N.B. The first row elements have atomic numbers 3 to 10.

This increases from left to right across a period, although the number of electrons increases, the shielding each electron experiences does not, so, due to the increase in the number of protons, the effective nuclear charge actually increases.

This property will increase slightly down a group, then remain constant.

There are various different scales for measuring this, all of which have different assumptions, therfore different levels of accuracy. One popular version is Slater's, which is simple due to its assumption that s and p orbitals make no difference, but provides suitably representative results.

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