Question

Which of the following is formed when calcium combines with oxygen?
A. Ca
B. CaO
C. \[Ca{{O}_{2}}\]
D. \[C{{a}_{2}}{{O}_{2}}\]

Answer 1

Hint: The oxidation state of calcium is +2 means it tends to give two-electron to another element (octave complete with by releasing two free electrons)whereas oxygen has an oxidation state of -2 and wants to get two electrons to complete its octave. So, due to the tendency to give two electrons, calcium forms a bond with oxygen which is in need of two electrons.

Complete step by step solution:
The atomic number of calcium is 20 so in its atom first shell (K) will be filled with two electrons, next shell (L) filled with 8 electrons such as
\[1{{s}^{2}},\text{ }2{{s}^{2}},\text{ }2{{p}^{3}}\]
The Left 10 electrons will occupy the next third shell (M). In this shell, there are three subshells, s, p, and d. In a subshell out of 10 two electrons will get filled. In the p orbital maximum number of an electron can be filled is 6 so, when two electrons get filled in the s of the third shell, the p subshell gets filled with 6 electrons leaving 2 electrons and that filled in the 4s subshell (one electron from paired electron will jump to 3d orbital). From this, we can conclude that the general valency of calcium in the ground state is two such as
\[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{2}},4{{s}^{2}}\]
OR
\[1{{s}^{2}},2{{s}^{2}},2{{p}^{6}},3{{s}^{2}},3{{p}^{2}},4{{s}^{1}},3{{d}^{1}}\]
On the other hand, oxygen whose atomic number is 8 and its electronic configuration is
\[1{{s}^{2}},2{{s}^{2}},2{{p}^{4}}\]

In the p subshell there are three orbitals and thus can have a maximum of 6 electrons but in oxygen, p is filled with 4 so two-electrons is needed to complete its octave. Thus, the valency of oxygen is 2. On one side, calcium has two free electrons in the 4s and 3d orbital and oxygen is in demand for two-electrons to full 2p orbital. So both can form bonds with a donation of electrons from calcium and acceptance of electrons by oxygen. One bond formed with two electrons. So, two free electrons of calcium form two bonds with two unpaired electrons of oxygen.

So when calcium reacts with oxygen in the atmosphere calcium oxide forms such as
\[Ca\text{ }+\text{ }1/2O\text{ }\to \text{ }CaO\] (oxidation state of calcium is +2 and oxidation state of -2, the positive sign represents the donation of electron and negative sign represent acceptance of electron).
Thus, the correct option is A.

Note: It is important to note that not a single element is present in the simplest form (Oxygen as a single element like O) but in a molecular form such as O₂ (molecule). But the valency of calcium is +2 so it can form a bond with only one oxygen thus, we take half of one molecule of oxygen to react with calcium.