What happens when we heat an atom?
If we take an atom of any element and then supply heat energy to it
  then what will happen?


A single atom can move in three dimensions (x, y, z) through space, it cannot rotate about an external axis like a diatomic molecule can, nor can it vibrate.  As you supply heat (or any form of energy) to the system containing the atom, it's kinetic energy will increase and it will move faster through space.  If you supply enough energy, then the electrons surrounding the atom will absorb that energy and undergo electronic transitions from the ground state to various excited states.  This process can be diagrammed using a Morse potential diagram , I've drawn a simple one below.  The diagram is meant to show promotion of an electron from the ground electronic state to higher and higher electronic levels as more energy is applied to the system.  Note that as the electron is promoted to higher energy levels, the average separation distance between it and the nucleus increases.  If enough energy is supplied, an electron will dissociate from the atom and we will be left with an ion.  Continue to supply even more energy and more electrons will be removed from the atom. Pumping even more energy into the system will ultimately lead to transformations in the nucleus of the atom.