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

This increases across a period, but the Noble gases aren't included as they do not form molecules and this property is exclusive to molecules. The increase is almost linear, mirroring the smooth increase in effective nuclear charge, the difference of p and s orbitals seeming not to effect it, but this is another result of it refering to molecules, as all the orbitals will be filled in a molecule, so their starting configuration won't matter. This is why they follow the effective nuclear charge predicted from Slater's rules so exactly, as these too don't distinguish between p and s orbitals.

This property will also decrease down the groups.

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