Atom is the second smallest form of any matter behind elementary particles. They are not visible to human eye. It consists of protons and neutrons in the atom core and electrons outside its core. All protons, neutrons and electrons are a part of atomic nuclei.
The more neutrons atom haves the bigger the mass of an atom is.
Number of electrons determines which element does atom represent. The more electrons atom haves the heavier element atom represents.
Different elements have different properties.
Hope this helped you understand what atom is better.
