Atomic Structure

• In the 5th Century BC, Democritus believed that all matter, whatever it was, was made up of identical lumps called 'atomos' which is where we get the word 'atom' from. 'Atomos' means uncuttable
• In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres ("atoms") that couldn't be broken up, but he reckoned that each element was made up of a different type of "atom"
• Nearly 100 years later, J.J. Thomson discovered particles called electrons that could be removed from atoms. So Dalton's theory wasn't quite right. Thomson suggested atoms where spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding - the plum pudding model.
• However, in 1909, scientists in Rutherford's lab tried firing a beam of alpha particles at thin gold foil - this was the alpha scattering experiment. From the plum pudding, they expected the particles to pass straight through the gold sheet, or only to be slightly detected. But although most of the particles did go straight though the sheet, some were deflected more than expected, and a few were deflected back they way they had come - something the plum pudding model could not explain
• Because a few alpha particles were deflected back, the scientists realised that most of the mass of the atom must be concentrated at the centre in a tiny nucleus. This nucleus must also have positive charge, since it repelled the positive alpha particles
• They also realised that because nearly all of the alpha particles passed straight though, most of the atom is just empty space. This was the first nuclear model of the atom.
• The nuclear model that resulted from the alpha particle scattering experiment was a positively charged nucleus surrounded by a cloud of negative electrons.
• Niels Bohr said that electrons orbiting the nucleus do so at certain distances called energy levels. His theoretical calculations agreed with experimentla data
• Evidence from further experiments changed the model to have a nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus
• About 20 years later after the idea of a nucleus was accepted, in 1932, James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers.
• Electrons occupy very small regions around the nucleus
• Protons are found inside the nucleus
• Neutrons are found inside the nucleus
• In the 5th century BC, Democritus believed that all matter was made up of millions of tiny, uncuttable pieces of that same matter. Atom comes from the word 'atomos' which means uncuttable
• In 1804, John Dalton agreed with Democritus that matter was made up of tiny spheres that couldn't be broken up, but he reckoned that each element was made up of a different type of atom
• In 1897, J.J. Thomson proposed the atom looked like a plum pudding (forming the plum pudding model) which suggested that atoms were spheres of positive charge with tiny negatively charged electrons stuck in them like fruit in a plum pudding - hence the name
• In 1909, the Alpha Particle Scattering Experiment was conducted by Ernest Rutherford, by firing alpha particles at a thin sheet of gold.
• The alpha particle scattering experiment - most alpha particles go straight though the gold sheet, but some were deflected more than expected, and a few were deflected back the way it came. Rutherford realised that most of the mass in an atom was concentrated in the centre, proving the existence of a nucleus. This nucleus was positively charged. He also realised that because most of the alpha particles went straight through, that most of an atom was just empty space.
• In 1913, Niels Bohr suggested that electrons orbited the nucleus at fixed differences called energy levels. Bohr's 'solar system' model is how most people think of atoms today
• In 1932, James Chadwick discovered the neutron, which was a particle that had a mass but no charge. This explained the imbalance between the atomic and mass numbers
• Currently:

  • The nucleus is tiny but makes up most of the atom's mass
  • The nucleus contains protons and neutrons, with electrons orbiting at fixed distances - energy levels
  • The radius of a proton is 10,000 times smaller than the radius of an atom
  • The radius of an atom is 1 x 10^-10
• If energy is gained by absorbing electromagnetic radiation, electrons move to a higher energy level which is further away from the nucleus. If energy is lost by the emission of electromagnetic radiation, electrons move to a lower energy level which is closer to the nucleus
• Relative charge of a proton is +1
• Relative charge of a neutron is 0
• Relative charge of an electron is -1
• Relative mass of a proton is 1
• Relative mass of a neutron is 1
• Relative mass of an electron is almost 0 (1/2000)
• Isotopes of an element are atoms with the same amount of protons but a different amount of neutrons
• Mass number is the number of protons + the number of neutrons
• Atomic number is just the number of protons
• Number of protons = number of electrons
• All elements have different isotopes, but there are usually only 1 or 2 stable ones. The other unstable isotopes tend to decay into other elements and give out radiation as they try to become stable. This is called radioactive decay
• Radioactive substances spit out one or more types of ionising radiation from their nucleus - alpha, beta and gamma. They can also release neutrons when they decay as they rebalance their atomic and mass numbers
• Ionising radiation is radiation that knocks off electrons from atoms, creating positive ions. The ionising power of a radiation source is how easily it can do this
• Alpha particles are helium nuclei
• Beta particles are high speed electrons
• Gamma rays are waves of electromagnetic radiation with a short wavelength
• Alpha particles are weakly penetrating
• Beta particles are moderately penetrating