# 15.2 Emission and absorption spectra

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## 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.

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.

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 $E=hf$ . 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.

why mass not change
because it is irreversible
Jiluis
what is different between work and power in details
When a force is exerted to an object and causes it to move in the direction of the force the work is done whereas power is the rate at which work is done.
ISRAEL
Hlayiselo
i also want to be helped
Tobeka
Why is alcohol flameable?
What is interconversion reaction for? ... under substitution reaction
Moloto
how can u tell whether the compound is an alkane, alkene or alkyne
alkane compounds are saturated, consists of single bonds between atoms and alkenes are unsaturated consists of one double bond between two adjacent carbon atoms .alkynes are also unsaturated but they consists of a triple bond between 2 adjacent carbon atoms.
Andile
you can tell simple by looking at the bonds which exist between the carbon-carbon atoms of the compound, so if a compound consists of double bond between its carbon-carbon atoms it is an alkene etc.
Tatyana
can it say it cause I understand it better
Hey, what is momentum?
the product of mass n its velocity
Albert
And what's simple harmonic?
Inocent
What is organic compounds
are compounds which consists of carbon atoms
Mkhize
Thanks bro
Gaba
How many industrial processes are there?
A homologous group is a group with the same amount of electrons in the outermost shell.
What are hydrocarbons
Its a compound of hydrogen and carbon atoms
Luyanda
do hydrocarbons get unsaturated?
yes
Thembi
they are saturated
Sinoty
Yes
Gugu
what is monomer
what is polymerism
Tobeka
I think a polymer is a chemical compound with molecules bonded together in long, repeating chains. Because of their structure, polymers have unique properties that can be tailored for different uses.
Kagiso
Louis
monomers are the building blocks of polymer.
Nandipha
which essays are expected in march exams
what do you look at when you want to successfully compare the boiling points of two|three organic molecules
1-check the homologous group 2-number of carbons or chain length n their intermolecular forces between the bonds
Lesego