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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

can you mix kc nd pressure
Khanyisile Reply
equation for dissociation
Amanda Reply
under which topic
Phila
acids and bases
Amanda
Newton's law
Anna
How does a catalyst affect the rate of chemical reaction
Rebone Reply
it speeds up the reaction without being by the reaction. in other words it increases the Reaction rate by lowering the activation energy of a reaction
Keabetsoe
hey
Siphokazi
ey
Keabetsoe
🙄
Holy
it only speed up the reaction not to affect the reaction
John
what is a compound?
Sithembiso Reply
define Doppler effect
Emihle Reply
It is the change in frequency of a wave in relation to an observer who is moving relative to the wave source.
Junior
Doppler effect is the change in frequency or pitch detected by a listener because the velocity of the sound source is different from that of a listener relative to the medium of sound propagation
John
explain how color is an unreliable clue to.the identify minerals
Camille Reply
Hi I want to know where the topic for electric circuits are?
Zahra Reply
hello.. How to deal with Reactions rate?. i dont Understand them
Simphiwe Reply
Firstly you must know how energy changes during chemical reactions...secondly the collision theory.... Rate and extent of reactions.... And factors that affect the rate of chemical reactions... If u capture all the topic then this chapter will be easy for you the is no way you can fail it...
Pearly
thank you
Petty
Heii.. I'm struggling with chemical equilibrium.
Ronny
how do I attempt chemical equilibrium?
Ronny
doppler effect question
Mabikinyane Reply
How do I differentiate from the formula of a listener who is moving away and who is moving towards the source
Njabulo
if the its away you will subtract on the numerator and add in the denominator but if its towards its the opposite
Ashley
how cn I calculate sound of a source if it's unkown
Simphiwe Reply
Use what you were given. What were you given?
Bandile
where can I find acids and bases
muravha Reply
how to calculate avarage power?
Bongani Reply
good result for bursary
Soluiswayi Reply
Good day Sir
noma
Good day Mam
Aobakwe
good day
clifort
How are you
Aobakwe
4. A builder of mass 75 kg is carrying bricks up a flight of stairs. There are 40 steps, each 25 cm high and he takes 10 s to climb the stairs, carrying 15 kg of bricks at a time. Calculate the total power output of the builder during his climb at constant speed.
Asanda Reply
I want to know how to calculate.
Tuauaa
mm I think it will be zero work an power due to a vertical climbing
CHRISTOPHER
How it was calculated 😕
Nthabeleng
1stly describe Hydrocarbon for me please
Magadi Reply
Compounds that consists of carbon and hydrogen atoms only.
Learner
Compounds that consists of carbon and hydrogen atoms only.
Learner

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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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