the atom

• made up of three subatomic particles - protons, neutrons, electrons
• protons

  heavy

  positively charged
• neutrons

  heavy

  neutral
• electrons

  hardly any mass

  negatively charged
• the nucleus is in the middle of the atom and contains protons and neutrons. it has a positive charge because of the protons.
• electrons move around the nucleus in electron shells. they are negatively charged. even though they are tiny, their shells cover a lot of space.
• in an atom no. of protons = no. of electrons
• atomic number

  tells you how many protons (so also electrons) an atom has
• mass number

  total no. of protons + neutrons.
• to work out the no. of neutrons in an atom subtract the atomic number from the mass number.