Atomic Structure

• How the model of the atom has changed overtime:

  1804 - Dalton stated that atoms were tiny spheres that could not be divided.

  1877 - JJ Thompson discovered the electron and the Plum Pudding Model was created. This was a ball of positive charge (protons) with electrons dispersed throughout it.

  1909 - Rutherford carried out the alpha particle scattering experiment. He discovered most of the atom was empty space. This led to the Rutherford Model in 1913. There was a positive nucleus at the centre of the electron and a cloud of electrons surrounding it.
• How the model of the atom has changed overtime:

  1913 - Bohr created the final model of the atom. If Rutherford had been correct, the cloud of electrons would get attracted to the nucleus and the atom would collapse. Therefore, electrons existed in "energy levels". It was later discovered that the nucleus could be divided into smaller particles with the same amount of positive charge (protons). James Chadwick provided evidence 20 years later to prove the existence of neutrons (neutral charge).
• Gold Foil Experiment

  Most 𝛼 particles went straight through the gold sheets - So most of atom is empty space

  Some 𝛼 particles were slightly deflected - So nucleus must be charged, deflecting positive 𝛼

  Few 𝛼 particles were deflected by >90° - So nucleus contained most of the mass
• Current model of the atom

  • Tiny nucleus that makes up most mass
  • Nucleus contains protons and neutrons
  • The rest of the atom is just empty space
  • Electrons whizz around the outside of the nucleus
  • Number of protons = number of electrons
  • Atoms have no overall charge
• Electrons in energy levels can move within or leave the atom. If they gain energy by adsorbing EM radiation, they move to a higher energy level (further away from the nucleus). If they release EM radiation they move to a lower energy level (closer to the nucleus). If outer electrons leave atom, the atoms becomes a positively charged ion.
• Atomic number = number of protons

  Mass number = number of protons + neutrons
• Isotopes are atoms of the same element with the same number of protons but different number of neutrons.
• Isotopes

  • All elements have isotopes but only one or two stable ones.
  • Unstable isotopes tend to decay into other elements and give out radiation as they try to become stable (try to balance protons and neutrons). This is called radioactive decay.
  • Radioactive substances release ionisng radiation from their nucleus. Alpha, beta and gamma.
  • They can also release neutrons to rebalance.
• Ionising power: the ability of each type of radiation to convert an atom to an ion by adding or removing electrons.

  Penetrating power: the ability of each type of radiation to pass through matter.
• Alpha


  • symbol = 𝛼
  • An alpha particle is two neutrons and two protons (helium nucleus)
  • Don't penetrate far into materials
  • Stopped quickly
  • Can only travel a few cm in air
  • Absorbed by a sheet of paper
  • Strongly ionising
• Beta

  • symbol = β
  • A fast moving electron released by the nucleus
  • Beta particles have no mass and -1 charge
  • Moderately ionising and moderately penetrating
  • Range of few meters in air
  • Adsorbed by a sheet of aluminum
  • For every beta particle, a proton has been turned to a neutron
• Gamma

  • symbol = γ
  • Waves of electromagnetic radiation
  • Penetrate far into materials
  • Travel a long distance in air
  • Weakly ionising as they pass through atoms
  • Adsorbed by thick sheets of lead or concrete
• Changes in the nucleus - Alpha

  • Nucleus loses 2 protons and 2 neutrons
  • Atomic number decreases by 2
  • Mass number decreases by 4
  • A new element is formed
• Changes in the nucleus - Beta

  • Electron emitted and a
  • Neutron changes into a proton
  • Atomic number increases by 1
  • Mass number stays the same
  • No new element is formed
• Radioactivity can be detected with a Geiger Counter.

  A radiometer gives a reading in counts per second.
• Half-life is the time it takes half the atoms in a sample to decay.
• Sources of radiation:

  • Radon gas
  • Food
  • Medical tests (ultrasound, MRI)
  • Cosmic rays (sun, uv rays)
  • Nuclear tests
  • Rocks and soil
• Risks from radiation:

  LOW DOSES:

  • minor damage to cells
  • can cause mutations leading to cancer

  HIGH DOSES:

  • tends to kill cells
  • causes radiation sickness and death
• Effect of radiation on body

  • Eyes: cataracts
  • Thyroid: build up causes cancer
  • Lungs: breathing in damages DNA
  • Stomach: isotopes sit and radiate
  • Reproductive organs: infertility or mutations
  • Skin: burns or cancer
  • Bone marrow: leukemia or other blood diseases
• Irradiation:


  • When an object is exposed to radiation on the outside
  • Doesn't cause the object to become radioactive
  • Can be blocked by suitable shield
  • Stops as soon as source is removed
• Contamination:

  • If the radioactive source is on or in the object
  • Lasts as long as the source is on or in it
  • Once contaminated, radiation can not be blocked from the object
  • Can be difficult to remove all the radiation
• Seriousness of irradiation and contamination

  • Beta and gamma are the most dangerous (can penetrate the body)
  • Alpha is less dangerous as it can't penetrate the skin
  • High levels of irradiation are dangerous
  • Inside the body, alpha is the most dangerous
  • Contamination is a major concern with alpha
  • Beta and gamma is less damaging