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9.8 The Chemistry of Art: 2.The colours and spectra of elements

Syllabus reference (October 2002 version)
2. By the twentieth century, chemists were using a range of technologies to study the spectra, leading to increased understanding about the origins of colours of different elements	Students learn to: identify Na⁺, K⁺, Ca²⁺, Ba²⁺, Sr²⁺, and Cu²⁺ by their flame colour explain the flame colour in terms of electrons releasing energy as they move to a lower energy level explain why excited atoms only emit certain frequencies of radiation distinguish between the terms spectral line, emission spectrum, absorption spectrum and reflectance spectrum describe the development of the Bohr model of the atom from the hydrogen spectra and relate energy levels to electron shells explain what is meant by n, the principal quantum number identify that, as electrons return to lower energy levels, they emit quanta of energy which humans may detect as a specific colour outline the use of infra-red and ultra-violet light in the analysis and identification of pigments and their chemical composition explain the relationship between absorption and reflectance spectra and the effect of infra-red and ultra-violet light on pigments including zinc oxide and those containing copper	Students: perform first-hand investigations to observe the flame colours of Na⁺, K⁺, Ca²⁺, Ba²⁺, Sr²⁺, and Cu²⁺ gather and process information from secondary sources to analyse the emission spectra of sodium and present information by drawing energy level diagrams to represent these spectral lines gather, process and present information about a current analytical technology to: describe the methodology involved assess the importance of the technology in assisting identification of elements in samples and in compounds and provide examples of the technology's use solve problems and use available evidence to discuss the merits and limitations of the Bohr model of the atom

Extract from Chemistry Stage 6 Syllabus (Amended October 2002), © Board of Studies, NSW.

[Edit 9 Jul 09]

Prior Learning: Preliminary module 8.2.3, HSC module 9.4.3.

Background: When an element is vaporized and thermally or electrically excited, it emits light. If dispersed by a prism, the light produces a line spectrum (a series of fine lines of individual colours separated by colourless spaces); the wavelengths at which the coloured lines occur are characteristic of the element. Some elements produce a very intense spectral line (or several closely spaced ones) that serves as a marker for the elements presence. This is the basis of flame tests. Some of the colours of fireworks and flares are due to the emissions from the same elements shown in flame tests.

perform first-hand investigations to observe the flame colours of Na⁺, K⁺, Ca²⁺, Ba²⁺, Sr²⁺, and Cu²⁺

To perform this investigation, plan it carefully so as to minimize hazards and wastage of resources. If you are using dangerous chemicals make sure you protect your skin and eyes, eg wear goggles.
Record your results in a table similar to the one below.
You might decide to repeat the experiment to verify the colours you observed.

identify Na⁺, K⁺, Ca²⁺, Ba²⁺, Sr²⁺, and Cu²⁺ by their flame colour

Name of element Cation Flame Colour

sodium

potassium

calcium

barium

strontium

copper

Na⁺

K⁺

Ca²⁺

Ba²⁺

Sr²⁺

Cu²⁺

yellow

violet

orange - red

apple green

red

green - blue

An example of a method of doing flame tests can be found at The Flame Test Student Worksheet, Imagine the universe, NASA/Goddard Space Flight Center

Name of element	Cation	Flame Colour
sodium potassium calcium barium strontium copper	Na⁺ K⁺ Ca²⁺ Ba²⁺ Sr²⁺ Cu²⁺	yellow violet orange - red apple green red green - blue

explain the flame colour in terms of electrons releasing energy as they move to a lower energy level

A flame colour is produced when a granule of an ionic compound or a drop of its solution is placed into a flame.
An atom does not radiate energy when its electrons are in the fixed orbits.
Electrons can move to other orbits, by absorbing or emitting a photon. A photon is an energy packet of electromagnetic radiation. The photons energy equals the difference in energy between the two stationary states or orbits.
The colour of a flame is created by the strong emission in the line spectrum of the element.
An emission spectrum is produced when electrons of atoms that have been excited to higher energy levels emit photons characteristic of the element as they return to lower energy levels.

Spectroscopy: Light and element identification , Dr Walt Volland, Science Division, Bellevue Community College, Bellevue, Washington, USA.

explain why excited atoms only emit certain frequencies of radiation

When an electron drops from an orbit farther from the nucleus to one closer, it emits a photon of specific energy.
The greater the difference in orbit radius, the greater is the energy of the emitted photon.
The energy of an atom is not continuous but quantised: it exists only in fixed amounts.
The energy packet is called a quantum. A quantum of electromagnetic energy is a photon.
The photon frequencies absorbed or emitted by an atom are fixed by the differences between energy levels of the orbits.

gather and process information from secondary sources to analyse the emission spectra of sodium and present information by drawing energy level diagrams to represent these spectral lines

An emission spectrum, such as that of sodium, is produced when electrons that have been excited to higher energy levels emit photons characteristic of sodium as they return to lower energy levels.

400 nm Sodium emission spectrum 750 nm

Only the two most intense lines are shown here.
In the next part (Part 3 Periodic Table and electron distribution), where you cover energy level diagrams, there is a box explaining:
- the connection between the emission spectrum of sodium and energy level diagrams
- how you can see the emission spectrum of sodium using a yellow street light and a CD-ROM.

Spectra LMP, Centre for Learning Innovation, DET, NSW. You can move the cursor over any element listed to see the emission spectra for that element.

distinguish between the terms spectral line, emission spectrum, absorption spectrum and reflectance spectrum

Spectral line: a gaseous sample of an element is dissociated into atoms and excited by an electric discharge. The emitted light passes through a slit and then a prism or diffraction grating, which disperses the light into individual wavelengths. The individual wavelengths are represented as spectral lines.
Emission spectrum: is produced when atoms that have been excited to higher energy levels emit photons characteristic of the element as they return to lower energy levels. For example in the visible part of the electromagnetic spectrum, the emission spectrum is seen as bright coloured lines that are specific for an individual element.
Absorption spectrum: is produced when atoms absorb photons of certain wavelengths and become excited from lower to higher energy levels. The absorption spectrum appears as black lines against a bright background.
Reflectance spectrum: certain atoms and molecules absorb and reflect energy at wavelengths related to their atomic structures. This takes the form of a reflectance spectrum, with distinctive features that can be used to identify minerals.

Spectral line Absolute Astronomy, AbsoluteAstronomy.com,Seattle, Washington, USA

gather, process and present information about a current analytical technology to:

describe the methodology involved

assess the importance of the technology in assisting identification of elements in samples and in compounds and

provide examples of the technology's use

Gather information by going to the Internet and using a service provider or going to the library and checking encyclopedias or CD ROM encyclopedias. You could either state a particular technology that you want to investigate or could research several technologies and then decide which one you want to choose.
Process the information by evaluating the relevance of the information from the various sources. Make sure that the information you have obtained describes the methodology of the technique.
In choosing a technology you should assess the importance of the technology in assisting identification of elements in samples and in compounds.
The example below has informationon on one technology- laser microspectral analysis.
When a powerful pulsed laser is focused on a surface, a tiny amount of the material is vaporised and through photon absorption, it heats up and ionises. This laser induced plasma is a micro source of light that can be analysed by a spectrometer.
The obtained emission spectra consist of lines corresponding to the elements evaporated from the sample surface. This technique is useful for determining trace elements in solid and liquid samples. It is important as it is a non-destructive technique with high sensitivity and minimal sample preparation.
Laser microspectral analysis is used to analyse the elemental composition of pigments used in restoring paintings. A microscopic amount of a colour of the painting is vaporised using laser energy. Atomic emission spectroscopy is used to identify and measure the amounts of each element. From the analysis a synthetic substitute for this colour can be prepared.
Analysis of the elements present in the pigments can also be used to identify the validity of an art work.

solve problems and use available evidence to discuss the merits and limitations of the Bohr model of the atom

describe the development of the Bohr model of the atom from the hydrogen spectra and relate energy levels to electron shells

Spectroscopists studying the spectrum of hydrogen identified several series of spectral lines in different regions of the electromagnetic spectrum. Each series showed a similar pattern: the spaces between spectral lines became smaller at the short wavelength or high energy end of the series.
Rutherford's model of the atom did not explain the existence of atomic line spectra, as the model implied an electron would spiral smoothly toward the nucleus releasing a continuous spectrum.
Niels Bohr developed a model that explained the observed line spectra.
Bohr proposed that the hydrogen atom had only certain allowable energy levels or stationary states. Each state was associated with a fixed circular orbit (electron shell) of the electron around the nucleus. While in this stationary state the electron does not radiate energy.
Bohr stated that only by emitting or absorbing a photon can an electron move from one electron shell to another. The spectral line relates directly to the photon absorbed or emitted.
An atomic spectrum appears as lines as the atom's energy has only certain discrete levels or states.
Despite its success in accounting for the spectral lines of the hydrogen atom, the Bohr model failed to predict the spectrum of any other element, even helium, the next simplest atom.

explain what is meant by n, the principal quantum number

In Bohr's model, the principal quantum number, n (1, 2, 3, ?) is associated with the radius of the electrons orbit, which is directly related to the electrons energy. The lower the quantum number, the smaller is the radius of the orbit and the lower is the energy of the atom.
When the electron is closest to the nucleus (n=1) the atom is in its lowest energy level or ground state.

identify that, as electrons return to lower energy levels, they emit quanta of energy which humans may detect as a specific colour

An emission spectrum is produced when electrons that have been excited to higher energy levels, emit photons characteristic of the element as they return to lower energy levels.
For example in the visible part of the electromagnetic spectrum, the emission spectrum is seen as bright coloured lines that are specific for an individual element.
Some elements produce a very intense spectral line (or several closely spaced ones) that serves as a marker for the element's presence. This is detected by humans as flame tests where individual elements have a specific colour, as seen in fireworks and flares.

Bohr Atom, showing atomic excitation and atomic de-excitation, University of Tennessee, Knoxville, Tennessee, USA

outline the use of infra-red and ultra-violet light in the analysis and identification of pigments and their chemical composition

Ultraviolet light can reveal changes in elemental composition on the surface of objects because it can cause specific fluorescence in materials depending on the chemical composition and age.
UV reflectance spectroscopy compares reflected radiation from a pigment surface with reflection from a material that does not absorb UV radiation, such as SiO₂.
Infra-red spectroscopy uses the properties of infra-red radiation to go beyond the thin layers of surface colour, and to identify the colours and therefore pigments below. This technique identifies the artist's preliminary drawings.
IR reflectance spectroscopy compares reflected radiation from a pigment surface with reflection from a material that does not absorb IR such as NaCl.

explain the relationship between absorption and reflectance spectra and the effect of infra-red and ultra-violet light on pigments including zinc oxide and those containing copper

Most absorption spectra are measured by passing light through a solution of the substance in a simple spectrometer. Pigments are insoluble solids so the spectrometer used to study absorption spectra of these solids is more complicated.
Colour that is seen is not absorbed. An ultra-violet spectrometer measures ultra-violet and visible light absorption versus wavelength, producing an absorption spectrum.
The interesting features of an absorption spectrum include: the wavelength of the radiation absorbed (colour not seen), the intensity of the absorption (relates to the quantity of pigment) and the shape of the absorption band (relates to shade and purity of pigment).
A reflectance spectra is useful when the substance cannot be dissolved in a colourless solvent. In this case light is shone onto the surface pigment and the spectrum of light not absorbed, but reflected, is measured.
Both ultra-violet and infra-red reflectance spectroscopy are also used to investigate changes in composition, such as sketches or painted-over attempts made by the artist. These techniques are useful for identifying pigments containing copper and zinc oxide as zinc oxide fluoresces in ultra-violet light, and changes from white to yellow in infra-red light.