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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

ketones's functional group
Moloi Reply
Why does the carboxyl group have acidc properties?
Evi Reply
carboxylic acids
tyrique
Besides improving appearance what is another reason why medal is electro plated
Elizabeth Reply
it is made of metal
Reunity
thanks
Elizabeth
what is different between Dependent variable and independent variables
Elizabeth
difference between A.c and D.c
Gerrard
AC has slip rings DC has split rings commutator
Mohludi
are UV rays dangerous?
Khathutshelo Reply
no
Lianda
what's this?
Lianda
if there is extensive exposure, it can lead to skin cancer
tyrique
i hv a problem with this chapter because i also have problem with Newton's laws of motion
Kearabetswe Reply
what is the wave model of atom ?
Trazy Reply
Can you state the Doppler effect in words
Nombuyiselo Reply
State Coulomb's law in words
Nombuyiselo
the magnitude of the electrostatic force exerted point on each other is directly proportional to the product on magnitude of charges and inversely proportional to the square distance between them
Cassiey
the change in frequency of the sound detected by a listener because sound source and listener have different velocity
Elizabeth
Newton's first law of motion
Nnuso Reply
When an object is at rest or traveling at a constant velocity it will remain at rest unless an unbalanced force acts upon it.
Tumelo
yes that's true
Mphoko
Always know Newton's second law of motion. It appears to be in every question paper
Botho
Is the normal force always 0
Mpilo Reply
no. newtons 3rd law states that if something exerts a force into something else it will experience that same magnitude of force but in the opposite direction. so the net force is equal to the force the object applies to the surface but in the opposite direction
Courtney
define the term rate of reaction in word
David Reply
I think it a chemical process in which substance act mutually on each other.
Sphesihle
how many bones are in the human body?
Asali
it's the rate at which the reactants are able to change to products
Courtney
206 bones
Elizabeth
what is wave lengh
mama Reply
is the de broglie wavelength of the particle
Hope
calcium and magnesium, which one can displace aluminum from its compound?
Markia Reply
how is aluminum ion formed?
Dickens
a substance that has the lower electronegativity which will be in this case the Ca -1.0 this is because the Ca would have a lower ionization energy -needs less energy to fill its outer most shell and therefore will cause a displacement of the Al
Courtney
an Al ion would form when it has lost electrons and will normally be a cation -positively charged (3+)
Courtney
molecules with the same molecular formula but different chain
Malaza Reply
what will be the answer
.angel
chain isomer
Anele
what is structural isomer?
Kearabetswe
Hey if you define an isomer it will come as an organic molecule with the same number of atoms,same molecule formula ,same molecular mass but different STRUCTURAL FORMULA so what we have is only the chain ,positional and functionally isomer.Never heard of STRUCTURAL isomer
Phethego
what does the newton's law says?
there are 3 newtons laws which are newton's law of universal gravitation, law of cooling, law of motion
Imagine
Did the feather or leaf hit the ground first?
Shaloom Reply
leaf
Pradip
both at the same time. air friction ignored
Courtney

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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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