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Atomic number I

May 29, 2013

The atomic number is the number of unit positive charge on the nucleus of an atom of the element. As we know that the positive charge of nucleus is due to the presence of proton which carries the one positive charge, so the atomic number is equal to the number of protons in the nucleus of atom.

The number of proton in the nucleus is equal to the number of electrons, so we can also say the atomic number is equal to the number of electrons that present on the extra nuclear part of the nucleus. It is denoted as the letter Z. So it can also be represented as Z = number of protons = number of electrons.

The atomic mass is the mass of the atom which present in the nucleus. As we know that the protons and neutrons present in the nucleus, so the atomic mass is equal to the total number of protons and neutrons that are combined called the nucleons.

It is denoted with the letter A. So the A = number of proton + number of neutron or total number of nucleons. So the Atomic Number vs Atomic Mass are related to each other. It is easy to find the number of electrons and protons and neutron by the value of both A and Z. The values of atomic mass can be seen by the Atomic Mass Table.

Now we discuss the Bohrs Model of the Atom. Various models were given for explanation the internal structure of atom. But after the failure of Rutherford model of atom, Bohrs gives his model which is based on the radiation quantum theory. There are some postulates of this model. First is that the electrons are revolved around the nucleus in some circular orbitals. These specific energy orbitals are called energy levels.

These energy levels are denoted as K, L, M, N…etc or 1, 2, 3, 4…etc. Only those revolution are permitted in which electrons are revolved only in those orbitals which has the angular momentum of electrons is whole number multiple of h/2π. So he described the angular momentum of electron as a quantised. In revolution process, electrons neither gain or loss the energy, the energy of electrons remains constant.

This is also called stationary state. So electrons can jump to higher energy level or drop to lower energy level, only when some amount of energy is absorbed or released respectively. He defined the quantisation of energy in terms of a quantity which can change discontinuously to have some specific values.

So this model explains the stability of atoms, helps in calculating the electron’s energy for hydrogen atom, and atomic spectrum of hydrogen atom. But it fails to consider the Heisenberg Uncertainty Principle, the spectra of larger atoms, Zeeman Effect, and fine and hyperfine structure in spectral lines. These are some limitations of this model but it is very successful mode for atoms.


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