Model of atomic structure
We use a simple models that simplifies what the inside of the atom looks like. The model shows that atoms have 3 types of subatomic particles:
protons have a charge of +1 and a mass of 1 and is found in the nucleus
neutrons have a neutral charge and a mass of 1 and are found in the nucleus.
electrons have a charge of -1 and have a mass of 0.00055 and surround the nucleus in energy shells.
The nucleus is described by it's mass number and it's proton number.
The proton number = no of electrons
Mass number = protons + neutrons
Radioactivity depends on how unstable the electron configuration of an element's isotope is. An isotope is a variaton of an element with a different number of neutrons and a different mass number. As their nuclei decay they emit particles called emissions. Radioactive decay occurs of it's own accord and can not be stopped by outside forces.
There are 3 types of radioactivity:alpha α, beta β and gamma γ.
Alpha has a relative charge of +2, a relative mass of 4 because it is a charged helium ion. It can only travel a few centimetres in air and it is easily deflected by paper.
Beta has a relative charge of -1, and a relative mass of 0.00055 because it is an electron. It can only travel a few metres in air and is deflected by tin foil or cardboard.
Gamma does not have a relative charge or mass because it is a type of electromagnetic radiation. It can travel a long distance in air and is only stopped by lead or thick concrete.
Nuclear reactions show the effect of each type of radiation decay on the element:
The element loses two protons and two neutrons as the mass number of helium is four and the proton number is two so the mass number is x - 4 and the proton number is x - 2
In beta, a neutron changes into a proton and an electron and the electron is emitted from the nucleus. This means that the proton number goes up by one as a proton number is created and mass number stays the same as a neutron is lost but a proton is created and electrons do not effect the mass number.
In nuclear fusion, the nuclei of two light elements are forced together to make one new heavier element. Fusion needs a lot of energy to overcome the repulsive forces between the nuclei so it only occurs at very high tempertaures but the reaction releases huge quanities of energy. Scientists hope that nuclear fusion can be used on earth to generate electricity as it produces no polluntants and the raw material is hydrogen which is easily obtainable. However, the reaction is still not efficient enough as it requires more energy to meet the required level of heat for the reaction to take place than we get from the reaction as electricity.
Half-life is half the time it takes for a radioactive nucleus to decay.
Example: strontium-90 has a half life of 28 years. How many years will it be until 1/8 of srontium-90's nuclei is left?
Every 28 years, what ever is left from the previous period is reduced by half at the end of the period. So at the beginning of the 28th year there will only be 1/2 of the nuclei left and this will be halved again in another 28 year period and so on. So, each half life is the exact same amount of time as the amount of time the nuclei took to decay to a half. (28 years)
1st period 28 years: 1/2 of mass of strontium-90
2nd period 56 years: 1/4 of mass of strontium-90
3rd period 84 years: 1/8 of mass of strontium-90
It will take 84 years for strontium-90's nucleus to decay to an 1/8 of it's original mass.
Use of radioisotopes
In geology: There are some radioactive isotopes in rocks and their half life can be used to calculate the rock's age. The measure of time can be found out by finding out the stage of decay by finding out the ratio of mother:daughter products that are formed at the end of decay. A higher amount of mother products indicate that the age of the rock is comparatively young. If the ratio of daughter products is higher then that rock is comparatively old.
- The half life of the radioisotope must be known for the results to be accurate.
- We don't know whether there has been a net movement of mother or daughter products out of the mineral since the time of crystalisation.
- Deformation or heating of the rock may mean that radioactive isotopes are lostso half life is reset again.
Radioisotopes can be used as medical tracers. To be a medical tracer, the isotope must satisfy a number of conditions. The half life of the isotope must be long enough to find the problem but short enough so as not to cause damage to the healthy body tissues.
Electronic structure: shells
Electrons reside in energy shells that are given a quantum number. The smaller it's quantum number the closer it is to the nucleus.
n=1: 2 electrons reside in the first energy shell
n=2: 8 electrons reside in the second energy shell
n=3: 18 electrons reside in the third energy shell
n=4: 32 electrons reside in the fourth energy shell
These energy shells are further split up into subshells labelled s,p,d and f.
subshell s can hold a maximum of two electrons
subshell p can hold a maximum of six electrons
subshell d can hold a maximum of ten electrons
subshell f can hold a maximum of 12 electrons
Order of subshells
Each subshell can hold a maximum of two electrons and they only occupy the same orbital if they have opposite or paired spins.
The order of subshells:
1s Example: Hydrogen has one electron that occupies one orbital so it's electron configuration is 1s1 Example: Helium has two electrons. Since an s subshell can occupy two electrons the electron pair occupy the first subshell 1s2
2s For elements with between 3-4 electrons as two electrons fit in each orbital.
2p For elements with up to 10 electrons as 1s and 2s occupy a maximum of 4 electrons and a p subshell can occupy 6 electrons.
3s An s subshell can occupy another 2 so this electron configuaration is reached with elements that have up to 12 electrons.
3p A p subshell occupies 10 electrons so this subshell is reached for elements with an electron number of up to 22.
3d A d subshell occupies 12 electrons so this subshell is reached for elements with an electron number of up to 44.