An element is a substance made up of atoms that all have the same number of protons in their nucleus

Elements consist of atoms with the same atomic number

• Atoms can have different numbers of protons, neutrons and electrons.

• It’s the number of protons in the nucleus that decides what type of atom it is

  • For example, an atom with one proton in its nucleus is hydrogen and an atom with two protons is helium.

    • There are about 100 different elements

• So all the atoms of a particular element have the same number of protons and different elements have atoms with different numbers of protons

Atoms can be represented by symbols

• Atoms of each element can be represented by a one or two letter symbol-it’s a type of shorthand that saves you the bother of having to write the full name of the element.

  • e.g. C=carbon, O-oxygen, Mg=magnesium

  • or e.g. Na=sodium, Fe=iron, Pb-lead

Isotopes are the same except for extra neutrons

• Isotopes are different forms of the same element, which have the same number of protons but a different number of neutrons

• So isotopes have the same atomic number but different mass numbers

• A very popular example of a pair of isotopes are carbon 12 and carbon 13

• Carbon 12:

  • 6 protons

  • 6 electrons

  • 6 neutrons

• Carbon 13

  • 6 protons

  • 6 electrons

  • 7 neutrons

• Because many elements can exist as a number of different isotopes, relative atomic mass is used instead of mass number when referring to the element as a whole.

  • This is an average mass taking into account the different masses and abundances of all the isotopes that make up the element

• You can use a formula to work out the relative atomic mass:

  • Relative atomic mass = sum of(isotopes abundance x isotope mass number) / sum of abundance of all the isotopes