Zeff  ∝   electronegativity