Q. ) An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a
divalent halide. Write the formula of the compound so formed. { with reference to the lesson : periodic classification of elements }

Dear Student,
If atomic number of the element X is 17, that means we call identify it as Chlorine 
Or its electronic configuration is 1s²2s²2p⁶3s²3p⁵
So it has 7 valence electrons 3s²3p⁵
Thus it needs 1 electron to complete its octet; Hence its valency is 1
Similarly, the element Y with atomic number 20 is Calcium
or its electronic configuration is 1s²2s²2p⁶3s²3p⁶4s²
So it has 2 valence electrons 4s²
Thus it needs to lose 2 electrons to complete its octet; Hence its valency is 2

Therefore, the formula of the compound between X and Y is YX₂ i.e CaCl₂
Regards.