In the MCQ test of this chapter there was a question:

• According to Mendeleev's Periodic table, the oxide of which of the following elements bears the formula RO₂ ?

1. Silicon 2. Sodium 3. Magnesium 4. Aluminium

I neither understood the question nor the answer

Please help me out..

Since group 4 element exhibit a valency of 4 and oxygen exhibits a valency of 2, so covalent bond between silicon and 2 oxygen atom helps it in completing the octet of these atoms. Sodium with a valency of 1, Magnesium with a valency of 2, Alumnium with a valency of  3 cannot gain an octet state by RO₂ formula.