What period number is the following atom in?

How did planetesmals form planets? a. they broke apart into smaller chunks. b. they collided and stuck together. c. they cooled and pulled ice together. d. they began to rotate.

Fill in the coefficients that will balance the following reaction: a0cr2(so4)3 + a1agno3 -> a2cr(no3)3 + a3ag2so4

The chemical equation below shows the reaction of sodium (na) and chlorine (cl) to form sodium chloride (nacl). 2na + cl2 → 2nacl in this equation, which of the following is a reactant? i. sodium ii. chlorine iii. sodium chloride