OCR AS Chemistry Unit 1: Atomic Spectra

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  • Created on: 22-08-13 17:25
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Atomic Spectra
Atoms can become excited by absorbing energy.
Atoms in their ground state have all their electrons at their lowest possible energy levels.
Ground state When the electrons in an atom are all at their lowest
possible energy levels
If an atom's electrons take in energy from their surrounding they can move to higher energy
levels, further from the nucleus.
At higher energy levels, electrons are said to be excited.
Electrons release energy by dropping down to a lower energy level.
The energy levels all have certain fixed values ­ they're discrete.
Electrons can jump from one energy level to another by absorbing or releasing a fixed
amount of energy.
When the excited atoms lose energy and return to their ground state the energy is often
emitted as electromagnetic radiation.
This radiation is usually in the infrared, visible or ultraviolet regions.
The emitted light can be split up into an atomic spectrum by passing it through a prism or a
diffraction grating.
Energy levels get closer together.

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Absorption Spectra
Energy is related to frequency.
So when electromagnetic radiation is passed through a gaseous element, the electrons only
absorb certain frequencies, corresponding to differences between the energy levels.
That means the radiation passing through has certain frequencies missing.
A spectrum of this radiation is called an atomic absorption spectrum.
The missing frequencies show up as dark bands on a coloured background.
Each element has a specific line that is the same in both spectra.
The lines become closer at higher frequencies.…read more

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Emission Spectra
When electrons drop to lower energy levels, they give out certain amounts of energy.
This produces lines in the spectrum.
The emission spectrum has a black background with coloured lines on it.
For any particular element, the frequencies in an emission spectrum are the same as those
missing in an absorption spectrum.
Each element has a different electron arrangement, so the frequencies of radiation
absorbed and emitted are different.
This means the spectrum for each element is unique.…read more

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Bohr's Theory
The basic idea behind Bohr's theory was:
Atomic spectra are caused by electrons in atoms moving between different energy
When an atom is excited, electrons jump into higher energy levels again ­ and emit
the extra energy as electromagnetic radiation, which gives an emission spectrum.
Bohr's theory not only explains how we get absorption and emission spectra, it also gave
scientists a model for the electronic structure of atoms.
However, Bohr's theory was controversial.…read more

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The rings represent the energy levels of the electron
in the hydrogen atom.
The further away from the nucleus an electron is, the
higher the energy level.
Energy levels are labelled with numbers, starting at 1
for the lowest level ­ the ground state.
The frequencies of the lines correspond to change in electronic energy from various upper
levels to one common lower level.
Each line corresponds to a particular energy level change, such as level 4 to level 1.…read more



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