Explain the concept of a compound.

(N/A) When two or more atoms of different elements combine,the molecule of a compound is obtained. Examples include water $(H_2O)$,ammonia $(NH_3)$,carbon dioxide $(CO_2)$,and sugar $(C_{12}H_{22}O_{11})$.
The molecules of water and carbon dioxide are represented in the figure.
According to the figure,a water molecule comprises two hydrogen atoms and one oxygen atom. Similarly,a molecule of carbon dioxide contains two oxygen atoms combined with one carbon atom.
Thus,the atoms of different elements are present in a compound in a fixed and definite ratio,and this ratio is characteristic of a particular compound.
The properties of a compound are different from those of its constituent elements.
For example,hydrogen and oxygen are gases,whereas the compound formed by their combination,i.e.,water,is a liquid. Hydrogen burns with a pop sound and oxygen is a supporter of combustion,but water is used as a fire extinguisher.
Moreover,the constituents of a compound cannot be separated into simpler substances by physical methods. They can be separated only by chemical methods.