P4 Atomic Structure


History of the Atom

New experimental evidence may lead to a scientific model being changed or replaced - History

1) Everything was made of atoms and they could not be divided.

2) Electrons were discovered in the atom. (are not tiny spheres and do have internal structures)

3) Plum pudding model (ball of positive charge with electrons embedded into it)  ***Alpha scattering experiment took place (used gold because it could be hammered into the very thin layer). Fired alpha particles at the foil. Most passed through clearly, but some were deflected and some were reflected. (Bohr)

4) Atoms must be mainly empty space, as the alpha particles went straight through. The centre of the atom must be positive as the alpha particles were deflected. The mass of the atom must be concentrated in the centre as the alpha particles bounced straight back. (nuclear Model)

5) Electrons orbit the nucleus at specific distances (orbits/shells)

6) The positive charge in the nucleus is due to positive protons.

7) Neutrons were discovered in the nucleus.

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Atomic Structure

Nuclear model, meaning a nucleus in the centre surrounded by electrons.

The radius of the nucleus is less than 1/10,000 of the radius of the whole atom.

Nucleaus is made up of protons(+ charged) and neutrons (- charged).  So neuleaus is positively charged.

Electrons have a negative charge, they move around nucleus at different distances.  These distances are known as energy levels.  Energy levels that are further from the nucleus are at a higher energy than those which are closer to the nucleus.

If an atom absorbs electromagnetic radiation an electron can move from a lower energy level to a higher energy level. The atom then emits electromagnetic radiation and the electron returns back to the lower energy level.

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Isotopes are atoms of the same element which have the same number of protons but a different number of neutrons.

Protons are held together by the strong nuclear force. It is strong enough to hold the nucleus together in spite of the protons' electric force repelling them from each other.

The presence of neutrons affects the balance between these forces so too many or too few neutrons make the nucleus unstable. Unstable nuclei eventually decay which gives out energy and possibly alpha or beta particles.

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Ionising Radiation

Some isotopes have an unstable nucleus. to become stable it must give out radiation. (radioactive decay).  radioactive decay is totally random.

The activity is the ate at which a source of unstable nuclei decay. activity is measured in becquerel (Bq)  1Bq = 1 decay per second.

geiger muller tube used to measure the activity of a radioactive source. count rate is the number of decays recorded each second by the detector.

Alpha = Helium atom.

Beta = High-speed electron emitted from the nucleus. (neutron changes into a proton and an electron.)

Gamma rays = Electromagnetic radiation from the nucleus.


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Properties of Alpha, Beta and Gamma Radiation

When radiation collides with atoms, that can cause the atoms to lose electrons and form ions.

Alpha particles =

  • are large
  • can travel for 5cm before colliding with an air particle.
  • stopped by a single sheet of paper.
  • very strongly ionising (can form a lot of ions) and are dangerous if inhaled or swallowed.

Beta particles = 

  • can travel for 15cm in the air before colliding with an air particle.
  • stopped by a few mm of aluminium.
  • Quite strongly ionising (can form a few ions), can penetrate the skin.

Gamma radiation = 

  • travels several metres before it is stopped in air.
  • stopped by a few cms of lead.
  • Weakly ionising (forming very few ions), can pass into and out of the body easily.
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Half life

The half-life of a radioactive isotope is the time for the number of nuclei of the isotope in a sample to halve.

the half-life is also the time it takes for the count rate (or activity) from a sample containing the isotope to fall to half its initial level.

1000 = 20 minutes

500 = 20minutes

250 = 20 minutes

125 = 20 minutes

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Irradiation and contamination

Ionising radiation can increase the risk of cancer in humans, as it can damage cells.

Exposure to raidation is called irradiation, meaning radiation from source reaches an object.  Irradiated object will not become radioactive themselves.  This is because it comes in contact with the radiation, but not the radioactive isotope itself. 

But the the further you are from the object the less exposure. So

  • Store radioactive sources in lead-lined bxes
  • Alpha radiation can be stopped by wearing gloves.
  • beta and gamma radiation have a longer range so precaution should be taken to stay as far away as possible. B & G radiation can be stopped by wearing a lead apron and staying behind lead walls and glass. B & G radiation can travel through your body

Contamination is when unwanted radioactive isotopes end up on other materials. This is hazardous as the radioactive atoms decay and release radiation which could harm you

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