First 151 words of the document:
Half the distance between two identical atoms bonded together by a single covalent
bond. Nobel gases do not have a covalent radius because they do not form covalent
Across a period, covalent radius decreases because effective nuclear charge increases
(due to an increase in no. of protons and little increase in shielding) therefore shells
are pulled closer towards the nucleus.
Covalent radius increases down a group due to the constant effective nuclear charge
and increase in no. of shells at increasing distances from the nucleus.
Covalent radius is constant across the first row of transition metals because despite
an increase in no. of protons across the period, electrons enter the inner 3D subshell,
therefore the increase in nuclear charge is shielded so all the transition metals feel
the same effective nuclear charge.