Arrangement of electrons in the atom

Once all the parts of the atom had been discovered scientists needed to understand how the electrons were arranged around the nucleus

It was thought that electrons whizzed around the nucleus like planets do around the sun This is impossible as electrons would run out of energy and fall into the nucleus

Niels Bohr A Danish physicist provided an insight into the arrangement of electrons by studying spectra

Bohr’s study of spectra A spectrum describes the array of colours formed when white light is passed through a prism

This kind of spectrum from red to violet is called a continuous spectrum or rainbow Bohr performed this experiment but used light coming from a hydrogen discharge tube (which is a tube filled with hydrogen at low pressure through which a current is passed)

Fig 3.3 in text book

A line spectrum Since there are distinct lines this is called a line spectrum Because it is formed when a current is passed through Hydrogen it is more accurately called an emission line spectrum

If hydrogen in the discharge tube is replaced with different elements different line spectra are produced

Each element has its own unique line spectrum It is like a finger print for the element This is how chemists analyse materials to look for certain elements for example to look for other elements in metal

Explaining the evidence Fill in p 16 Carol

Atomic absorption spectrometry Scientists have shown how atoms emit light when electrical or heat energy is supplied to them They will also absorb light if white light is passed through a gaseous sample of the element

When the white light that comes out through the gaseous sample of an element is examined it is noticed that certain wave lenghts are missing What you see is missing lines in the spectrum

Continuous Spectrum Emission Spectrum Absorption Spectrum

The missing wave lenghts are absorbed by the sample and correspond exactly to the wave lenghts that would be detected if an emission spectrum of that sample were recorded The absorption spectrum is like a photographic negative of the emission spectrum

The reason for this is that the atoms in the ground state absorb the same amount of radiation as they would emit in the excited state

Absorption spectrometry is used in practice to determine whether certain elements are present in a sample eg Heavy metals in water It is also used to determine the concentration of a sample of a particular element Eg. The concentration of sodium in a sample of sodium isolated White light is not normally used instead scientists use the light of the element they are examining Eg. Sodium light when looking fore sodium

Energy Sublevels Inser p 19 Carol

Wave Nature of the Electron 1924 French scientist Louis De Broglie suggested all moving objects have a wave motion In tiny particles such as electrons this wave motion is significant If an electron has a wave motion then Bohr’s idea of it travelling in a fixed path at a fixed distance from the nucleus is not strictly correct

Consider this image and fig 3.17 in book

Werner Heisenberg treated this problem mathematically and proposed Heisenberg’s Uncertainty Principle Which states: It is impossible to measure at the same time both the velocity and position of an electron

Heisenberg reasoned that when you try to find the location of an electron by firing a beam of light at it the electron is so small it will automatically be deflected out of position so you can never have a certain fixed path like the Bohr model suggests

If an electron is not travelling in a fixed path we can only talk about the probability of finding an electron in a certain region in an atom

From this we get the definition of an orbital as being: An orbital is a region in space where there is a high probability of finding an electron NB Fig 3.19

In a hydrogen atom there is only one electron so the orbital is the area where it is likely or probable to find this electron It is called the 1s atomic orbital because it is spherical and near the nucleus

Luckily instead of drawing all the dots we simply draw a sphere to indicate that within that sphere there is a high chance of finding an electron!

Erin Schrodinger Swedish but worked in Dublin and became an Irish citizen Used Maths to work out the probability of finding an electron in any particular sublevel in an atom His equations give 4 different shape orbitals s, p, d and f

All s orbitals are spherical 1s, 2s , 3s and 4 s are all spherical and only differ in size

All p orbitals are dumbbell shaped There are 3 parts to a p sublevel px , py and pz The letters x, y and z stand for the axes they occupy in space

Within the dumb bell there is high probability of finding an electron There is virtually no probability of finding one outside it