Which non-metal below is most reactive? (2 points)
a. chlorine (cl)
b. helium (he)
c. silicon (si)
d. argon (ar)

The answer is: a. chlorine (Cl).

Chlorine is halogen element.

Halogen elements are in group 17: fluorine (F), chlorine (Cl), bromine (Br) and iodine (I). They are very reactive and easily form many compounds.

They have high electronegativity because they have seven valence electrons in their outermost energy level, so they can gain an electron to have the octet rule.  

Chlorine has atomic number 17, it means it has 17 protons and 17 electrons.

Electron configuration of chlorine atom: ₁₇Cl 1s² 2s² 2p⁶ 3s² 3p⁵.

Chloride is negative ion of chlorine. Chloride is formed when chlorine gain one lectron.

Chloride anion has 17 protons and 18 electrons (like argon-noble gas).

The electron configuration for the chloride ion: ₁₇Cl⁻ 1s²2s²2p⁶3s²3p⁶.