Question

An atom with atomic number 20 is most likely to combine chemically with the atom whose atomic number is _____________ .

Answer 1

Write the electronic configuration of the atom with atomic number 20. With this we can determine whether the atom will lose electrons or gain electrons to attain stability. Then write down the electronic configuration of the atoms with the atomic number mentioned in the options. The number of electrons gained by one atom should be equal to the electrons lost by the other atom in order to form a chemical bond.

Complete step-by-step answer:
As suggested in the hint above we will write the electronic configuration of elements with atomic number 20.
E.C = $\text{1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{6}}}\text{3}{{\text{s}}^{\text{2}}}\text{3}{{\text{p}}^{\text{6}}}\text{4}{{\text{s}}^{\text{2}}}$
Since the last electron enters the s subshell, the element belongs to the s block of the modern periodic table. The atom will attain complete octet configuration when it loses two electrons.
Now we will write the electronic configuration for the atoms given in options. The atom which will attain complete octet configuration after gaining 2 electrons is the correct answer.
(A) Z = 11
E.C = $\text{1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{6}}}\text{3}{{\text{s}}^{1}}$
This is not a desirable configuration because the atom has to lose an electron to gain stable configuration. Hence bond formation cannot take place.

(B) Z = 16
E.C = $\text{1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{6}}}\text{3}{{\text{s}}^{\text{2}}}\text{3}{{\text{p}}^{4}}$
This is a desirable electronic configuration. This is because the atom needs to gain 2 electrons to attain stable configuration. Hence bond formation can take place between the two atoms.
(C) Z = 18
E.C = $\text{1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{6}}}\text{3}{{\text{s}}^{\text{2}}}\text{3}{{\text{p}}^{\text{6}}}$
The atom has a stable electronic configuration without the need to lose or gain an electron. Hence this atom is inert and bond formation cannot take place.
(D) Z = 10
E.C = $\text{1}{{\text{s}}^{\text{2}}}\text{2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{6}}}$
The atom has a stable electronic configuration without the need to lose or gain an electron. Hence this atom is inert and bond formation cannot take place.

Therefore, the correct answer is option (B).

Note: The name of the atom having atomic number 20 is Calcium. The name of the atom having atomic number 16 is Sulphur. So, the compound formed from the atoms is calcium sulphide and its symbolic representation is CaS.