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  • Nucleus = protons (positive) + neutrons (no charge)
  • Nucleus surrounded by electrons (negative). Atom has no overall charge.
  • Isotopes = atoms of the same element with different number of neutrons.
  • Atomic number = number of protons and neutrons in an atom.
  • Atoms arranged in periodic table in groups and periods.

Alkali metals = 1 electron in the outer shell.

Halogens = 7 electrons in the outer shell.

When atoms form chemical bonds, their outer shell of electrons changes.

Key words - atomic number, proton, neutron, electron, alkali, isotopes, chemical bonds.

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Ionic and Covalent Bonding

  • Ionic compounds = giant structures held together by strong forces of attraction between oppositely charged ions.
  • Ionic bonds ------> metal and no-metal atoms ------> electrons are transferred
  • Covalent bonds ------> non-metal atoms ------> electrons are shared

Giant covalent structures = strong, covalent bonds and high melting points, e.g. diamond, graphite, silicon dioxide.


Metals can conduct heat and electricity because they have delocalised electrons.


Compound = two or more elements chemically bonded together.

Mixture = two or more elements or compounds not chemically bonded together.

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