Atomic number and mass number
5 marks · HardThe element chlorine occurs naturally as two main isotopes: chlorine-35 and chlorine-37. The relative abundances of these isotopes are 75% chlorine-35 and 25% chlorine-37.
Define the term 'isotope'.
State the atomic number of chlorine.
Explain, using your knowledge of atomic structure, why chlorine-35 and chlorine-37 are isotopes of the same element.
Calculate the relative atomic mass of chlorine to 1 decimal place. Show your working.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Atoms of the same element with the same number of protons but different numbers of neutrons.
(b) 17
(c) They have the same number of protons (atomic number 17) but different numbers of neutrons.
(d) Relative atomic mass = (35 × 75/100) + (37 × 25/100) = 26.25 + 9.25 = 35.5 · must show Working must be shown for calculation
Also accept: Alternative ways of writing the calculation are accepted.
Common mistakes: Mixing up mass number and atomic number; Incorrectly calculating the weighted mean; Defining isotope as different element
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Marking points
B1 (a) · 1 mark · Correct definition of isotope: atoms of the same element with the same number of protons but different numbers of neutrons.
B2 (b) · 1 mark · Correct atomic number for chlorine: 17
B3 (c) · 1 mark · States correct reason: same number of protons, different numbers of neutrons.
M1 (d) · 1 mark · Shows correct use of the weighted mean formula with correct numbers or method shown.
A1 (d) · 1 mark · Correct answer to 1 decimal place: 35.5 (follow-through)
Worked solution
a) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.
b) The atomic number of chlorine is 17.
c) Chlorine-35 and chlorine-37 both have 17 protons, so are atoms of the same element. They differ only in the number of neutrons (Cl-35 has 18, Cl-37 has 20).
d) Relative atomic mass = ((35 × 75) + (37 × 25)) / 100 = (2625 + 925) / 100 = 3550 / 100 = 35.5