The atomic number of calcium (Ca) is 20, and the atomic number of sulfur (S) is 16. This means that calcium has 20 protons in its nucleus, while sulfur has 16 protons.
When calcium and sulfur bond, they form an ionic bond. This occurs because calcium is an alkaline earth metal and has two electrons in its outermost shell. To achieve stability, it tends to lose these two electrons, becoming a positively charged ion (Ca²⁺). On the other hand, sulfur is a non-metal that needs two more electrons to complete its outer shell and achieve a full octet. When calcium loses its two electrons, sulfur gains them, resulting in the formation of a negatively charged ion (S²⁻).
The electrostatic attraction between the positively charged calcium ions and the negatively charged sulfide ions leads to the formation of an ionic bond. This type of bond is typically strong and results in the formation of ionic compounds, such as calcium sulfide (CaS).