Archbishop McGrath Catholic High School Ysgol Uwchradd Gatholig Archesgob McGrath
The interaction of electromagnetic radiation and matter
Name ……………………………………………………………………………………..
1 The interaction of light and matter Most of our understanding of the electronic structure of atoms has come from the area of science known as spectroscopy — the study of how light and matter interact. To understand this we must understand the nature of light. Chemists use two models to describe the behaviour of light - the •
wave model and the
•
particle model.
Neither theory fully explains all the properties of light. Some are best described by the wave model: the particle model is better for others. We choose the theory which is most appropriate to the situation. Usually this depends on the dimensions – •
for large scale, the particle theory is better
•
for very short distances, the wave model is more appropriate
When we use the term 'light' we normally mean the visible light to which our eyes respond. But visible light is only a small part of the electromagnetic spectrum, which includes all the different forms of electromagnetic radiation. There are other regions, for example: radio waves, ultra-
γ
violet, infra-red and -rays as shown on the right.
Complete the following table, noting the wavelengths of different colours of light Colour red orange yellow blue green indigo violet
Wavelength (nm)
2 Different ideas about the true nature of light The wave theory of light Light is one form of electromagnetic radiation. Like all electromagnetic radiation, it behaves like a wave, with a characteristic wavelength and frequency. Amplitude, Wavelength, Frequency, and Speed The amplitude (a) is the total distance betweeen the crest of a wave and the centre line. It's a distance, so can be measured in metres, centimetres, millimetres or smaller units of length.
The wavelength (l) is the distance between one peak of the wave and the next peak. It's a distance so can be measured in metres, centimetres etc. It is sometimes given the Greek letter
λ (lambda).
It's also the distance
between one part of the wave and the next part.
The frequency (f) is the number of complete waves passing a point each second. It's a 'number per second' so it's measured in /s or s-1; usually called hertz (Hz) after a German physicist. 1 kilohertz = 1 kHz = 1000 Hz 1 megahertz = 1 MHz = 1,000,000 Hz
The speed of a wave (v) is just what it says - the speed at which the vibrations in the wave move from one point to the next. Wave speed is measured in metres per second (m/s, ms-1). Speed of light Frequency and wavelength are very simply related. Multiplying the wavelength and frequency together gives us a constant you get a constant. In the case of light, this constant is the speed of light. A wave of light will travel the distance between two points in a certain time. It doesn't matter what kind of light it is, the time is always the same.
The speed at which the wave moves, the speed of light (symbolised by c). is the same for:•
all kinds of light, and for
•
all kinds of electromagnetic radiation.
It has a value of 2.99 x 108ms-1 when the light is travelling in a vacuum.
3 The speed of light is given by:-
c=f位 where: c= speed of light f = frequency 位 = wavelength There is another parameter we use in infra red (ir) spectroscopy: wavenumber. This is simply the reciprocal of the wavelength and is the number of waves per unit of distance. It is useful in ir spectroscopy since it gives convenient numbers to handle and, in ir spectroscopy, has units cm-1.
Waves of different wavelength
Waves of different frequency
Waves of the same frequency but different amplitude
4
Theory of electromagnetic radiation All electromagnetic radiation has fundamental properties and behaves in predictable ways according to the basics of wave theory. The Victorian theoretical physicist, James Clerk Maxwell, deduced the nature of electromagnetic radiation to be a combination of magnetic and electric fields Electromagnetic radiation consists of an electrical field (E) which varies in magnitude in a direction perpendicular to the direction in which the radiation is traveling, and a magnetic field (M) oriented at right angles to the electrical field. Both these fields travel at the speed of light. Light which is radiating in all directions is termed non-polarised light.
It is possible to effectively filter the light and permit through only light radiating in a particular plane. This is called plane-polarised light and is extremely important in Chemistry since the only difference in the properties of optical isomers is the ability of optical isomers (d- and l- or R- and S-) to rotate plane polarised light in opposite directions by the same angle of rotation. It is also the reason why PolaroidTM sunglasses work – by effectively restricting the total amount (intensity) of light passing through the lens.
5 The particle theory of light In some situations, the behaviour of light is easier to explain by thinking of it not as waves but as particles. This idea, originally suggested by Isaac Newton, (who referred to ‘corpuscles’ of light) was used by Albert Einstein in 1905. As the name suggests, the particle theory of light regards light as a stream of tiny 'packets' of energy called photons. The energy of the photons is related to the position of the light in the electromagnetic spectrum. For example, photons with energy 3 x 10-19 J would correspond to red light.
Combining the wave and particle theories of light The two theories of light - the wave and photon models - are linked by a relationship which was proposed by Max Planck around the turn of the century:
E = hf where •
E = the energy of a photon
•
f = frequency of the light on the wave model multiplied by a constant
•
h is a constant (the Planck constant and has a value of 6.63 x 10-31 J Hz-1.
Example For a photon of red light with an energy of 3 x 10-19 J, the frequency would be given by
3 x 10-19 = 6.63 x 10-31 f
and so f = 3 x 10-19 / 6.63 x 10-31 = 4.5 x 1011 Hz
6 Emission spectra It has been known for hundreds of years that white light contains light of all the colours of the rainbow.
It has also been known for a long time that white light can be split into its constituents colours using a prism.
It has also been known for a long time that different elements may have different characteristic colours associated with them. We can use this property in the flame tests where we note any characteristic colour. Equally, we could pass the emitted light through a prism and notice the different individual colours.
Atoms can become excited by absorbing energy, for example, from:-
•
flames
•
an electric discharge or from
•
radiation in the stratosphere or in outer space.
When atoms lose energy, it is often emitted as electromagnetic radiation. This radiation is usually in the infra-red visible or ultra-violet region. The emitted light can be split up into an emission spectrum by passing it through a prism or a diffraction grating (see below).
7 The spectrum of white light contains a continuous series of lines and so a continuous spectrum contains all the colours of the rainbow.
Different elements may have different colours and it has long been known that some elements can be identified in this way. Splitting the light emitted by different elements into the spectrum demonstrates that all elements have different, characteristic spectra as shown by the absorption spectra of H, Hg and Ne below.
•
If a cold gas is given energy, then the atoms can absorb light of particular frequency – this as an absorption spectrum.
•
If a gas is hot then it can emit light of particular frequency. This is an emission spectrum.
8 •
In absorption spectra, the lines appear black since the light is absorbed and then re-emitted in an infinite number of directions. Effectively, almost no light of these particular wavelengths then get to the detector and so these wavelengths appear to be black. The black lines in the absorption spectrum are at exactly the same wavelength as the coloured lines in the emission spectrum.
The sequence of lines is characteristic of each element and, like human fingerprints, can be used to identify the element.
The composition of stars, for example, can be determined from vast distances away by using this technique. This is also how helium was first detected and the element was named after the Greek word for the sun, ‘helios’.
9 The spectrum of hydrogen atom
The electromagnetic spectrum contains many wavelengths not visible to the human eye. This does not mean there are no absorptions or emission outside of the visible region.
There are.
The
spectra are often named after the physicist who discovered them:•
Lyman
•
Balmer
•
Paschen
•
Brackett
•
Pfund
The Balmer series is shown schematically below.
It is very important to notice that:•
There are discrete lines
•
As the wavelength decrease, the lines get closer and closer together
The full hydrogen spectrum contains the other series of lines, one in the visible and several in the infra-red region. The spectrum was interpreted in 1913 by the Danish scientist, Niels Bohr. Bohr's theory explained why the hydrogen atom only emits a limited number of specific frequencies. The frequencies - predicted by Bohr's theory matched extremely Well with the observed lines.
10 How Bohr's theory explained emission spectra The basic idea behind Neils Bohr-s theory is this - atomic emission spectra are caused by electrons in atoms moving between different energy levels (also known as shells and sub-shells). When an atom is excited, electrons jump into higher energy levels. Later, they drop back into lower levels and emit the extra energy as electromagnetic radiation, which gives an emission spectrum.
Bohr-s theory not only explained how the cause of absorption and emission spectra, it also gave a new model for the electronic structure of the atom.
However, Bohr's theory was controversial – •
It made use of Planck’s new idea of quantisation of energy which was at odds with much of what was then thought about energy.
•
It only explained the spectra of hydrogen. Bohr himself stated that he could not use it to explain the spectra of helium or larger elements however since he could calculate the spectrum of hydrogen. Since the theory fitted completely with experimental observation, there was clearly something significant in it even if the theory was not complete.
11 Summary of Bohr’s theory of the hydrogen atom
• The electron in the H atom is only allowed to exist in certain definite energy levels • A photon of light is emitted or absorbed when the electron changes from one energy level to another • The energy of this photon is equal to the difference between the two energy levels (∆E) • The frequency of the emitted or absorbed light is related to
∆E by:
∆E = h f where •
h is Planck’s constant
•
f is the frequency of the absorbed (or emitted) light.
There are refinements to be made to Bohr’s model since the lines in emission and absorption spectra get closer together with decreasing wavelength and eventually converge, producing a continuum. This suggests that the shells are not equally spaced:-
and that, eventually, the electron has enough energy to escape the atomic environment entirely. The frequency where the continuum begins represents the 1st ionization energy of the atom.
12 Ionisation energy Notice that as energy increases the levels become more closely spaced until they converge. After this point, which corresponds to the electron breaking away from the atom, the electron is free to move around with any energy. The H atom has lost its electron and become an H+ ion. This is ionisation and the energy difference between this point and the ground state is called the ionisation energy. Ionisation can be represented by the equation X(g)
X+(g) + e-
where X stands for an atom of any element and e- for an electron. Notice that X is shown as X(g), indicating that the atoms are separated from one another, in the gaseous state. In the case of hydrogen, we can work out the ionisation energy.from the point where the lines of the Lyman series converge together.
The entire spectrum of hydrogen (Lyman, Balmer, Paschen etc;) can be explained by this theory
13 Summary •
In the Bohr model, the rings represent the energy levels of the electron in the hydrogen atom.
•
The further away from the nucleus, the higher the energy.
•
Levels are labelled with numbers starting at 1 for the lowest level — the ground state.
•
The lines of the Lyman series correspond to changes in electronic energy from various upper levels to one common lower level, level 1. Each line corresponds to a particular energy level change, such as level 4 to level 1.
•
The series of lines which lies in the visible region, the Balmer series, arises from changes to level 2 from levels 3, 4, 5 .... etc.
Example questions The diagram below shows a part of the atomic emission spectrum of hydrogen
Explain why it consists of a series of sharp lines and is not a continuous spectrum
[2] QWC [1]
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14 What happens when radiation interacts with matter? Energy interacts with matter Electromagnetic radiation can interact with matter, transferring energy to the chemicals involved. The chemicals absorb energy, and the absorbed energy can make changes happen in the chemicals. Just what changes occur depend on •
the chemical involved
•
the amount of energy involved_
We must also remember that molecules are doing energetic things all the time. They:•
move around
•
rotate and
•
the bonds in the molecule vibrate.
•
The electrons in the molecule have energy too and they can move bebween the different electronic-energy levels.
A molecule has energy associated with several different aspects of its behaviour, including: • energy associated with translation (the molecule moving around as a whole) • energy associated with rotation (of the molecule as a whole) • energy associated with vibration of the bonds • energy associated with electrons.
These different kinds of energetic activities involve different amounts of energy-- for example, making the bonds in a molecule vibrate generally involves more energy than making the molecule as a whole rotate. The energy needed increases in the general order shown below.
electronic energy vibrational energy
rotational energy
translational energy
Increasing energy
15 Quantisation of energy
Max Planck originally suggested that energy could exist as definite, fixed packets which were termed quanta. At the time this was a revolutionary idea since classical physics assumed that bodies could have energies of any amount i.e. there was a continuum of energy.
Niels Bohr suggested that electrons in the hydrogen atom could only have fixed amounts of energy and that the proposed transitions between energy levels would only happen if the electron was given precisely the correct amount of energy to make the transition – too much or too little and the transition would simply not occur. Likewise, when the electrons moved from a higher energy level to a lower level, a fixed amount of energy was released.
We can extend these ideas to other types of energy – generally all other types of energy (translational. rotational and vibrational) are quantised as well.
Change occurring.
Size energy change (J)
Type of radiation absorbed
1 x 10-22 to 1 x 10-20
microwave
Change of −
rotational energy level
-20
to 1 x 10
-15
−
vibrational energy level
1 x 10
−
electronic energy level
1 x 10-15 to 1 x 10-16
infra red ultra violet and visible (uv/vis)
The greater the transition, the larger the energy change and the higher frequency (lower wavelength) of light absorbed or emitted.
Excited state
Large jump – corresponds to uv light
Excitation energy
Small jump – corresponds to visible light
Ground state
16 Colours of materials or why are plants green?
We learn lower down the school that objects are a particular colour because of absorption of the other colours. For example, a red object is said to be red because it does not absorb red but simply reflects that colour.
Examining the amount of uv/visible light absorbed by a chemical compound is termed uv/visible spectroscopy. The basic principle is straightforward. A sample tube is placed in a beam of light. The amount of light absorbed at each wavelength examined is measured and a graph is drawn. sample
light source
detector
This is the uv/vis spectrum of chlorophyll.
The higher the peak at any wavelength, the more light of that wavelength is absorbed.
It should be clear that chlorophyll absorbs blue light and red light. Since chlorophyll a does not absorb the green light, chlorophyll a and hence plants are green.
17 Interpreting a spectrum uv/vis spectra can be deceptively simple since we are examining electronic transitions. However, bearing in mind that the molecule can also be moving and rotating, the broad peaks show electronic transitions with all these other transitions superimposed. When recording the spectrum, chemists often give the wavelength of the maximum absorption (λmax). We are usually only really concerned with three features of a uv/vis spectrum: • the wavelength of the radiation absorbed (remember, for a compound to he coloured, at least
part of the absorption band must he in the visible region) • the intensity of the absorption • the shape of the absorption band.
For example, carotone has the following spectrum:
λmax = 453nm (blue region) and hence carrots are orange. The intensity of the absorption depends on the concentration of the solution and on the distance the light travels through the solution. Standard molar values are quoted so that values for different compounds can be compared.
It is enough to know that the higher the peak, the more intense the absorption. With substances used as dyes, the intensity of the absorption is important commercially since because it determines the amount of pigment or dye needed to produce a good colour. The shape and width of the absorption band is important because it governs the shade and purity of the colour seen.
18 This is the spectrum of oxygenated and de-oxygenated haemoglobin.
Why is blood red in healthy people? ………………………………………………………………………………………………………… …………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………….. ……………………………………………………………………………………………………………………………………………………………………..
Why do people suffering from lack of oxygen appear to have a blue colouring? …………………………………… …………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………….. …………………………………………………………………………………………………………………………………………………………………….. ……………………………………………………………………………………………………………………………………………………………………..
Mark scheme for specimen question