Atoms

• all substances are made of atoms. They're extremely small and can't be seen without a microscope. They have a radius of 0.1 nanometres
• Nucleus:

  • in the middle of the atom
  • contains protons and neutrons
  • positive charge
  • almost the whole mass of the atom is in the nucleus
• Electrons:

  • move around the nucleus in electron shells
  • negatively charged and very small
  • cover a lot of space
  • has virtually zero mass
• Number of protons equals number of electrons:

  1. Atoms are neutral- they have no charge overall
  2. This is because they have the same number of protons and electrons
  3. The charge on the electrons is the same size as the charge on the protons but opposite so charges cancel out
• Atomic number and mass number describe an atom:

  1. The nuclear symbol of an atom tells you its atomic (proton) number and mass number
  2. The atomic number tells you how many protons there are
  3. The mass number tells you the total number of protons and neutrons in the atom
  4. To get the number of neutrons, just subtract the atomic number from the mass number