Elements with 1/2 filled valence shells give or take electrons to make a complete outer shell these many form covalent (shared electrons) bonds because it requires too much energy to completely strip or add 4 electrons.
Elements with less than 1/2 filled valence shells release electrons as it becomes much more difficult to strip electrons to fill their valence shell.
Elements with more than 1/2 filled valence shells strip electrons from other atoms.
So as one moves to the left on the periodic chart, there are fewer electrons to release and the elements do so. It becomes more energetic to strip the 2nd and 3 electron. As one moves to the right, fewer electrons need to be stripped from others and so the do so more easily. With lower nuclear charge, elements requiring more electrons find higher energetics in adding the to an alread negatively charged orbital system.
Elements with the same number of electrons in the valence shell behave more similarly to each other than they do to their neighbors who have more or fewer electrons. Thus the halogens behave similarly as do the alkaline earth elements.
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