chemistry revision chapter 1 pt1

chemistry revision chapter 1 pt1

1. Elements and compounds

   • An element is a substance that cannot be broken down chemically
   • A compound is a substance that contains at least two different elements, chemically combined in fixed proportions.
   • A chemical reaction is needed to make an element into a compound
   • Chemical reactions always involve the formation of one or more new substances.

2. Atoms, formulae and equations

   • Elements are made up of atoms that are all the same
   • If two or more atoms join together by sharing their electrons the atoms form a molecule.
   • You cna see which elements are in a compound by looking at its formula

3. Mixtures

   • Mixtures can easily be separated because the chemicals in them are not joined together
   • mixtures can be separated by filtration, crystallisation, simple distillation, fractional distillation and chromatography
   • no new substances are made from the separation of mixtures.
   • A mixture consists of 2 or more elements or compounds not chemically combined together

4. changing ideas about atoms

   • 400 BC Democritus gave the idea of the atom with no proof.
   • 1803 John Dalton used his investigations to describe the atom in further detail.
   • 1879 J.J. Thomson discovered the electron and so using the plum pudding model
   • 1909 Geiger and Marsden had surprising results with the gold leaf and alpha particles which resulted in Marsden and Rutherford said that atoms has a nucleus.
   • 1913 Niels Bohr discovered the nuclear model which is the model we use today but it has been adjusted since then
   • 1932 James Chadwick found the neutron which finishes the Nuclear Model

5. Modelling the atom

   • Individual atoms are very small.
   • An atom is made up of a nucleus that is surrounded by electrons. the nucleus carries the positive charge. electrons, which surround the nucleus, each carry a negative charge
   • The diameter of an atom is about 0.0000001mm.
   • the RADIUS of an atom is 1 X 10 to the minus power 10 meters

6. relating charges and masses

   • An atom always has the same number of protons as electrons so the atom is always neutral
   • if an atom losses electrons and becomes charged, this charged particle is called a positive ion
     • If an atom GAINS electrons and becomes charged, this charged particle is called a negative ion