Isotopes, atomic mass and mass spectronomy

Question 1

Isotopes and Carbon 12

Answer 1

• All elements have atoms of different masses due to having different numbers of neutrons. These are called isotopes of the element and their mass is called their isotopic mass.
• To measure the mass of an atom (or isotope) it is compared to a standard (just as we compare all lengths to the metre).
• This is carbon-12, the most common isotope of carbon that has a mass of 12 units exactly. The masses of isotopes are quoted relative to this standard.

Question 2

Relative isotopic mass

Answer 2

• The relative isotopic mass (Ir) of an isotope is the mass of an atom of that isotope relative to the mass of an atom of carbon-12 taken as 12 units exactly.
  eg. Helium has 2 protons and 2 neutrons, meaning that it has an atomic mass of 4. This is compared to carbon-12 so its approximate mass relative to a carbon-12 atom is 4/12 of 12 = 4.

Question 3

The mass spectrometer

Answer 3

• The mass spectrometer is an instrument that not only provides us with information about the relative isotopic mass of each isotope but also how much (percentage abundance) of that isotope is in a sample of the element.
• This composition is the same no matter what the source of the element.

Question 4

How the mass spectrometer works

Answer 4

• Gaseous sample is bombarded with electrons making mainly singly postively charged ions.
• Positive ions are then accelerated at high speeds by an electric field.
• 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.

Question 5

The mass spectrum

Answer 5

• A mass spectrometer produces a mass spectrum, which is like a graph showing the relative isotopic masses of an isotope against its relative abundance.

Question 6

Calculating Relative Atomic Mass

Answer 6

• The relative atomic mass (Ar) of an element represents the average mass of one atom, taking into account the number of isotopes, their relative isotopic mass and their relative abundance.
• To calculate this, the relative isotopic mass is multiplied by its relative abundance and is added to the mass by abundance of the other existing isotopes of that element, then everything is divided by 100.

Question 7

Calculating relative atomic mass- chlorine example

Answer 7

Ar = 34.969 x 75.80 + 36.966 x 24.20/100

 = 35.452

• This represents the average mass of one atom. Because it is relative to the mass of the carbon-12 isotope it does not have any units.
• Refer to mass spectrum graph