Electronegativity decreases down a group in the periodic table due to increased distance of bonding pairs from the nucleus caused by additional electron shells and increased shielding. Conversely, electronegativity increases across a period as the atomic number rises, which enhances the nucleus's positive charge, attracting electrons more strongly. Therefore, the effective nuclear charge plays a crucial role in these trends.

1. why does an atoms electronegativity decrease down a column in the period table but increases
across the row in the periodic table?
Solution
The electronegativity decreases down the group because the bonding pair is far away from
the nucleus because of addition of extra shells. This also leads to the pure shieling because
the second period element will only be shielded 1s2 electrons whereas higher period
elements will be shielded by 1s2 2s2 2p6 electrons. Hence the electronegativity decreases
down the group
When we go from left to right in a period, the distance of presence of electrons remain the
same, since we are in same shell but the charge on nucleus (increases due to increase in
atomic number), the increase charge lead to attraction of electrons pair tightly making it
more electronegative in nature