Because the third-shell electrons are farther from the nucleus, they are held more weakly.

The first ionization energies, plotted at the top of this page, demonstrate this.

Within each shell, the first IE increases with atomic number because of the increased charge on the nucleus.

However, each third-row element has a lower first IE than its second row analogue as the outer electrons are further from the nucleus so much more easily removed.

Electronegativity values also follow this trend (see next page).