An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a compound.
(i) Write the position of these elements in the modern periodic table.
(ii) Write the formula of the compound formed. Justify your answer in each case.

Explanation

(i) The position of elements X in the modern periodic table is group number 17 and period number 3. The position of element Y in the modern periodic table is group number 2 and period number 4. Electronic configuration of element X = 2, 8,7.
It has 3 shells so period number 3.
Halogens are kept in group 17.
Electronic configuration of element Y = 2, 8, 8,2
It has 4 shells so period number is 4.
The valence shell has 2 electrons so the group number is 2.
(ii) Formula of the compound :
`{:("Valency of element Y","Valency of element X"),("2","1"):}`
`thereforeYX_(2)`