20. Naturally occurring bromine has two isotopes: bromine-79 and bromine-81.
(a) State what is meant by the term isotopes.
(1)
ANSWER
Isotopes are atoms of the same element that have the same number of protons (i.e., atomic number, "Z") but a different number of neutrons, meaning that their mass number, "A", varies.
(b) Complete the table to show the numbers of subatomic particles in the two isotopes of bromine.
(2)
ANSWER
(c) The mass spectrum of a sample of bromine is obtained. (i) Draw a dot-and-cross diagram to show the bonding in a molecule of bromine. Only the outer electrons should be shown.
(2)
ANSWER
(ii) Describe the formation of the molecular ion of bromine in the mass spectrometer. Include an equation. State symbols are not required.
(2)
ANSWER
Beam of high energy electrons is made to strike the gaseous bromine molecule.
The equation is as followed:
Br2→ Br2 + e (−)
(iii) On the mass spectrum grid, draw the peaks for the bromine molecular ions, showing the relative peak heights. The bromine isotopes in this sample have the same relative abundance.
(2)
ANSWER
The choices we have are as follows:
79Br 79Br
81Br 81Br
79Br 81Br
81Br 79Br
(d) The percentage abundances of the isotopes in a different sample of bromine are shown.
Calculate the relative molecular mass of this sample of bromine, giving your answer to two decimal places.
(3)
ANSWER
