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

(a) Element X (atomic number 17)
So, Electronic configuration of X: 2, 8, 7
No. of electrons in outermost shell = 7 So, Group number = 17
No. of shells = 3
So, Period to which the element belongs = 3

Element Y (atomic number 20)
Electronic configuration of Y: 2, 8, 8, 2
No. of electrons in outermost shell = 2 So, Group number = 2
No. of shells = 4
So, Period to which the element belongs = 4

(b) X has 7 valence electrons so, it needs 1 electron to complete its octet and Y has 2 valence electrons so, it can donate its 2 electrons to acquire the octet configuration. Hence, X will gain 1 electron and Y will lose 2 electrons, so the chemical reaction is:
$X_2 + Y \rightarrow YX_2$
X = Cl (At. No. = 17) and Y = Ca (At. No. = 20)
So, $Cl_2 + Ca \rightarrow CaCl_2$

Updated on: 10-Oct-2022

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