WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

WJEC Chemistry for AS Level

Emission spectra When atoms are given energy by heating or by an electrical field, electrons are excited and the additional energy promotes them from a lower energy level to a higher one. When the source of energy is removed and the electrons leave the excited state, they fall from the higher energy level to a lower energy level and the energy lost is released as a photon (a quantum of light energy) with a specific frequency. The observed spectrum consists of a number of coloured lines on a black background.

Study point A spectrometer is an instrument that separates light into its constituent wavelengths.

Wavelength

The fact that only certain colours appear in an atom’s emission spectrum means that only photons having certain energies are emitted by the atom. If the electron energy levels were not quantised but could have any value, a continuous spectrum rather than a line spectrum would result.

12 Knowledge check Give two differences between absorption and emission spectra.

The hydrogen spectrum An atom of hydrogen has only one electron so it gives the simplest emission spectrum. The atomic spectrum of hydrogen consists of separate series of lines mainly in the ultraviolet, visible and infrared regions of the electromagnetic spectrum. There are six series, each named after their discoverer. Only one series, the Balmer series, is in the visible region of the spectrum.

13 Knowledge check Which letter represents the transition that causes a line of the lowest frequency in the emission spectrum of atomic hydrogen? A Second energy level to first energy level. B Third energy level to first energy level. C Third energy level to second energy level. D Fourth energy level to second energy level.

Paschen

Balmer

Lyman

∞ λ /nm

700

400

200

100

Infrared

Visible

Ultraviolet

01.02.18 AS Chemistry Eduqas Barking Dog Art DRAFT ▲ Diagram showing part of the emission spectrum of atomic hydrogen. When an atom is excited by absorbing energy, an electron jumps up to a higher energy level. As the electron falls back down to a lower energy level it emits energy in the form of electromagnetic radiation. The emitted energy can be seen as a line in the spectrum because the energy of the emitted radiation is equal to the difference between the two energy levels, Δ E , in this electronic transition, i.e. it is a fixed quantity or quantum. Since Δ E = hf , electronic transitions between different energy levels result in emission of radiation of different frequencies and therefore produce different lines in the spectrum. Study point Electronic transition is when an electron moves from one energy level to another. 28