As we move across a period from left to right, we generally find that each element has a smaller covalent radius than the element preceding it. It might seem counterintuitive because it implies that atoms with more electrons have a smaller atomic radius. It explains with the concept of effective nuclear charge, Z_eff. It is the pull exerted on a specific electron by the nucleus, taking into account any electron-electron repulsions. For hydrogen, there is only one electron, and so the nuclear charge (Z) and the effective nuclear charge (Z_eff) are equal. For all other atoms, the inner electrons partially shield the outer electrons from the pull of the nucleus, and thus:

Zeff  = Z − shielding