Four elements P, Q, R, and S have atomic numbers 12, 13, 14, and 15, respectively. Which of these are metal, non-metals, and metalloids?

P and Q have 2 and 3 valence electrons, respectively. So, they lose their electrons to attain a stable electronic configuration and act as metals. 

R has 4 valence electrons, so it can either gain or lose them and show intermediate properties of metals and non-metals, acting as a metalloid.

S has 5 valence electrons, and it needs to gain 3 more to attain noble gas configuration, so it acts as a non-metal.

Hence, P, Q are metals, R is metalloids and S is non-metal.