Question

What is the formula for lithium sulfide?

Answer 1

Hint: Lithium sulphide is an inorganic compound and generally leads to the formation of a solid deliquescent powder of yellow-white colour. In air, it readily hydrolysed in order to release the hydrogen sulphide gas (having rotten egg odour).

Complete step by step answer:
As the name suggests, the given compound lithium sulfide comprises a metal lithium (\[Li\]) and a non-metal sulfur (\[S\]). We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. lithium sulfide. From the periodic table, it is clear that lithium is a metal and it belongs to group 1 thus, it possesses +1 cationic charge and sulfur is a non-metal and it possesses -2 anionic charge. It will look like $L{i^{1 + }}{S^{2 - }}$. As lithium sulfide is neutral, the net charge on the compound should be zero. So in the present case, $( + 1) + ( - 2) = - 1 \ne 0$. Thus, this can’t be the formula for lithium sulfide as one to one ratio of the charges makes the ionic formula. Now, we can change the subscripts in order to balance the charge. We will change the subscript of lithium to neutralise the charge i.e. $Li_2^{ + 1}{S^{2 - }}$. Now the charges are balanced as $(2 \times ( + 1)) + ( - 2) = 0$.
Hence, the formula for lithium sulfide is $L{i_2}S$.

Note: Lithium sulfide is commonly employed in lithium sulphur batteries. It is generally prepared by the reaction of lithium with sulfur. The reaction is conducted in the presence of anhydrous ammonia. The reaction is depicted below:
\[Li{\text{ }} + {\text{ }}S{\text{ }} \to {\text{ }}L{i_2}S\]