Atomic Mass

Question 1

What atom do we compare other atoms to to find their mass?

Answer 1

Carbon-12

Question 2

Define relative isotopic mass

Answer 2

The mass of an atom of an isotope compared with 1/12th of the mass of an atom of carbon-12.

Question 3

Define relative atomic mass

Answer 3

The weighted average mass of an atom of an element compared with 1/12th the mass of an atom of carbon-12.

Question 4

How do you measure relative atomic masses?

Answer 4

Using a mass spectrometer.
- Sample of element placed in mass spectrometer and vaporised.
- The sample is bombarded with electrons forming positive ions.
- The positive ions are accelerated using an electric field.
- The positive ions deflected using a magnetic field.
- Ions of lighter isotopes deflected more than ions of heavier isotopes. This separates the isotopes.
- Ions are detected to produce a mass spectrum.

Question 5

What does a mass spectrum provide?

Answer 5

• the relative isotopic masses of the isotopes in an element
• isotopic abundances

Question 6

Define relative molecular mass (Mr)

Answer 6

The weighted mean mass of a molecule of a compound compared with 1/12th the mass of an atom of carbon-12.

Question 7

Define relative formula mass

Answer 7

Used when referring to substances with giant structures and is the sum of the atomic masses of all elements in the formula of that substance.
e.g. CuSO4.H2O