Chemistry

Chemistry

1. Atoms and their structure

   • At the centre of an atom is a nucleus containing protons and neutrons. Electrons are arranged around the nucleus in energy levels or shells. Make sure you can label a simple diagram of an atom like this one.

2. Mass

   • Protons have a relative mass of 1 Electrons have a relative mass of (1/1836) Neutrons have a relative mass of 1
   • Mass number and atomic number The mass number of an atom is never smaller than the atomic number. It can be the same, but is usually bigger: The mass number of an atom is the total number of protons and neutrons it contains The atomic number of an atom is the number of protons it contains Notice that most of the mass of an atom is found in the nucleus.

3. Isotopes

   • Isotopes are atoms of an element with the normal number of protons and electrons, but different numbers of neutrons. Isotopes have the same atomic number, but different mass numbers

4. Chemical equations

   • • In chemical formulas the number of atoms in a molecule is written as a subscript, so we write H2O for water which has two atoms of hydrogen for each one of oxygen.
   • the coefficients give the number of molecules (or atoms) involved in the reaction.

5. Mixtures

   • A mixture is a combination of two or more pure substances in which each pure substance retains its individual chemical properties. Mixtures can be composed of solids, liquids, or gases.

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