1.2 ABSORPTION AND EMISSION SPECTRUM
- Chemistry at glance
- Jan 21, 2020
- 6 min read
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In this topic I would like to give you one important sentence that will help you understand what I am saying later in that post. And this sentence is:
LIGHT= EM RADIATION = ENERGY TRAVELLING AS A WAVE
We are going to pass light through a sample of atom to see what happens to the electrons in that atom and light (visible light) is a part of the electromagnetic spectrum. What I am sure you know is that EM radiation is actually energy which travels as a wave. Therefore, in this unit we will look at light travelling as a wave and it being the energy that an atom needs for the electrons to move.
You will also need to remember two equations that relate energy, wavelength and frequency together with two constants (you will get these in the exam). Sometimes examiners give you the equations as well however it is better to remember them if they don't. They are not that difficult to remember anyway. And here they are:
As in the photo, you can also rearrange these equations depending on what the question asks you to find. These equations are very relevant to the topic and this is because every wave has its frequency and wavelength and we know that light travels as a wave. When we know the frequency of the light (wave) we can find the energy related to this frequency.
This is a little bit confusing but I am sure it will all be more understandable after we introduce some examples. But first of all you need to know the relationships between frequency, wavelength and energy:
1. Energy and frequency- since E=hf and h is Planck's constant (it is always the same), as frequency increases the energy also increases. Therefore, the relationship is directly proportional.
2. Energy and wavelength- as you can see in the photo, I rearranged the equation of energy so that I have the wavelength instead of frequency present. (If you are not sure how I did this, go back to the previous photo and everything is explained there). Now, imagine that you increase the wavelength. As the number at the bottom increases the energy decreases and therefore this is an inversely proportional relationship.
3. Frequency and wavelength- you only need one equation for this one and this is c=f x wavelength and when you rearrange this you can see that as you increase the wavelength, the frequency will decrease. Therefore, the relationship is again inversely proportional.
And I also included two examples but I think they are pretty much clear on the photo therefore I will not explain in words what I did. However, what you need to be aware is that you will need to be able to rearrange both of these equations and mix them together.
Once you know the equations, we are going to look at the electrons in atoms and how they work with heat which is a form of energy. Just like I said at the beginning.
1. Electrons in an atom are found in shells and they are in the ground state until they get energy.
2. When energy is provided to an atom for example in the form of heat, the electrons jump to higher energy levels.
3. Once the heat is taken away from them, the electrons will fall back down to lower energy levels and to the ground state. By doing so, they release a packet of energy ( it is called a photon, a particle representing light). For example, when we heat sodium,we get an orange colour.
4. You can find the frequency of the energy emitted by using energy difference in energy levels and putting it into E=hf.
ALL ATOMS ABSORB ENERGY (IN ANY FORM E.G. HEAT, LIGHT) AT CERTAIN WAVELENGTHS.
And now we have two spectrums. One is absorption (when electrons jump up) and one is emission (electrons fall back down). I found this difficult to understand when I was reading through textbooks however I think I found the right explanation that is easty to understand.
Absorption spectra:
- When energy is passed through an atom (in the form of light) at certain wavelengths, the energy will be absorbed by the atom and the electrons will jump to higher energy levels;
- The places where energy will be absorbed, will leave black lines in the spectrum.
Emission spectra:
- After absorption, when the energy (light) is removed, the electrons fall back down and release energy (photon/ a packet of energy)
Photon - discrete packet of energy related to EM radiation (light).
- They will emit that energy (photon) at a certain wavelength and a coloured line will be shown on the spectrum.
When the energy causes the electrons to jump to higher energy levels, they drop back down when energy is taken away from them. However when you don't take away this energy electrons will jump up and up until they reach a convergence limit.
Convergence limit- an energy level where atom has no longer control over the electron and the atom becomes ionised. This is where the biggest energy difference is seen and therefore the light has the highest frequency.
Here is a EM spectrum for you to visualise all that is happening:
A few points about this photo:
- Visible light has a wavelength between 400nm-700nm. Anything below this is UV radiation and anything above that is infrared radiation.
- As the wavelength increases, the frequency decreases.
- Highest frequency in the UV radiation.
- Highest wavelength at the infrared radiation.
You will need this information for the next part of this topic. Furthermore, only these three radiations are present because they are the ones that we need to focus on for this part of the course.
Hydrogen emission spectrum
For an AS-level, you are going to concentrate on hydrogen emission spectrum. This is because it is the easiest emission spectrum as hydrogen only has one electron. Every time an elctron falls to a particular energy level is called series.
There is one electron but it can do many things. This depends on what energy level it is initially before emission and how far it is going to fall.
- > If it falls to energy level 3 (n=3), it is called Pashen series;
- > If it falls to energy level 2 (n=2), it is called Balmer series;
- it is in the Visible region of the spectrum
- > If it falls to energy level 1 (n=1), it is called Lyman series;
- it is in the UV part of the spectrum
- it has a bigger energy difference than Balmer series as the electron drops lower, meanig the frequency is higer (E=hf). Since the frequency increases to the left of visible light (photo above), Lyman series is in the UV part of the spectrum.
You don't need to know anything more about Pashen series but you need some more information about the other two.
Lyman series
We know now that Lyman series is when the electron drops to n=1 and we also know that at n= infinity atom loses all control over the electron (becomes ionised). Therefore, it is clear that when atom loses control over its atom in Lyman series, it is the inisation of atom.
We can find the ionisation energy if we multiply the energy difference (n=1 -> n= infinity) by Avogadro's number (constant found in data booklet).
And as always I prepared an example for you guys to see exactly what is going on.
1. Use frequency and Planck's ccnstant to find energy.
2. Multiply the answer by Avogadro's constant.
3. Then you can divide it by 1000 to get the answer in kJ instead of J.
And this is all for this topic. As always I have more past paper questions prepared on my youtube channel and if you would like a copy of these questions let me know. All contact details at the top of this page. See you soon!
PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!
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