This phenomenon is known as the periodic trends in reactivity. It occurs because of the arrangement of electrons in orbitals.
As one moves from left to right across a period, the number of electrons in the outermost shell increases. These outermost electrons are involved in chemical reactions.
Therefore, as the number of outermost electrons increases, the reactivity of the element increases. However, the elements have filled shells, so beyond a certain point, the outermost shell will be full.
The reactivity then decreases as the electrons are no longer available to take part in chemical reactions. Therefore, the reactivity of the elements decreases from left to right in a period and then increases as the outermost shell begins to fill up again.
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