An atom is the smallest element that can be extracted from materials. Many types of Atoms are present on earth, and these has been classified in the famous Mendeleiev table.
All atoms can be represented as a heavy nucleus full of neutral ( neutrons in black ) and positively charged elements ( protons in red ). Around this nucleus, a continous flow of much smaller,lighter and negatively charged elements called electrons (in purple).
There is one electron for one proton, so that all the charges neutralise each other, and the atom is said to be neutral and stable. However, electrons are often shared and exchanged between atoms, for instance by chemical reactions, or when specific materials are submitted to an electromagnetic field.