Atoms

• The atom is made up of three subatomic particles - protons, neutrons and electrons.
• Protons are heavy and positively charged.
• Neutron are heavy and neutral.
• Electrons have hardly any mass and are negatively charged.
• The Nucleus

  • It's in the middle of the atom.
  • It contains protons and neutrons.
  • It has a positive charge because of the protons
  • Almost the whole mass of the atom is concentrated in the nucleus.
  • Compared to the overall size of the atom, the nucleus in tiny.
• The Electrons

  • Electrons move around the nucleus in energy levels called shells.
  • They're negatively charged.
  • They're tiny, but their orbitals cover a lot of space.
  • The size of their orbitals determines the size of the atom.
  • Electrons have virtually no mass.
• Number of Electrons equals Number of Protons.
• Neutral atoms have no charge overall.
• The charge on the electrons is the same size as the charge on the protons - but opposite.

  This means the number of electrons always equals the number of protons in a neutral atom.
• If some electrons are added or removed, the atom becomes charged and is then an ion.
• Atomic Number and Mass Number describe an Atom
• The atomic number tells you how many protons there are.
• Atoms of the same element all have the same number of protons - so atoms of different elements will have different numbers of protons.
• The mass number is the total of protons and neutrons in the atom.
• To get the number of neutrons, just subtract the atomic number from the mass number.
• Molecules are groups of atoms.
• Atoms can join together to form molecules.
• Some molecules are made from just one element, while others are made up of more than one element.