Isotopes

1. Isotopes Isotopes are atoms of the same element that contain different numbers of neutrons. Copper has 2 isotopes. (copper – 63 , copper – 65) Aluminium has 1 isotope (aluminium – 27) Carbon has 3 isotopes (carbon -12, carbon – 13, carbon – 14)

2. Mass Spectrometer • The mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules.

3. Stage 1: Ionisation • The atom is ionised by knocking one or more electrons off to give a positive ion. Stage 2: Acceleration • The ions are accelerated so that they all have the same kinetic energy. Stage 3: Deflection • Different ions are deflected by the magnetic field by different amounts. The amount of deflection depends on: -the mass of the ion. Lighter ions are deflected more than heavier ones. -the charge on the ion. Ions with 2 (or more) positive charges are deflected more than ones with only 1 positive charge. • Stage 4: Detection • The beam of ions passing through the machine is detected electrically.

6. What the mass spectrometer output looks like • The stick diagram for molybdenum looks like this: • That means that molybdenum consists of 7 different isotopes and their masses of the 7 isotopes on the carbon-12 scale are 92, 94, 95, 96, 97, 98 and 100.The commonest ion has a mass/charge ratio of 98.

7. R.A.M • Relative Atomic Mass( Ar) • The relative atomic mass of an element is the average mass of the isotopes in a naturally occurring sample of the element, taking into account the proportion of each isotope present.

8. Practise • Calculating Relative Atomic Mass • Naturally occurring silver is 51.84% silver-107 and 48.16% silver-109. • Calculate the relative atomic mass of silver.

9. Answer • R.A.M. (Ag) = (51.84/100 x 107) + (48.16/100 x 109) • = 55.469 + 52.494 • = 107.96