The atomic numbers of three elements, X, Y and Z are 9, 11 and 17 respectively. Which two of these elements will show similar chemical properties? Why?

Atomic no. of X = 9

Electronic configuration = 2,7

Valence electrons = 7

Valency of X =1

Atomic no. of element Y = 11

Electronic configuration of Y = 2,8, 1

Valency of Y= 1

Atomic no. of element Z = 17

Electronic configuration of Z = 2,8, 7

Valency of Z= 1

The element X and Z have same number of valence electrons and since chemical properties depends on the valence electrons. So, Element X and Z will show similar chemical properties.