Emission Spectra

Student Expectation

The student is expected to compare and explain the emission spectra produced by various atoms.

Key Concepts

• • A hot gas under low pressures will emit light of specific colors. The light can be seen in a spectroscope as bright lines.

  • The positions of the bright lines emitted by each element are unique and well defined, and thus can be used to identify and analyze the gaseous element.

  • Electrons in an element can have only certain discrete levels of energy.The emission spectrum of an element depends upon the configuration of its electrons.

  • When energy is added to a gaseous element, the electrons move to a higher state of energy. After a period of time the electrons emit the extra energy as light and fall back to their initial ground state.

EMISSION SPECTRA

Emission Spectra are Unique

The emission spectra are composed of electromagnetic radiation. They can be emitted by elements’ atoms. The positions of the bright lines emitted by each element are unique and well defined. The property of uniqueness can thus be used to identify and analyze different elements, such as a gaseous element. For example, a hot gas under low pressure will emit light of specific colors. This light can be seen in a spectroscope as bright lines.

Causes of Emission Spectra

What is the reason that a gaseous element can emit spectra? When energy is added to a gaseous element, the electrons move to a higher state of energy. This higher energy level is not stable. The electrons tend to stay at the stable energy level. After a period of time, the electrons emit the extra energy as light and fall back to their initial ground state. This initial ground state is more stable than the higher energy level. During the process of electrons jumping from these levels, the bright lines are emitted.

Do the lines of the spectra have the same position every time? If an electron is under the same conditions, the position will be the same. Electrons as an element can only have certain discrete levels of energy. The possible jumps for the electrons between energy levels are limited, not infinitely possible. Although emissions could occur in each atom and there may be billions of atoms in an object during an experiment, those electrons can only jump between certain energy levels, which can be expressed as ΔE. This equation shows the emission spectra are the same:

Formula for the energy of an emitted electron.

Emission Spectra Depend on Electron Configurations

In addition, the emission spectra of an element depends upon the configuration of its electrons. Different configurations or structures of the electrons will result in different spectra. The range of different waves that can be released when electrons fall from a higher energy level to a lower energy level in an atom is called the emission spectrum for that atom. Consider millions of hydrogen atoms in a region. A scientist could apply a high voltage to the electrons and excite them to different energy levels in different atoms. In this case, there would be a specific set of light waves that would be emitted by these atoms as electrons eventually fall to their ground state. A scientist can use a spectroscope to observe these waves.

If the configurations of electrons change, the energy level changes with it. Electrons in an element can have only certain discrete levels of energy. We know that:

Formula for the speed of light.

So we can conclude:

Formula for the energy of an emitted electron.

Different Configurations Produce Different Wavelengths

This equation tells us the emission energy is based on the emission spectrum, which only happens at certain discrete frequencies or wavelengths. Different configurations of electrons will produce different frequencies or wavelengths in emission.