Question

Difference between atom and molecule.

Answer 1

A molecule is the smallest part of a substance, that has all the properties of the substance.
We can break down the molecules further into multiple atoms, but those will have different properties from the original substance. An atom was long believed to be the smallest piece of matter.
The water molecule is $H_{2}O$, meaning that each water molecule is build up of one oxygen atom, and 2 hydrogen atoms. One molecule of $H_{2}O$ is the smallest amount of water we can have. Going smaller, it will not be water anymore, but atoms of oxygen and hydrogen.
Chemical reactions can break down and modify molecules, but not the atoms. The ethanol (alcohol) molecule is composed of 2 carbon, 6 hydrogen, and 1 oxygen atom $\left(C_2{H}_{6}O\right)$. When we burn ethanol the chemical reaction destroys the ethanol molecules and uses extra oxygen $\left(O_2\right)$ molecules to produce carbon dioxide and water molecules:$C{O}_2$ and $H_{2}O$. And because the atoms themselves don't change, we keep the same amount of C, H and O atoms in the end product.