Atomic Emission Spectroscopy & Flame Test
This is part of preliminary HSC Chemistry course under the topic of Atomic Structure and Atomic Mass.
HSC Chemistry Syllabus
- Model the atom's discrete energy levels, including electronic configuration and SPDF notation (ACSCH017, ACSCH018, ACSCH020, ACSCH021)
- Investigate energy levels in atoms and ions through:
Bohr Model - Flame Test, AES, Electron Configuration
Bohr's Model of the Atom
Bohr's model of the atom builds on the contribution by Rutherford. In this model, Bohr postulated:
1. Electrons revolve around the nucleus in circular orbits with discrete radii and quantised energies. In these orbits, electrons exist in 'stationary states' and do not emit energy.
Bohr’s model of the atom describes electrons orbiting in stable energy levels as opposed to Rutherford's model in which electrons' motion was not described.
2. An electron can transition between orbits by absorbing or releasing energy that is exactly equal to the difference in energy of orbits, consistent with the law of conservation of energy.
Electron excitation occurs when an electron absorbs energy to move to an orbit of higher energy.
Electron relaxation occurs when an electron moves to a lower orbit, releasing energy in the form of electromagnetic radiation (photon).
If the photon's energy is less or greater than the difference in energy of orbits involved in the electron transition, no transition will occur. This interaction between electrons and energy is referred to as spectroscopy.
When electrons absorb energy to move to orbits of higher energies, absorption spectroscopy occurs. Conversely, when electrons release energy while moving to orbits of lower energies, emission spectroscopy occurs.
Atomic Emission Spectroscopy (AES)
Atomic Emission Spectroscopy (AES) is a method of chemical analysis that identifies elements within a sample by examining the intensity of light emitted from a flame at a specific wavelength. The wavelengths found in the atomic emission spectra help determine the element's identity. Notably, the intensity of the lines generated by the emitted light is proportional to the quantity of the element's atoms present in the sample.
Emission spectra are typically produced when a low-pressure gas's atoms are heated or otherwise excited, such as by a strong electric field.
When a granule of an ionic compound or a droplet of its solution is placed in a non-luminous flame, the electrons within absorb the flame's heat energy. This absorption causes the electrons to jump to higher energy levels - the energy absorbed must match the discrete energy difference between the orbits.
When an electron transitions from a higher excited state to a lower one, it must emit energy corresponding to the energy difference between the two orbits. Here, higher energy photons correspond to a higher frequency (lower wavelength, i.e., the violet end of the visible spectrum). The larger the energy difference between the two shells, the greater the energy of the photon, and therefore, the higher its frequency.
Each element possesses a unique set of possible energy transitions. As a result, each element has a unique emissions spectrum, which becomes visible when electrons release radiation in the visible light spectrum as they transition back to their ground state after excitation. The overall colour is determined by the wavelengths of the emitted photons, which correspond to their colour.
An emission spectrum is a pattern of coloured lines superimposed on a dark background, with each line representing radiation released by an excited electron that has returned to a lower energy level, or its ground state.
The emission spectrum of some elements are shown below.
Hydrogen Spectrum and Bohr's Model of the Atom
The hydrogen emission spectrum is the collection of visible light lines produced by a sample of hydrogen gas when it is excited e.g. heated. This spectrum is one of the earliest evidence for Bohr's model of the atom.
Emission spectrum of heated gas. Credit: libretext
The emission spectrum of hydrogen consists of specific visible light lines on a black background. It is produced when light from a heated sample of hydrogen gas undergoes dispersion through a glass prism. This can also be produced using other gases and discharge tubes.
When electrons in hydrogen atoms absorb energy (e.g. heat), they can transition to orbits of higher energy. These excited electrons will emit energy in the form of electromagnetic radiation (photons) when moving to orbits of lower energies.
Bohr explained that the emission spectral lines are the visible light photons released when electrons return to orbits of lower energies. The emission lines have specific wavelengths as they correspond to the specific amount of energy associated with each photon emitted.
There are multiple emission lines for hydrogen (and other elements) because an Bohr-model atom consists of orbits of many energy levels, which in turn allows for electron transitions that absorb/release different amounts of energy.
Spectral lines observed in the Balmer Series or hydrogen spectrum (part of) are a result of electronic transition from higher energy levels to n = 2.
Electromagnetic radiation that is absorbed/released during electron transitions it not limited to visible light. For example, in a hydrogen atom, electron transitions to n = 1 orbit will produce radiation in the ultraviolet spectrum (Lyman series). Electron transitions to n = 3 orbit will produce radiation in the infrared spectrum (Paschen series).
Flame Test
The flame test is an application of atomic emission spectroscopy. When a sample of metal or metal ion is heated with a Bunsen burner, electrons in the atom or ion are excited to a higher orbit. When these electrons transition back to their ground state (original orbit), they release energy in the form of electromagnetic radiation. When this radiation is in the visible light spectrum, it gives the flame a particular colour.
The flame test is a simple approach to identify metals or metal ions. In this process, a sample of the solution is introduced to the flame of a Bunsen burner. However, it's important to note that not all metal ions produce discernible colours when subjected to a flame test; only metals whose electrons release energy in the form of visible light can be identified using the flame test.
The following table shows some common metals and their flame test colours.
| Metal | Flame Colour |
| Calcium | Brick red |
| Barium | Apple green |
| Strontium | Scarlet |
| Lithium | Crimson |
| Sodium | Orange yellow |
| Copper | Blue green |
| Potassium | Lilac |