CBSE Class 10 Science 2017 · Paper 31/2/C · Question 14

P (atomic number 11): K, L, M $=2,8,1$. It has $3$ shells, so it is in period $3$; it has $1$ valence electron, so it is in group $1$. Q (atomic number 19): K, L, M, N $=2,8,8,1$. It has $4$ shells, so it is in period $4$; it has $1$ valence electron, so it is in group $1$. (P is sodium and Q is potassium: both are alkali metals in group 1, with Q one period below P.)