Chemistry revision

• As the table above shows, effectively all the mass of the atom comes from the protons and neutrons, so the sum of the number of protons and the number of neutrons gives the mass number of an atom. The number of protons in an atom is called the proton number or atomic number. We represent the proton number and mass number of an atom numerically. 
• the proton number equals the electron number. Note that (unless there are no neutrons in the nucleus) the mass number is always larger than the proton number, as it combines the number of protons and the number of neutrons. We can therefore calculate the number of protons, neutrons and electrons in any atom if we know the proton number and mass number.
• We have seen how the nucleus of an atom contains protons and neutrons, and that the nuclei of different atoms have different numbers of protons and neutrons. The differences in proton number (also called atomic number) result in the different elements, and the differences in neutron number result in the different isotopes of elements. The number of electrons in the outer shell is equal to the number of protons.
• There are roughly 100 different types of atom, called the elements. The proton number of an atom determines which element it is. If two atoms have the same proton number, they are atoms of the same element. If two atoms have different proton numbers, they are atoms of different elements. We represent elements with a chemical symbol, either a capital letter, or a capital and lowercase letter.
• The elemental chemical symbol typically accompanies the mass number and proton number when elements are displayed. Usually the mass number is displayed above the proton number, but if in doubt remember that the mass number (unless there are no neutrons in the nucleus) will be larger than the proton number. 

  
• Atoms of the same element (having the same proton number) can have different numbers of neutrons – we call these isotopes. This means that the atomic masses of isotopes differ, which also causes their physical properties to be slightly different. As elements have the same number of electrons as isotopes of the same the element, the chemical properties of the isotopes of an element are the same as those of the element. Elements can have multiple isotopes, but the abundance of each isotope (the proportion of all the atoms of that element that exist as each isotope) varies.

  
• In order to make certain chemical calculations, we need to know the atomic mass of the elements involved, but isotopes complicate this. The solution is to use relative atomic mass – an averaged value of atomic mass which incorporates both the abundance and mass number of all known isotopes. The relative atomic mass of an element is referred to as the element’s Ar, and can be calculated as follows:=

  Relative Atomic Mass=(% Isotope 1×Mass Isotope 1)+(% Isotope 2×Mass of Isotope 2)+(% Isotope 3×Mass of Isotope 3)⋯Sum of All Isotope AbundancesSum of All Isotope Abundances
• the mass of the reactants is equal to the mass of the products. This is called conservation of mass.