What is an atom?

Hint: Since our early school days, we have been taught that anything that has a mass and occupies space is called a matter. Now, what are the things that give a matter it's property of mass or volume?

Complete step by step solution:

Atoms are the fundamental building block of a matter that retains its all chemical properties. Everything in this universe is made up of matter. Just to have a general idea, look at the flowchart.

The Universe→ Galaxies→ Systems with Stars →Planets →Biomes →Ecosystems →Populations→ Organisms→ Systems→ Organs→ Tissues→ Cells→ Cell organelles→ Macromolecules→ Molecules→ Atoms

Now, you can imagine how small atoms are. But we can go even further, atoms also consist of subatomic particles- proton, neutron and electron. Protons are the positively charged particles, electrons are the negatively charged particles and neutrons are neutral in nature, meaning they have no charge on them. Protons and neutrons together make up the nucleus at the center of the atom, a dense and positively charged core. And negatively charged electrons revolve around the nucleus forming electron clouds. As we can see, most of the mass of an atom is concentrated in its nucleus. The mass of an electron is only 1/1836 the mass of the nucleus of the lightest atom, that of hydrogen. Even though the nucleus is heavy, it is small in comparison to the overall size of the atom.
The concept of atom was first proved in the year 1808, by a famous English chemist, physicist, and meteorologist John Dalton.

Note: Hydrogen being the only exception as it generally has only one electron and one proton but no neutrons. And atoms may lose or gain electrons according to the requirements in the chemical reaction.