Atomic Spectra: Emission and Absorption

Emission and Absorption Spectra of the Hydrogen Atom

Emission Spectrum

The emission spectrum of hydrogen is produced when electrons in the atom release energy as they move from higher to lower energy levels. In the case of hydrogen, when an electron falls from a higher energy level to a lower one, it emits energy in the form of light. The emitted light forms distinct lines at specific wavelengths, showing the energy levels of the atom.

Balmer Series

This series is part of the hydrogen emission spectrum, where electrons fall from higher energy levels (n > 2) to the second energy level (n = 2).

The lines produced in the Balmer series are in the visible light spectrum, and include:

• H-alpha (n = 3 to n = 2): Red light, 656 nm.
• H-beta (n = 4 to n = 2): Blue-green light, 486 nm.
• H-gamma (n = 5 to n = 2): Violet light, 434 nm. These lines provide evidence that the energy levels in an atom are quantised—electrons can only occupy specific energy levels, and the energy difference between these levels corresponds to the wavelength of light emitted.

Absorption Spectrum

The absorption spectrum of hydrogen is produced when electrons absorb energy and move from lower to higher energy levels.

In this process, specific wavelengths of light are absorbed, which correspond to the energy required to move an electron from one energy level to another.

In the absorption spectrum, dark lines appear in the continuous spectrum at exactly the same wavelengths as the bright lines in the emission spectrum. This is because those specific wavelengths of light are absorbed by the hydrogen atom to excite electrons to higher energy levels.

For example, in the Balmer series, when hydrogen gas is exposed to white light, the electrons in hydrogen absorb specific wavelengths and move from the second energy level (n = 2) to higher levels (n > 2). The same wavelengths of light that appear as bright lines in the emission spectrum appear as dark lines in the absorption spectrum.

Atomic Absorption Spectrometry (AAS)

Atomic Absorption Spectrometry (AAS) is an analytical technique used to determine the concentration of specific metal elements in a sample.

It is based on the principle that atoms in the ground state can absorb light of a specific wavelength characteristic of that element. The greater the concentration of the element present, the more light is absorbed.

How AAS Works

1. Sample Preparation: The sample containing the metal is atomised, typically by heating it in a flame or an electrically heated graphite furnace.
2. Light Source: A hollow cathode lamp, made of the element being analysed, produces light of the correct wavelength for that element.
3. Absorption: The atomised sample absorbs some of this light as atoms are excited from lower to higher energy levels.
4. Detection: The decrease in light intensity is measured. The amount absorbed is directly proportional to the concentration of the element in the sample.

Application of AAS

• Detection of Metals: AAS is widely used for detecting metal ions, such as sodium, potassium, magnesium, and calcium, in various samples, including water, food, and biological fluids.
• Medical Use: Measuring levels of essential metals such as calcium, magnesium, sodium, and potassium in blood or urine samples

Sodium Street Lights

• Sodium vapour lamps produce the yellow-orange glow seen in street lighting. The light is generated by exciting sodium atoms in the gas phase, which emit light at a characteristic wavelength (around 589 nm).
• In an AAS setup, sodium atoms can also absorb light at this wavelength. This principle of specific light absorption is how AAS detects sodium and other elements.

Fireworks

• The bright colours in fireworks are produced by metal salts such as sodium, strontium, and copper. When heated in the fireworks explosion, these metals become excited and emit light at specific wavelengths.
• Sodium salts, for example, produce a yellow flame, while strontium salts emit red, and copper salts give off blue. These emissions are based on the excitation and subsequent de-excitation of electrons in the metal atoms, a process related to the same principles of light absorption and emission that AAS relies on.