ATOMIC PHYSICS

 Atomic physics deals with the study of the atom. Before studying the atom it is essential to know about the mass and charge of its constituent particles, the proton, the neutron and the election. These parameters are listed in Table C. 1.7.

Particle	Mass (kg)	Charge (coulomb)
Proton	1.672 x 10⁻²⁷	+ 1.6 x 10⁻¹⁹
Neutron	1.675 x 10⁻²⁷	Neutral
Electron	9.108 x 10⁻³¹	-61.6 x 10⁻¹⁹

Thus a proton and a neutron have almost the same mass, which is nearly 1836 times that of an election. Protons and election have equal but opposite charges.

Atomic Structure

An atom consists of a central nucleus surrounded by electronic at various energy levels. The most successful model of the atom is the wave mechanical model, which is basically a mathematical rather than a visual model. For our purpose, it will suffice to consider the Bohr model of the atom.

According to the Bohr Model, an atom consists of a central nucleus with electrons revolving around it in circular orbits. This is similar to planets revolving around the sun. The nucleus consists of closely packed protons and neutrons and contains almost all the mass of the atom. In a neutral atom, the number of electrons is equal to that of protons.

The hydrogen atom is the lightest with only one electron revolving around a single proton. The heaviest natural atom is that of uranium with 92 electrons.

Inside an atom, the electrons can revolve only in certain specified orbits. An election in the innermost orbit has the least energy and an electron in outer orbit has more energy. These orbits are specified as energy levels. In a normal atom, the electrons occupy their lowest energy levels. If the atom is supplied with extra energy – by strong heating or by bombardment with some fast moving particle – one or more electrons may jump from one energy level to a higher one. The atom is then said to be in an excited state. The atom, 

however, does not remain in this state for more than 10⁻⁸ s. 

The electrons fall back to their lower energy levels by emitting surplus energy in the form of electromagnetic wave pulses called photons. Depending on the difference between the energy levels, the emitted radiation may be of many frequencies, lying in the infiared, visible, ultraviolet and even X- ray regions.