Inorganic Chemistry: Atomic Spectra

Understanding Atomic Spectra

Atomic spectra refer to the spectrum of frequencies of electromagnetic radiation emitted or absorbed during transitions of electrons between energy levels within an atom.
Absorption spectra occur when energy is taken in by the atom and electrons are excited to a higher energy level.
Emission spectra occur when an excited atom returns to its ground state or a lower energy state, emitting energy.

The unique line spectrum produced by each element is due to the distinct energy levels within an atom.
These energy levels are fixed and can be likened to steps in a stairway where electrons can only exist on a step, not in between.
The difference in energy between two levels corresponds with the energy of the light absorbed or emitted.

The Bohr model of the atom quantises the electron’s energy levels.
When an electron absorbs energy, it gets ‘excited’ and moves to a higher energy level, further from the nucleus.
When it ‘de-excites’ or returns to its ground state, it emits energy in the form of light. The colour of the light relates to the energy difference of the levels.
The frequency (f) or energy (E) of the light absorbed or emitted is related to the energy difference between levels. The higher the energy difference, the higher the frequency and the shorter the wavelength of the light emitted.
Energy is related to frequency according to the equation E = hf, where h is Planck’s constant.

A continuous spectrum shows all possible colours or frequencies of light without any gaps.
A line spectrum shows only certain colours or frequencies related to particular atomic transitions. It looks like individual lines of colour against a dark background when seen through a spectroscope.
The sun or a light bulb emits a continuous spectrum, while a heated gas emits a line spectrum.

Spectroscopy, the study of atomic spectra, is vital in verifying atomic theory and studying chemical composition of samples.
It is used in astronomy to determine the chemical composition of stars and other celestial bodies by studying their emission spectra.
It’s also useful in forensic science, art, environmental monitoring amongst others to identify and quantify the elements and compounds present in a sample.