Neils bohar proposed his atomic model in 1913.
He describe that's electron travel in a circular orbit surrounds the neuclus of an atom.each orbit has quantized energy and size. Energy is transmitted when an electron jumps from one to other orbit near to nucleus.and it absorb energy when it jumbs away from nucleus.he also said that these electrons are restricted to fixed orbits. In excited state of electron, they absorb or emitt photon with specific wavelength.
A patient receiving an X-ray in a hospital
Isotopes are variants of a particular chemical element which differ in neutron number, and consequently in nucleon number. All isotopes of a given element have the same number of protons but different numbers of neutrons in each atom.
The term isotope is formed from the Greek roots isos ("equal") and topos ("place"), meaning "the same place".