The elements silicon (atomic number 14) and chlorine (atomic number 17) are both in Period 3. Which is the more reactive element?

Chlorine is more reactive than silicon. Chlorine has 7 valence electrons; nearly a full outer shell, while silicon has only 4 valence electrons

Answer;

-Chlorine is more reactive than silicon

Explanation;

-As you move across a period, the nuclear charge will increase; the number of energy levels will stay the same, so there is a stronger and stronger attraction for the electrons.

-The electrons are being held more tightly as you move across a period. It becomes more and more difficult to lose electrons and consequently the reactivity of non metals increases as you go from left to right across the periodic table; Therefore; chlorine is more reactive than silicon.