Oxygen isotopes and lakes

Isotopes are variations of a particular chemical element. It is all to do with the number of neutrons. Oxygen has two main isotopes: 18O which has 10 neutrons and 8 protons; and 16O which has 8 neutrons and 8 protons. Although these variants have a different number of neutrons (and therefore a different atomic mass), the number of protons remains the same, and they are still classed as the same element. Isotopes are analysed in terms of ratios such as 18O/16O which is shortened to δ18O (δ is the symbol for ‘delta’ and means ‘change’) and are measured in parts per thousand or ‘per mil’ (‰). 18O is said to be ‘heavier’ than 16O, because it is carrying more neutrons. The greater the number of neutrons, the heavier the isotope.