Ionisation Potential of a Hydrogen Atom

Hydrogen spectral series:

There are mainly five series and each one is named after its discoverer. They are as follows:

• The Lyman series: The series of lines emitted when an electron moves from the quantum number of n>1 to the first orbit of quantum number n’=1. Lies in the ultraviolet area.
• The Balmer series: This consists of the lines caused by the emission from the electron movement from n > 2 to the orbit n’ = 2.
• The Paschen series: All the lines of this series lie in the infrared band. It overlaps with the Brackett series.
• The Pfund series: This series too lies in the infrared band.
• The Brackett series: These spectral lines lie within the band of infrared wavelengths.

The wavelengths of the radiations emitted can be determined by a formula given by Rydberg. The Rydberg formula is as follows:

Where n1 is the principal quantum number of lower energy level, n2 is the principal quantum number of lower energy level and is the wavelength of the spectral line.

The wavelength of the first line is maximum, and the first line is called the first member. The wavelength of the last line is minimum, and this line is called the series limit.

Ionisation potential of a hydrogen atom

The ionisation potential of a hydrogen atom is the amount of energy required to remove its electron from its shell to form a cation. It is expressed in electron volts. The ionisation potential of a hydrogen atom is only the first ionisation since there is only one electron to be removed. However, with other elements, there may be more electrons so the removal of first, second, and third electrons will be first, second, and third ionisation. 

The ionisation potential of a hydrogen atom is 13.6eV.

Factors that influence ionisation potential

• Nuclear charge: The relationship between the ionisation potential and the effective charge of an atom is directly proportional. The greater the charge, the greater the ionisation potential. This is because if the nuclear charge of an atom is greater than the force of attraction binding, the electrons to the nucleus will be greater. So it will be more difficult to free the electron from the atom.
• Shielding or screening effect: The shielding or screening effect is caused by the electrons that lie between the nucleus and the outermost shell. This effect increases with an increase in the number of electrons that lie between the nucleus and the outermost shell. As the shielding effect increases, the force of attraction between the nucleus and the electron in the outermost shell decreases making it easy to free the electron with less energy. This means that as the shielding effect increases, the ionisation potential decreases. 
• Atomic radius: As the atomic radius of an atom increases, the ionisation potential decreases. This is simply because the greater the distance between the nucleus and the outermost electron, the lesser the force of attraction between the two. And the lesser the attraction, the lesser the ionisation potential.
• Penetration effect: As the distance from the nucleus increases, the energy required to remove an electron from its shell decreases. This is because the forces of attraction between the nucleus and the electron decrease. So the order for the value of the ionisation potential for s, p, d, and f orbitals is s>p>d>f. This is followed by the inference that the penetration power of 2s and 3s electrons is more than 2p and 3p electrons. So it will require more energy to remove 2s and 3s electrons.
• Stability of half or fully filled orbitals: Hund’s rule states that the energy levels of half-filled and completely filled orbitals are high. Hence, according to this rule, the ionisation potential of half-filled or completely filled orbitals is high.

Conclusion

The ionisation potential of a hydrogen atom is quite high because the size of the atom is small, there are no electrons shielding the electron from the nucleus so the force of attraction between the two is high as well. Ionisation potential determines the metallic character of an element. If the ionisation potential of an element is low, it means that it is electropositive in nature. So in the periodic table, the ionisation potential increases on going from top to the bottom of a group.