Predict the Formula of the Ionic Compound from the Electronic Configuration

The electronic configuration of two elements A and B are -

A- 1s2 2s2 2p6 3s2 3p6 4s2

B- 1s2 2s2 2p6 3s2 3p5

What is the formula of the ionic compound formed between these elements?

 Step 1: Identify the group

A belongs to group 2 as deduced from the valence shell electronic configuration of the element.

A- 1s2 2s2 2p6 3s2 3p6 4s2

 

B belongs to group 7 as seen from the valence shell electronic configuration of the element.

B- 1s2 2s2 2p6 3s2 3p5

Since the Group 2 elements have low ionization enthalpy, it can easily lose two electrons to achieve the stable octet configuration. As the principal quantum number of the outer most shell of element A is 4, we can safely conclude that the element A is Calcium (Ca) belonging to period 4 of the periodic table.

The Group 7 elements, the halogens have high negative electron gain enthalpy values due to which it can accept electrons. From the principal quantum number of the valence shell, we know that the element B belongs to period 3 and is Chlorine (Cl).

 Step 2: Predicting the Stoichiometry

As one Ca atom can lose two electrons,

 

Ca Ca2+ + 2e-

 

we need 2 Cl atoms to accept the two electrons to give

2Cl + 2e- 2Cl-

 Net Reaction:

Ca2+  +   2 Cl-  CaCl2

From the above reaction, we can see the need for one Ca atom and two Cl atoms for forming compound Calcium Chloride (CaCl2).

 Step 3: Deducing the Molecular Formula

The simplest method is employing the cross-multiplication strategy wherein, the valency of Ca is 2 and Cl is one from which we can arrive at the formula CaCl2.

Ca     2

Cl        1

Therefore, the formula of the compound is AB2 or CaCl2.

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