Relative Atomic Mass. Isotopes.

Question 1

Define isotopes.

Answer 1

Isotopes are atoms of the same element, (i.e. they have the same atomic number) which have different mass numbers due to the different number of neutrons in the nucleus

Question 2

What did Francis William Aston build?

Answer 2

He built an instrument called a mass spectrometer to measure the masses of atoms

Question 3

Summarise Aston’s work (3)

Answer 3

1. He found that a sample of neon gas constricted of two varieties of neon atoms.
2. He concluded that the neon gas constricted of atoms of neon that differed in the number of neutrons in the nucleus.
3. These two varieties of neon atoms are said to be isotopes of neon.

Question 4

How is the average mass of an atom measured?

Answer 4

It is measured relative to the mass of the carbon-12 isotope.
For this reason it is called its relative atomic mass.
The symbol for relative atomic mass is A.

Question 5

Define relative atomic mass.

4

Answer 5

The average of the mass numbers of the isotopes of the element,

as they occur naturally

Taking their abundances into account and

expressed on a scale in which the atoms of the carbon-12 isotope have a mass of exactly 12 units

Question 6

A mass spectrometer is used to analyse a sample of boron. It is found that the boron sample contains 18.7% 10B5 and 81.3% 11B5.
Calculate the relative atomic mass of boron.

Answer 6

In 100 atoms of boron these are
18.7 atoms of mass 10 = 18.7 x 10 = 187
81.3 atoms of mass 11 = 81.3 x 11 = 894.3
———————————————————–
Total mass of 100 atoms = 1081.3
Average mass of 1 atom = 10.813
i.e. relative atomic mass of boron = 10.813