Bohr's Theory and Spectral Mechanisms

Niels Bohr

In 1913, the Danish scientist Niels Bohr put forward his theory of energy levels in an Atom. He based his theory on the analysis of emission spectra he obtained when he excited hydrogen atoms in a discharge tube.

Main points of Bohr's theory

1. Electrons can only occupy certain areas within the atom, called energy levels.

2. The ground state of an electron is when it occupies the lowest energy level available to it.
3. When an electron absorbs energy, it jumps from a lower energy level to a higher energy level. The electron is now said to be excited.
4. However, the electrons are less stable in higher energy levels and tend to drop back down again to lower levels.
5. When an electron drops from a higher to a lower level a definite amount of energy is released in the form of a light of a particular frequency.
6. The definite amount of energy released when an electron drops from a higher to a lower level is equal to the difference in energy between the two energy levels this energy difference is given by the equation:

$$E2 – E1 = hf$$

Where:

• $E2$ = energy value of higher level
• $E1$ = energy value of lower level
• $H$ = Planck's constant
• $F$ = the frequency of light emitted

7. The energy levels are given a number called the principal quantum number the lowest energy level is numbered N = 1.

Spectroscopic evidence to support Bohr's theory

When negation elements such as hydrogen are electrically excited in a discharge tube the electrons of the hydrogen atoms absorb the energy and then give off energy in the form of light.

When this life was examined, a line spectrum was obtained.

Everyday uses of the emission spectrum

• Fireworks: salts of strontium and barium are used in firework displays.
• Lasers make use of light released by electronic transitions.

Atomic absorption Spectra

The atomic absorption spectrum is the spectrum that is obtained when light is passed through a gaseous sample of an element.

An atomic absorption spectrometer is used to analyse samples of water or blood for heavy metals such as lead or Mercury.

• The main principle of atomic absorption spectrometry is that atoms of an element absorb light of a certain frequency which is particular to that element.
• The amount of light absorbed is directly proportional to the concentration of that element.
• During the process, light which is characteristic of the element is passed through an atomised sample of the element in the spectrometer.
• The amount of light absorbed by the sample is measured by the spectrometer and this information can be used to estimate the concentration of that element present.

Modern Improvements in Bohr's theory

Bohr's model of the Atom where he envisaged electrons as particles revolving around the nucleus was developed and modified by scientists in the 1920s and 1930s.

In 1923 the French scientist Louis de Broglie proposed that electrons had wave properties, and this was shown experimentally to be correct.

• Electrons were now set to have both a particle and wave nature.

• If an electron has a wave nature, then it could not have a definite path about the nucleus as Bohr had proposed. The main difference between Bohr's theory and the modern theories are:

• Bohr thought the electron had particle form only, but it has now been shown to have a particle and a wave nature.

• Bohr proposed that the electron had definite orbits about the nucleus. However, the modern theory uses orbitals to describe the probable location of an electron.