Question

a element X atomic mass 17 is react with element by atomic mass 20 to form a compound
a. write the position of these element in the modern periodic table
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Answer 1

Atomic mass of X = 17

So X = Cl (chlorine)

Atomic mass of Y = 20 [ let other element be Y]

So Y = Ca (calcium)

Reaction:-

Ca + Cl = $CaCl{2}$

a. Position of X (cl) in periodic table : group 17,period 3

As electronic configuration of cl (17) = 2,8,7 outermost shell has 7 electrons so group no.17 and it has three shells so period no. 3

Similarly,

Position of Y (ca) in periodic table : group 2,period 4

As electronic configuration of ca (20) = 2,8,8,2 outermost shell has 2 electrons so group no.20 and it has four shells so period no. 4