7.1.3 Emission & Absorption Spectrum

Emission & Absorption Spectrum

Line spectra is a phenomenon that occurs when excited atoms emit light of certain wavelengths which correspond to different colours
This comes from differences in discrete energy levels when electrons move between energy levels within an atom
The emitted light can be observed as a series of coloured lines with dark spaces in between
- These series of coloured lines are called line or atomic spectra

Each element produces a unique set of spectral lines
No two elements emit the same set of spectral lines, therefore, elements can be identified by their line spectrum
There are two types of line spectra: emission spectra and absorption spectra

When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
Each transition corresponds to a different wavelength of light and this corresponds to a line in the spectrum
The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background
Each emitted photon has a wavelength which is associated with a discrete change in energy, according to the equation:

2.5.2 Difference in Energy Levels Equation

Therefore, this is evidence to show that electrons in atoms can only transition between discrete energy levels

hydrogen-emission-spectra

Emission spectrum of Hydrogen gas

An atom can be raised to an excited state by the absorption of a photon
When white light passes through a cool, low pressure gas it is found that light of certain wavelengths are missing
- This type of spectrum is called an absorption spectrum

An absorption spectrum consists of a continuous spectrum containing all the colours with dark lines at certain wavelengths
These dark lines correspond exactly to the differences in energy levels in an atom
When these electrons return to lower levels, the photons are emitted in all directions, rather than in the original direction of the white light
- Therefore, some wavelengths appear to be missing

The wavelengths missing from an absorption spectrum are the same as their corresponding emission spectra of the same element

hydrogen-absorption-spectra

Absorption spectrum of Hydrogen gas

A larger version of the hydrogen spectrum from the infrared to the ultraviolet region looks like this

The full hydrogen spectrum

In the spectrum, we can see sets or families of lines
Each element will have several series with electrons able to jump between specific energy levels producing specific energy photons
The Lyman series converges on the ground state for electrons
- The Balmer series converges on the second energy level n = 2
- The Ritz - Paschen converges on the third energy level n = 3 and so on

The Lyman series photons will have the most energy since they have the shortest wavelength
The Pfund series photons will have the least energy since they have the longest wavelength

7-1-3-electron-jumps-in-the-hydrogen-spectrum

Electron jumps in the hydrogen spectrum

The discovery of these electron jumps helped scientists to understand how the movement of electrons is able to produce photons of specific wavelength and energy