What is the trend for atomic radii across the periods and down the group?

So, in general, atomic radius decreases across a period and increases down a group. Across a period, effective nuclear charge increases as electron shielding remains constant.

Explanation:

Moving from left to right across a period, the atomic radius decreases. The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons.

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c. electrical charges of ionic bonds.