Question

When you write the formula for a molecular compound from its name, how do you know how many of each kind of atom is present?

Answer 1

Remember that the noble gas electronic configuration is quite stable all the time compared to other configurations. Generally, half filled and full filled electronic configurations are more stable than the other configurations. Every molecule will be trying to have half filled/full filled electronic configuration in order to be stable all the time.

In the same way every atom tries to satisfy the octet rule or to have noble gas electronic configuration to be stabel.

For example, H₂O ----------> 2H⁺ + O^-2 . So Oxygen gained 2 electrons from 2 Hydrogens and these two electrons will go the valance p orbitals of O.

The oxidatio state of every element is equal to its group number. So in any molecule, every atom tries to get that oxidation state. For example NaOH . Na belong to 1st group, it loses one eletron to OH

NaOH------------> Na+ + OH-.

If you take 2nd group element Ca, it would be Ca(OH)₂ . Ca(OH)₂ --------------> Ca⁺² + 2OH^-. so if somebody says calcium hydroxide then you have to think that the central atom belong to which group and based on that you have to write the formula.

In this way you can have clarity.

Please ask if u have any further doubts on it. Thanks.