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

 1.2 Basic ideas about atoms

Emission and absorption spectra Light and electromagnetic radiation Light is a form of electromagnetic radiation. Electromagnetic radiation is energy travelling as waves. A wave is described by its frequency ( f ) and its wavelength ( λ ). The frequency and wavelength of light are related by the equation: c = f λ  ( c is the speed of light) The frequency of electromagnetic radiation and energy ( E ) are connected by the equation: E = hf  ( h is Planck’s constant) Therefore, f ∝ E , and if frequency increases, energy increases. f ∝ 1/ λ and if frequency increases, wavelength decreases. The whole range of frequencies of electromagnetic radiation is called the electromagnetic spectrum.

Exam tip Since f ∝ E and f ∝ 1/ λ , the lower the wavelength, the higher the frequency and the greater the energy.

Study point

Wavelength (µm)

Light is electromagnetic radiation in the range of wavelength corresponding to the visible region of the electromagnetic spectrum.

10 −6 10 −5 10 −4 10 −3 10 −2 10 −1 10 0

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Study point Wavelength is the distance over which the wave’s shape repeats. Frequency (in Hz) is the number of times the wave is repeated in one second.

Gamma ray

X-rays Ultraviolet

Infrared

Microwaves Radio waves

Visible spectum

Violet Ultraviolet

Blue

Green Yellow

Red Infrared

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Wavelength (nm)

Knowledge check Two lines in the emission spectrum of atomic hydrogen have the following frequencies 4.6 × 10 14 Hz and 6.9 × 10 14 Hz. State which one has the higher (a) energy (b) wavelength. 11

▲ The electromagnetic spectrum

In this unit we are only concerned with the infrared, visible and ultraviolet regions. It is important to note that both energy and frequency increase going from the infrared through visible to the ultraviolet region. Thus, blue light is of a higher energy than red light. As frequency gets larger and therefore wavelength decreases, blue light must have a shorter wavelength than red light. Absorption spectra Light of all visible wavelengths is called white light. All atoms and molecules absorb light of certain wavelengths. Therefore, when white light is passed through the vapour of an element, certain wavelengths will be absorbed by the atoms and removed from the light. Looking through a spectrometer, black lines appear in the spectrum where light of some wavelengths has been absorbed. The wavelengths of these lines correspond to the energy taken in by the atoms to promote electrons from lower to higher energy levels. 27 DRAFT Increasing wavelength