Question

What is the formula for potassium sulfide?

Answer 1

Chemical formula for any compound represents the total number of atoms in that molecule. The chemical formula is written in such a way that cation (positive species) is written first and then the anion (negative species). A sulfide ion is made when sulfur loses two electrons, so it has a charge of 2.

Complete answer:
A chemical formula for any compound represents the total quantities of atoms involved in a molecule. While writing the chemical formula for any molecule, IUPAC has suggested that the cation is named and written before the anion of that molecule. The cation is the ion that carries the positive charge, while anion carries the negative charge.
We have been given potassium sulfide. This compound consists of potassium and sulfur as the atoms. The potassium ion is represented as, $K^+$, as it is a metal that has 1 electron in the outer shell which gets donated. While the sulfide ion is represented as, $S^{2-}$ when sulfur gains 2 electrons to complete its octet, potassium has a valence of positive of 1 while sulfur has a valence of negative of two 2. The chemical formulas of compounds are written in such a manner, as to compensate the valencies or charges present in the atoms of the element. For instance, one positive charge containing ions cannot form a compound with an element having a charge of negative two. Therefore, those positive ions are taken in a quantity of two, as to compensate the valence of the negative ions.
Therefore, to make the charge of -2 of sulfide neutral, potassium ions are taken in a quantity of 2. So, the formula of potassium sulfide becomes,${K_2}S$ .
Hence, the formula for potassium sulfide is ${K_2}S$.

Note:
Potassium is a highly reactive metal as it has only 1 electron in its valence shell, and is placed in group 1, so it has a group oxidation state of +1. While, sulfur is a non – metal, that is placed in group 16, and has a group oxidation state of +2 and +4. Potassium sulfide is an ionic compound that can hydrolyze in water to give hydroxide OH ion and HS ions.