Electron Energy Levels

Electrons in an atom exist in energy levels around the nucleus. According to the Bohr theory, it can be explained that there are energy levels around the nucleus. These energy levels are numbered as 1, 2, 3 … or K, L, M, N, … from the nucleus.

These energy levels are called principal quantum numbers (n) or principal energy levels. For each principal energy level, there are sublevels also known as subshells. These subshells are named s, p, d, f, g, … Each subshell consists of orbitals. If the principal quantum number is "n", the number of the orbitals present in the energy level is given by n².

Hydrogen emission spectrum

A hydrogen gas sample under low pressure is electrically energized. The emission spectrum of hydrogen is obtained by passing the resulting beam through a prism and diffracting it according to the frequency. This is known as the emission spectrum of hydrogen.

This spectrum was invented in 1885 by a Swiss scientist Johnathan Balmer. Therefore, this spectrum is also known as the Balmer series. There are four lines in this series and all the lines present in the visible range of the electromagnetic spectrum.

1. Hα (Red)
2. Hβ (Green)
3. Hγ (Blue)
4. Hδ (Violet)

The distance between two lines decreases in the direction of increase in frequency.

Bohr explanation of the Hydrogen emission spectrum

In 1913 Danish physicist Neil Bohr suggested a theory to explain the Hydrogen emission spectrum.

When Hydrogen gas (H₂) under low pressure is supplied energy, the bond between the two atoms of the H₂ molecule is broken and forms hydrogen atoms. There is only one electron in the hydrogen atom. This electron exists in the lowest energy level (n = 1 principal quantum number) in the hydrogen atom.

This state is called the ground state. When the energy is supplied, the electron in the ground state will absorb the energy and transit to a higher energy level. This state is called the "excited state". The excited state is unstable. Therefore, the electrons will emit the absorbed energy, and transit to the lower energy levels.

The Lyman series is obtained due to the radiation emitted by the electron transition between the higher energy level to the lowest energy level (n = 1). The lowest frequency line in the Lyman series is obtained due to the electron transition between n = 2 to n = 1.

The highest frequency line is obtained due to the electron transition between the highest energy level to the n=1 level. Lyman series is in the UV range of the electromagnetic spectrum.

When the electrons transit from the higher energy levels to the n = 2 level, the Balmer series is obtained. The lowest frequency level of the Balmer series is Hα line and it is red in color. This is obtained when the electrons transit from the n = 3 to n = 2 energy level.

When the electrons transit from n = 4 to n = 2, the Hβ line is obtained and the emitted radiation is green in color. When the electrons transit from n = 5 to n = 2, the Hγ frequency line is obtained and it is blue in color. The highest frequency line of the Balmer series is Hδ line. This is obtained when the electrons transit from n = 6 to n = 2 energy level. Hδ line is violet in color.

There are the Paschen series, Brackett series, and Pfund series in the IR range of the electromagnetic spectrum. Those series are obtained when the electrons transit from the higher energy levels to n = 3, n = 4, and n = 5 respectively.

When the transition electrons from higher energy levels to lower energy levels they always emit a specific frequency of radiation. That explains why there are energy levels in an atom with a specific energy. Also, the energy gap between the two lines is decreasing in the direction that increases the energy. This explains that the energy gap between two energy levels is decreasing in the direction away from the nucleus.

Bohr's theory suggests the following information on an atom.

1. There are energy levels around the nucleus. And those energy levels have a unique energy.
2. The energy of an energy level is increasing in the direction away from the nucleus.
3. The energy gap between two energy levels is decreasing in the direction away from the nucleus.
4. Electrons with low energy exist in low energy levels and electrons with high energy exist in higher energy levels. Also, electrons can transit to any energy level.
5. Electrons do not emit radiation as they rotate around the nucleus.

Failures of the Bohr model

Although the Bohr theory can explain the emission spectrum of hydrogen, it cannot be used to explain the spectrum of the elements with multiple electrons. The emission spectra of other elements are different from each other.

According to the Bohr explanation, electrons travel in a circular path. If it does the atom has to be flat. But that is not true.

The intensity of some frequency lines of the hydrogen emission spectrum is low. This also cannot be explained by Bohr's theory.