A smallest unit of element which consists of a dense, central positively charged nucleus surrounded by negatively charged electrons is called as atom.
Atom is divided into two parts – Atomic structure and extra nucleus part. Atomic structure consists of positively charged proton (P+) and neutral neutron (n°). Extra nucleus part contains negatively charged electrons (e-).
Atoms have mass as all elements and compounds have mass. The mass of atom is mainly concentrated in its nucleus because of protons. Protons has highest mass as compare to electrons and neutrons.
Hydrogen is the lightest of all elements and the nucleus of hydrogen atom contains one proton surrounded by one electron.
The nucleus of every atom contains fixed number of protons which attracts same number of electrons and an atom becomes electrically neutral. Ions can be formed either by adding or by removing electrons from the atom.