Isotopes

Isotopes of Chlorine

Chlorine atoms contain 17 protons and 17 electrons. About 75 per cent of chlorine atoms have 18 neutrons, while about 25 per cent have 20 neutrons.

Calculating relative atomic mass - Higher tier

Chlorine's Ar of 35.5 is an average of the masses of the different isotopes of chlorine. This is calculated by working out the relative abundance of each isotope. For example, in any sample of Chlorine 25% will be 37Cl and 75% 35Cl. The relative atomic mass is therefore calculated using the equation: (% of isotope 1 × mass of isotope 1) + (% of isotope 2 × mass of isotope 2) ÷ 100 So in the case of chlorine: (75 × 35) + (25 × 37)∕100 = 2625 + 925∕100 = 35.3

Relative atomic mass

Different atoms have different masses. Atoms have such a small mass it is more convenient to know their masses compared to each other. Carbon is taken as the standard atom and has a relative atomic mass (Ar) of 12. Atoms with an Ar of less than this have a smaller mass than a carbon atom. Atoms with an Ar which is more than this have a larger mass than a carbon atom. The table below shows some Ar values (you do not need to remember them: you will be given them in the exam if you need them to answer a question).

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. The different isotopes of an element have identical chemical properties.

Isotopes of Hydrogen

Most hydrogen atoms consist of just one proton and one electron, but some also have one or two neutrons.

Ar values of elements

These values tell you that magnesium atoms are twice as heavy as carbon atoms, and 24 times heavier than hydrogen atoms, while hydrogen atoms are 12 times lighter than carbon atoms. They also allow you to work out that three oxygen atoms weigh the same as two magnesium atoms.