Atomic Spectra

Core Concepts

In this tutorial, you will learn about atomic spectra, including the meaning behind this term, why spectra appear the way they do, and applications of this unique phenomena.

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What is an Atomic Spectra?

Atomic spectra or atomic emission spectra are plots that show the unique wavelengths of light which each element emits due to an electron transitioning from a higher energy state to a lower energy state in an atom. Every element on the periodic table contains a different number of electrons in its neutral state. Hence, each element has different gaps in energy between its highest occupied electronic energy level and subsequent higher energy levels in unoccupied orbitals. Once this highest energy valence electron is excited to a higher electronic energy level (excited state) via a photon, its relaxation back to ground state results in a photon being emitted. These photon’s wavelengths (which correspond to a specific energy) are what comprise an atomic spectra. You can view the characteristic atomic spectra for multiple elements from Boston University here.

Only the highest energy electron can be excited to an excited state, due to its intrinsic instability. Therefore, atomic spectra are the result of energy changes from valence electrons exclusively.

Quantization of Energy States

The reason only specific, discrete, wavelengths of light are emitted upon relaxation of an electron is due to the inherent quantization of energy levels within an atom. Quantized energy levels means that each energy level an electron can be excited to is a discrete value. Just as a ladder contains multiple rungs, and it is impossible to have a height between two rungs when climbing, electrons can only be stable in higher atomic orbital energies or the ground state, but never in between. Because the energy levels are discreet, the differences between energy levels also take on discreet values, which result in sharp emission lines at specific wavelengths or frequencies.

Visual Representation of Electronic Energy Levels

Upon excitation with a photon, the electron could jump to the next unoccupied orbital, or a higher energy orbital. This is completely dependent upon the energy of the photon which interacts with the electron. The multiple lines on the spectrum demonstrate the various differences in energy between the ground electronic state and higher energy excited states.

Hydrogen Spectra.jpg
The atomic emission spectrum of Hydrogen

Atomic Absorption Spectra

Just as every element has a unique emission spectrum, similarly each element also has a unique absorption spectrum. This is a measurement of what energies (or wavelengths) the atom absorbs based on different electronic energy levels.

Hydrogen Absorption Spectrum

The technique of atomioc absorption spectroscopy (AAS) uses this property to identify elements in samples. AAS is commonly used to analyze food and pharmaceutical samples.

Equation for Photon Energy

The difference between the electron’s ground state energy and the energy of the excited state is equivalent to the energy of the emitted photon. The equation below calculates the energy.

    \begin{gather*} { E=hc/\lambda } \end{gather*}

    \begin{gather*}  \par E=Energy \\ {h=Planck's} {constant} \\ c=Speed of Light \\ \lambda=Wavelength \end{gather*}

Generally, photon energy is expressed in electron-volt(eV), which requires you to divide the energy in Joules by the elementary charge of an electron(1.6E-19).

Applications of Atomic Spectra

Atomic spectra are important for a variety of analytical techniques. Techniques including atomic emission spectroscopy (AES) and atomic absorption spectroscopy (AAS) rely on this phenomena specifically. Sample characterization or quantitative analysis can be performed using these techniques, even with extremely small sample volumes. For example, some elements can be detected in parts per billion (ppb) or even parts per trillion (ppt). This characteristic makes these instruments especially useful in environmental, biological, industrial and forensic applications.