Atoms are the smallest particle that can be extracted from matter by chemical reaction. Eveything in and around us is made of Atoms that interact together to build matter.
Atoms are represented in classrooms with the Rutherford model: each atoms is composed of a nucleus filled with neutral and positively charged elements around which negatively charged particles (electrons) gravitate.
Electrons are very light and small when compared to the nucleus, and they seem (in classic physics) to be partially free and in constant movement on a given orbital, a bit like our earth rotates around the sun.
There is always as many protons as electrons so that each atom is stable and considered as neutral. If not, he will quickly get stabilised by another element through chemical reactions in which they can share electrons.
Atoms combine together to form molecules that constitute every solid, liquid or gazeous. These maerials are sensitive to the environnement and can have various type of behaviors under temperature or pressure variations, or when submitted to light or other electromagnetic energies.