Why do alkali metals and halogens react so strongly with other elements?

• A. Halogens want to lose a valence electron
• B. Alkali metals want to gain a valence electron
• C. They have full valence shells
• D. They are noble gases

Answer: (A)

Reactivity here comes from a strong drive to reach a stable electron arrangement. Alkali metals have just one electron in their outer shell and very low energy to remove it, so they readily lose that electron to become positively charged ions. Halogens, on the other hand, have seven electrons in their outer shell and a strong pull to complete that shell by gaining one electron, forming negatively charged ions. When these two types meet, electrons are transferred so the metal loses its outer electron and the halogen gains one, releasing a lot of energy as new bonds form. This pursuit of a noble-gas configuration makes their reactions especially vigorous, especially in forming ionic compounds like salts. Statements that halogens lose electrons, that they already have full valence shells, or that noble gases are reactive don’t fit how these elements actually behave.