GCSE AQA PHYSICS - atomic structure

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  • Created by: ML321
  • Created on: 23-03-21 18:07

structure of the atom

Protons and neutrons are the heaviest particles. Protons and neutrons have a relative mass of 1 and electrons have a relative mass of 0. the relative charge of a proton is (+)1 and the relative charge of a neutron is 0. An electron has a relative charge of -1, which makes the atom neutral (there is an equal number of protons and electrons). 

In a neutral atom, the number of electrons is always the same as the number of protons. However, if the atom becomes ionised, the number of electrons will change. This is because an ion is an atom that has lost or gained one or more electrons.

17 - Chlorine - Atomos, Atomosthis symbol shows that chlorine has 35 overall particles in the nucleus (protons and neutrons), 17 of which are protons. There are the same amount of protons and electrons so there is also 17 electrons. To find the amount of neutrons you do 35-17 which gives 18, so there is 18 neutrons.

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ISOTOPES AND IONS

An elements atomic number defines it, so an element with 17 protons will always be chlorine. An element's mass number can vary, which means that it could have different numbers of neutrons. So, although chlorine has a mass number of 35 which means it has 18 neutrons, it can also have a mass number of 37, which means it would have 20 neutrons. These different types of chlorine are called isotopes.

QUESTION:

How many neutrons does 146C contain?

IONS:

normally, atoms are neutral, but they can lose or gain electrons due to collisions or other interactions. When they do, they form charged particles called ions. 

  • if the atom loses one or more electrons, it becomes a positively charged ion
  • if the atom gains one or more electrons, it becomes a negatively charged ion.
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STABLE NUCLEI

an atom's nucleus can only be stable if it has a certain amount of neutrons for the amount of protons it has. Elements with fewer protons (near top of periodic table) are stable if they have the same number of neutrons and protons.

an example is carbon: carbon-12 is stable and has 6 protons and 6 neutrons. However, as the number of protons increases, more neutrons are needed to keep the nucleus stable.

For example lead: lead-206 has 86 protons and 124 neutrons.

Nuclei with too many or too few neutrons do exist naturally but are unstable and will decay by emitting radiation. 

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NUCLEAR RADIATION

Nuclear Radiation

There are 4 types of radioactive decay:

The Alpha Particle:

If the nucleus has too few neutrons, it'll emit a 'package' of 2 protons and 2 neutrons called an alpha particle. Alpha decay causes the mass number of the nucleus to decrease by 4 and the atomic number to decrease by 2.

The Beta Particle:

If the nucleus has too many neutrons, a neutron will split into a proton and an electron. It will then emit the fast moving electron, called a beta (β) particle. This process is called beta radiation. A beta particle has a relative mass of 0, so it's mass number is 0. Beta decay causes the atomic number of the nucleus to increase by one and the mass number stays the same.

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NUCLEAR RADIATION PT2

The Gamma Ray:

Ater emitting an alpha or beta particle, the nucleus will often still be too 'hot' and will lose energy in a similar way to how a hot gas cools down, by emitting infared radiation which is an electromagnetic wave. Gamma ray emission causes no change in the number of particles in the nucleus meaning that both the atomic number and mass number remain the same. 

Neutron Emission:

Occasionally it is possible for a neutron to be emitted by radioactive decay.A further example of neutron emission is in nuclear fission reactions, where neutrons are released from the parent nucleus as it splits. Neutron emission causes the mass number  of the nucleus to decrese by 1 and the atomic number remians the same.

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HALF LIFE

Radioactive decay is random. A rodioactive material will contain trillions of nuclei and not all of them will decay at the same time or rate so it is impossible to tell when a particular nucleus will decay. 

Half life is the time it takes for the acticity of a radioactive substance to fall by half.

The activity is the rate at which a source decays and is measured in Becquerels (Bq).

For example:

Iodine has a half life of 8 days. If the initial activity is 4000 counts/s what will it be after 16 days?

the initial activity is 4000 counts/s so after 8 days it will half to 2000 counts/s. For it to half again, it would take 16 days so therefore the answer is 100 counts/s

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NUCLEAR EQUATIONS

A nucleus changes into a new element by emitting alpha or beta particles. The changes are shown as nuclear equations. 

Alpha decay changes the mass number of the lement by -4 and the atomic number by -2. 

Beta decay changes the atomic number by +1 (the nucleus gains a proton) but the mass number stays the same. 

Gamma is pure energy and will not change the structure of the nucleus. 

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NUCLEAR EQUATIONS EXAMPLES

 Alpha Decay, Beta Decay, Gamma Decay - Electron Capture, Positron  Production - Nuclear Chemistry - YouTube                                                                     Nuclear equation and Alpha Decay: During Alpha decay a alpha particle (also  known as a helium particle) is ejected from the nucleus. The particle is  composed of 4 nucleons, two of which are protons. Because two protons are  ejected the element is ... (http://www.ipodphysics.com/resources/alpha%20Uri.PNG)      --> Alpha decay         

                                                                                                         Cyberphysics - Nuclear Equations  ---> Beta decay

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IRRADIATION AND CONATMMINATION

Irradiation - exposing objects to beams of radiation. Applies to all types of radiation including radiation from the nuclei of atoms. It is more dangerous when an object is exposed to a source of radiation outside the object. If the half life chosen is too long, the damaging effects of the radiation would last too long and the dose would continue to rise.

Advantages: 

  • Sterilisation can be done without high temperatures. 
  • It can be used to kill bacteria on things that would melt

Disadvantages:

  • It may not kill all bacteria on an object
  • it can be very harmful - standing in the environment where objects are being treated by irradiation could expose someone's cells to damage and mutate (can cause cancer)

 

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IRRADIATION AND CONTAMMINATION PT2

Contamination - occurs if an object has a radioactive material introduced to it. It is more dangerous when the source is oon or in the object. Use of isotopes with a short half life means the the exposure can be limited.

Advantages:

  • Radiocactive isotopes can be used as medical and industrail tracers.
  • use of isotopes with a short half life mean exposure can be limited

Disadvantages:

  • rodioactive isotopes may not go where they are wanted
  • it can be difficult to ensure that the contammination is fully removed so small amounts could be left behind
  • exposure to radioactive materails can damage healthy cells

 Example - an apple exposed to the radiation from cobolt-60 is irradiated but an apple injected with cobolt-60 is contaminated.

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NUCLEAR FISSION + FUSION

Nuclear Fission is where a neutron carshes into a large atomic nucleus (for example uranium or plutonium) and causes it to split into two more smaller nuclei and 3 neutrons. Lots of energy is released after the large nucleus splits into two more.

the neutons that were released then go onto hit other large atomic nuclei and cuase them to split and the process repeats and repeats (a chain reaction). To stop a chain reaction from occuring and causing an explosion, the metal rods in the reactor, called control rods, stop this from happening. 

Nuclear Fusion is where two smaller nuclei join together to make one larger, heavier nucleus. These reactions happen in stars where two hydrogen nuclei fuse together under high temperatures and pressure to form a nucleus of a helium isotope. 

The issue whith fusion (and the reason we can't do it on Earth) is that it requires the fusing of two nuclei, both of which have positive charges, so will repel each other (just like magnets do) The process of the fusion has to happen quickly so that the repulsion of the charges does not havw time to stop it from happening. Another reason that it can't be done on Earth is that it requires very hot temperatures (150,000,000 degrees celcius). 

If we could do it on Earth, it would mean lots green energy for us, though it is not renewable

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