Topic 4: Atomic structure

• Protons and neutrons

  In the nucleus of an atom
• Number of protons

  Determines the amount of positive charge in the nucleus
• The radius of an atom is about 0.1 nanometres or 1 x 10^(-10) m
• The radius of a nucleus is less than 1/10 000 of that of the atom, about 1 x 10^(-14) m
• Almost all of the mass of an atom is in the nucleus
• Protons and neutrons

  Have a relative mass of 1
• Electrons

  Have a very small relative mass
• Protons

  Have a relative charge of +1 (positive)
• Neutrons

  Have a relative charge of 0 (neutral)
• Electrons

  Have a relative charge of -1 (negative)
• Atoms have no overall charge
• In an atom, the number of protons equals the number of electrons, so the positive charges of the protons are cancelled by the negative charges of the electrons
• Electrons

  Orbit the nucleus in energy levels at specific distances
• Energy levels

  Further from the nucleus are at a higher energy than those which are closer to the nucleus
• Electrons changing energy levels

  Gain or lose energy
• Absorbing electromagnetic radiation

  Electron moves from a lower energy level to a higher energy level
• Emitting electromagnetic radiation

  Electron returns back to the lower energy level
• Mass number

  Number of protons and neutrons added together of atoms in that element
• Atomic number

  Number of protons in atoms of that element
• Isotope

  Atom of the same element with the same number of protons but different number of neutrons
• Ion

  Atom that can lose electrons from their outer energy level
• Ion with 1+ charge

  Atom has lost one electron, so there is one more proton than there are electrons
• Ion with 2+ charge

  Atom has lost two electrons, so there are two more protons than there are electrons
• Before the discovery of the electron, atoms were thought to be tiny spheres that could not be divided
• In 1897 scientists discovered that atoms have an internal structure as they contained tiny negative particles called electrons
• Plum pudding model

  Atom as a ball of positive charge with negative electrons embedded in it
• Alpha particle scattering experiment

  Firing tiny positive alpha particles at a sheet of very thin gold foil
• The alpha particle scattering experiment showed that atoms are mainly empty space because many alpha particles passed straight through the gold foil
• The alpha particle scattering experiment showed that the centre of the atom (nucleus) must have a positive charge because some alpha particles were deflected
• Alpha particles repel when they come close to the centre of an atom (nucleus) because they are both positively charged and like charges repel
• The alpha particle scattering experiment showed that most of the mass is concentrated at the centre of the atom (nucleus) because some alpha particles bounced straight back
• The results of the alpha particle scattering experiment meant that the nuclear model replaced the plum pudding model of the atom
• Nuclear model

  Most of the atom is empty space, there is a tiny positive nucleus in the centre of the atom containing most of the mass of the atom
• The nuclear model was adapted by Bohr suggesting that negative electrons orbit the nucleus at specific distances in energy levels
• After Bohr's discovery, Chadwick discovered that the nucleus also contains neutral neutrons
• Unstable nucleus

  To become stable, the nucleus gives out radiation, and this is called radioactive decay
• Radioactive decay is a totally random process, it cannot be predicted
• Activity

  The rate at which a source of unstable nuclei decays
• Activity

  Measured in becquerel (Bq). 1 Bq = 1 decay per second
• Geiger-Muller tube

  Used to measure activity