When scientists started exploring matter, they discovered that the smallest particle of an element that maintains its chemical identity through all chemical and physical changes is called and 'atom'.
Although Dalton's theory of the existence of atoms was published in 1808, there was no clear idea of what an atom might look like until the early part of the twentieth century.
Scientific investigation on the structure of atom and its subatomic particles is an area where still absorbs scientists.
The nuclei of atoms are made up of protons and neutrons. These two components of the nucleus are referred to as nucleons.
Electrons revolve around the nucleus in different energy levels or shells and each shell is associated with definite energy.
The outermost shell of an atom is called its Valence shell. It is the decisive shell during a chemical reaction.
It is interesting to note that atoms of a given atomic number can have different number of neutrons.
The nucleus contains protons and neutrons collectively called nucleons. It is positively charged. The mass of an atom is concentrated in the nucleus.