Bohr's Model of an Atom

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Rutherford's model had a major drawback. It could not explain why ultimately electrons did not fall into the nucleus by taking a spiral path. This was in concurrence with the electromagnetic theory that states 'if a charged particle undergoes accelerated motion then it must radiate energy (lose) continuously'.

In order to explain the stability of an atom Neils Bohr in 1913 gave a new arrangement of electrons in the atom. According to Neils Bohr, the electrons could revolve around the nucleus in only 'certain orbits' (energy levels), each orbit having a different radius. When an electron is revolving in a particular orbit or particular energy level around the nucleus, the electron does not radiate energy (lose energy) even though it has accelerated motion around the nucleus.

Arrangement of energy levels around the nucleus

Bohr's concept of atom

• An atom is made up of three particles, electrons protons and neutrons. Electrons have a negative charge and protons have a positive charge whereas neutrons have no charge. They are neutral. Due to the presence of equal number of negative electrons and positive protons the atom as a whole is electrically neutral.

• The protons and electrons are located in a small nucleus at the centre of the atom. Due to the presence of protons the nucleus is positively charged.

• The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The 'energy levels' or 'shells' or 'orbits' are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by letters K, L, M, N, O and P. The energy levels are counted from centre outwards.

• Each energy level is associated with a fixed amount of energy. The shell nearest to the nucleus having minimum energy and the shell farthest from the nucleus having maximum energy.

• There is no change in the energy of electrons as long as they keep revolving in the same energy level. But when an electron jumps from a lower energy level to a higher one, some energy is absorbed while some energy is emitted. When an electron jumps from a higher energy level to a lower one, the amount of energy absorbed or emitted is given by the difference of energies associated with the two levels. Thus if an electron jumps from orbit 1 (energy E₁) to orbit 2 (energy E₂) the change in energy is given by E₂ - E₁. The energy change is accompanied by absorption of radiation energy of E= E₂ E₁ = h where h is a constant called 'Planck's constant' and is the frequency of radiation absorbed or emitted. The value of h is 6.626 x 10^-34 J^-s. The absorption and emission of light due to electron jumps are measured by use of spectrometers.

This model of the atom was able to explain the stability of the atom. It also explained the phenomenon of atomic spectra and ionization of gases.