These are two forms of the Hydrogen atom. Such atome, having the same number of protons yet differ in massare called Isotopes.

Fig. 1.  A Heavy Hydrogen atom.   Normal Atom.

An atom can be described as the smallest particle of an Element.  There are only  118 Elements in the whole of the Universe. They  include gases like Nitrogen, Oxygen  and non metallic solids like Carbon and Sulphur and the metals like Gold , Iron and Copper. 

All substances that end in ‘ium’  such as Aluminium, Calcium are metallic elements except Helium which is a non metal

We cannot  see atoms. Their nature and the properties have been discovered by indirect methods The present day idea of the Atom is based mainly on the views of Sir Ernest Rutherford. According to his investigations, the atoms are mostly empty space with some electrical charges here and there. The center of the atom is called the Nucleus and harbours the positive charges (Protons). The Negative charges (Electrons) circle round at a distance forming a shell. According to calculations, if the nucleus is compared to a cricket ball placed in the middle of the ground, the first electron orbits  in a globe as far away as the boundary  line. In-between, it is simply empty space. The electrical charges are usually described as Sub – atomic particles.

A mass of a proton is taken as one unit when comparing masses of atoms. The mass of an electron is negligible. There is another sub atomic particle called the Neutron which is also embedded in the nucleus. This does not show a charge and as such it can be considered as a particle formed by the merging of an electron and a proton.

An atom of Heavy hydrogen, sometimes called Deuterium has a Neutron in addition to the proton. The chemical properties of the two types are identical. The difference is in the mass due to the presence of the Neutron. Such Atoms of the same element are called Isotopes.