Atomic Structure
6 marks · NormalChlorine exists in nature as two main isotopes: and .
Define the term isotope.
Explain the difference between an atom and a molecule, using chlorine as an example.
Given that the relative abundances of and are 75% and 25% respectively, calculate the relative atomic mass of chlorine. Show your working.
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(a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
(b) An atom is a single particle of an element (e.g. one atom), while a molecule is made of two or more atoms bonded (e.g. a chlorine molecule contains two chlorine atoms bonded together).
(c) 35.5
Also accept: Alternative phrasing for atom/molecule and for isotope.
Common mistakes: Confusing mass number with atomic number; Stating molecules as single atoms; Incorrect percent decimals in calculation
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Marking points
B1 (a) · 1 mark · States isotopes have same number of protons.
B2 (a) · 1 mark · States isotopes have different numbers of neutrons.
B3 (b) · 1 mark · Correct statement of atom as single particle.
B4 (b) · 1 mark · Correct example given for molecule.
M1 (c) · 1 mark · Writes correct formula/method for relative atomic mass calculation.
A1 (c) · 1 mark · Correctly calculates the relative atomic mass (35.5).
Worked solution
a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
b) An atom is a single particle of an element, like one chlorine atom (). A molecule is two or more atoms chemically bonded together, for example, a chlorine molecule () which has two chlorine atoms joined.
c) Relative atomic mass = (75/100 35) + (25/100 37) = 26.25 + 9.25 = 35.5