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Emission and absorption spectra

Emission spectra

You have learnt previously about the structure of an atom. The electrons surrounding the atomic nucleus are arranged in a series of levels of increasing energy. Each element has its own distinct set of energy levels. This arrangement of energy levels serves as the atom's unique fingerprint.

In the early 1900s, scientists found that a liquid or solid heated to high temperatures would give off a broad range of colours of light. However, a gas heated to similar temperatures would emit light only at certain specific colours (wavelengths). The reason for this observation was not understood at the time.

Scientists studied this effect using a discharge tube.

Diagram of a discharge tube. The tube is filled with a gas. When a high enough voltage is applied across the tube, the gas ionises and acts like a conductor, allowing a current to flow through the circuit. The current excites the atoms of the ionised gas. When the atoms fall back to their ground state, they emit photons to carry off the excess energy.

A discharge tube ( [link] ) is a glass gas-filled tube with a metal plate at both ends. If a large enough voltage difference is applied between the two metal plates, the gas atoms inside the tube will absorb enough energy to make some of their electrons come off i.e. the gas atoms are ionised. These electrons start moving through the gas and create a current, which raises some electrons in other atoms to higher energy levels. Then as the electrons in the atoms fall back down, they emit electromagnetic radiation (light). The amount of light emitted at different wavelengths, called the emission spectrum , is shown for a discharge tube filled with hydrogen gas in [link] below. Only certain wavelengths (i.e. colours) of light are seen as shown by the thick black lines in the picture.

Diagram of the emission spectrum of hydrogen in the visible spectrum. Four lines are visible, and are labeled with their wavelengths. The three lines in the 400–500 nm range are in the blue part of the spectrum, while the higher line (656 nm) is in the red/orange part.

Eventually, scientists realized that these lines come from photons of a specific energy, emitted by electrons making transitions between specific energy levels of the atom. [link] shows an example of this happening. When an electron in an atom falls from a higher energy level to a lower energy level, it emits a photon to carry off the extra energy. This photon's energy is equal to the energy difference between the two energy levels. As we previously discussed, the frequency of a photon is related to its energy through the equation E = h f . Since a specific photon frequency (or wavelength) gives us a specific colour, we can see how each coloured line is associated with a specific transition.

In this diagram are shown some of the electron energy levels for the hydrogen atom. The arrows show the electron transitions from higher energy levels to lower energy levels. The energies of the emitted photons are the same as the energy difference between two energy levels. You can think of absorption as the opposite process. The arrows would point upwards and the electrons would jump up to higher levels when they absorp a photon of the right energy.

Questions & Answers

formula of heat reaction
Anelisiwe Reply
final energy-initial energy of substances
what is the difference between ethene and ethane
Joy Reply
Ethane is a saturated organic molecule while ethene is unsaturated Ethane has 2 carbon atoms and 6 hydrogen atoms, while ethene has 2 Carbons and 4 hydrogens Ethane has single bond between the 2 Carbon atoms while ethene has double bond
How do you know that the reaction is in acid medium or basic medium
Miss Reply
What formular is used when source and Observer are moving towards to each other
Esihle Reply
put a + on top n - on the bottom
what is doppler effect?
Ansiwe Reply
doppler effect is the perceived change in frequency of a sound caused by either the source or the listener
moving relative to each other
A Catalyst is a chemical substance that speeds up a chemical reaction.
Sheron Reply
It does this by lowering the activation energy.
what is exothermic
Nambile Reply
reaction that releases energy
what is a catalyst
Catalyst is to speed up the reaction
catalyst is a substance added into a solution to speed up the process
As it speeds up the reaction by lowering the activation energy, it does not take part in the reactants or products formed
reaction that release energy
what is exothermic
Nambile Reply
reaction that gives off heat to the surroundings
formula for calculating the heat of reaction
I need help in electrostatic
Muthuphei Reply
in exactly in electrostatic
send the question on electrostatic maybe I can help
factors affecting the rate of reaction
thabo Reply
I'm xiluva ...do we help those who are having challenges on their studies?
I mean on physical science
I believe it's supposed to be like that. Helping those who are having educational challenges
Yep physical sciences more specifically
ok ....nna I need some help in physics electricity ...just hints how we do what n how to solve some of the problem... just hints
The first thing you can do is to understand ohm's law and the difference between a series and parallel connection concerning current, resistance and potential difference
Hi Xiluva,not sure if this will be helpful but to understand electricity better I suggest you download Siyavula physical science books there's both a learner book and a teacher's guide I was struggling too they really did help you might be able to do some exercises after reading siyavula material
what diode mean
diode s a part of electrical current tht allows only moves to one direction ..only ...
allows current .....to flow to one direction I meant .
ok thanks
ok thanks guys
temperature surface area concentration addition of a catalyst
thank you mr
How to calculate e.p
zama Reply
What is polymer
Kwinda Reply
chemical compound with molecules bonded together in long ,repeating chains
hey guys i need help with the topic of forces and their application
Monica Reply
A force is a pull or push and it can be applied in rocket launchers moving cars & stoping (breaking them).
A force is that Agency which changes the object's state of rest or motion. A push or pull are the effects of force.
are you sure u wantd to ask tht or u wantd to say " how many bonds are there in alkane nd their names?" ?
Frank Reply
I also think that's what she meant 'coz I don't really understand her question
yeah nd if that's th case , there are only single bonds wch I could say one of em tht I know is methane
I mean methane,ethane they end up to what
well I also assume ths is wah u lookin for .. methane , ethane , propane, butane, pentane, hexane , Heptane, octane , nonane , decane . is ths wah u wanted ?
how many alkane are there n there names
Sylvia Reply

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Source:  OpenStax, Siyavula textbooks: grade 12 physical science. OpenStax CNX. Aug 03, 2011 Download for free at http://cnx.org/content/col11244/1.2
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