Absorption and Emission Spectrum
From the OCR B specification:
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The electromagnetic spectrum in order of increasing frequency and energy and decreasing wavelength.
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Transitions between electronic energy levels in atoms
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the occurrence of absorption and emission atomic spectra in terms of transition of electrons between electronic energy levels
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the features of these spectra, similarities, and differences
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the relationship between the energy emitted or absorbed and the frequency of the line produced in the spectra, ∆E = hν
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the relationship between frequency, wavelength and the speed of electromagnetic radiation, c = ν λ
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fame colours of Li+, Na+, K+, Ca2+, Ba2+, Cu.
Electromagnetic spectrum:
The electromagnetic spectrum is the range of all the types of EM radiation. It is a continuous spectrum of various wavelengths and frequencies which correspond to a specific type of wave which will have specific properties and colour.
(Figure 1)
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As shown in figure 1, you can see that the electromagnetic spectrum is made up of 7 different types of waves which in order of increasing wavelength and decreasing frequency and energy is:
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Gamma
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X-ray
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Ultraviolet
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Visible light
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Infrared
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Microwave
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Radio waves
-A mnemonic which you can use to help you remember the order of the electromagnetic spectrum is:
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Good
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Xylophones
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Usually
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Vibrate
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In
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My
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Room
Transitions between electronic energy levels in atoms:
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Every atom contains shells, these shells hold and contain the atoms electrons which orbit the nucleus. These shells are at discrete energy levels and which is unique for every element which exists on the periodic table.
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Atoms ideally want to be as stable as possible so when there is external energy electrons will transfer between energy levels by emitting or absorbing a photon of electromagnetic radiation in a process called atomic electron transition.
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This can only happen if the energy absorbed or emitted exactly equals the energy difference between two energy levels (the distance between the shells of an atom) Therefore this specific amount of energy will be unique for each type of element and will correspond to different wavelengths.
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Energy level diagrams show the different energy levels in an atom that are ‘quantized ’which in the diagram are represented by distances of horizontal lines where n=1 is the ground state, and the other levels are the excited states. The arrows represent the transitions of energy levels, and these will be different for each type of element.
(figure 2)
Emission spectrum:
An Emission spectrum is a black spectrum with coloured lines. These coloured lines correspond to the frequencies of electromagnetic radiation emitted due to an atom or molecule during the transition from a higher energy level to a lower energy level. When heat is applied, it causes the electrons to become excited and gain kinetic energy enabling more collisions to occur per unit of time. This enables the electron to go to a higher energy level state. When the electron then loses energy and moves to a lower energy level it emittes a photon of radiation which the emission spectrum will detect and is a unique value of frequency. The coloured lines in the spectrum represent the wavelengths present when the photon is emitted. Hence E=hv.
(figure 3)
Absorption spectrum:
An absorption spectrum is a coloured spectrum with black lines. These black lines correspond to the frequencies of electromagnetic radiation absorbed due to an atom or molecule making a transition from a lower energy level to a higher one. This occurs when the element is heated causing the electrons to become ‘excited’ due to kinetic energy absorbing a specific photon of radiation. This enables the electron to moves to a higher energy level which the emission spectrum will detect and will be shown by black lines.
(figure 4)
As shown by figures 3 and 4 (which both represent the transition of energy levels of a carbon atom) the absorption and emission spectrums have many similarities and differences:
Similarities:
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For the same type of element, lines will show up at the same points on the spectrum since the energy absorbed is equal to the energy emitted for each type of element.
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You can use both spectrums to identify a specific element by looking at the lines in both spectrums.
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Differences:
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Emission spectrum uses emitted energy to create the spectrum whereas an absorption spectrum uses absorbed energy.
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Emission has colours as its indicator to show the energy waves present whereas the absorption shows this with black lines.
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Emissions is a back spectrum with coloured lines and an absorption spectrum is a coloured spectrum with black lines.
Calculations:
We can identify types of elements by looking at the lines in the spectrum and relating it to the energy emitted or absorbed and find unknown compounds. This can be done using the equation: ∆E = hν
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Where ∆E is the energy of a photon,
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h is Planck's Constant and is equivalent to 6.63x10-34
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v is the frequency of the light found by looking at the spectrum lines.
We can also use another equation to work out the wavelength of electromagnetic radiation c = ν λ
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c represents the speed of light which all electromagnetic waves travel at and is equivalent to 3x108 m/s
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v represents the frequency of the electromagnetic radiation ( Measured in Hz)
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λ represents the wavelength of electromagnetic radiation. (Measured in meters)
An example of a type of question may be asked in an exam is: A photon of light has a wavelength of 0.050cm. Calculate its energy in Joules (J).
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To calculate the answer, you would have to do the following steps:
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Firstly, since the question is asking about energy and mentioning wavelength you must recognize that it wants you to use two equations to find the answer which are c = ν λ and ∆E = hν (for OCR B these are not given in the exam, and you must know these equations by memory so make sure you know them!)
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Next you would use the equation c = ν λ to find the frequency (v) you would do this by rearranging the equation, so v is the subject of the formula hence v= c/ λ
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Then convert your units. So you convert your given wavelength into meters. Therefore you would divide the 0.050cm by 100 to get 5x10^-4m
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Then you would substitute in the numbers. C will always be 3x10^8 m/s for any electromagnetic radiation and wavelength is always measured in m for this equation . It should look like this: v= 3x10^8/5x10^-4 so v= 6x10^11Hz
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Now that you have found v, you can use the other equation to find its energy (∆E = hν) where h is Planck's constant and v is 6x10^11hz. It should look like this: ∆E = 6.63x10^-34 x 6x10^11
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So therefore the answer will be 3.98x10^-22 J. (Sometimes exam equations will want the answer to a certain number of significant figures and the appropriate units make sure you don’t miss out on these simple marks)
Flame tests:
When you place curtain samples of specific elements under a flame they produce different colours. This is due to the Bunsen burner causing the electrons to become excited due to the increase in kinetic energy. This causes the electrons to collide more frequently leading to the electrons transitioning to a higher energy level. When the excited electrons lose energy, they emit a photon of light. This is the same reason why fireworks are different colours depending on the metal they contain as the frequency of light is unique for every element. Because each element has its signature line spectrum, they will produce different colours under a flame, and we can use this to identify curtain ions. For OCR B you need to know these colours and ions by memory:
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Li+ is a red flame
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Na+ is an orange flame
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K+ is a purple flame
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Ca2+ orange/red
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Ba2+ is a green flame
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Cu2+ turquoise/green flame