Principles of Chemistry
7 marks · EasyThis question is about atomic structure and drawing diagrams of electron configuration.
Define the term isotope.
Chlorine has two naturally occurring isotopes: 35Cl (75% abundance) and 37Cl (25% abundance). Calculate the relative atomic mass (A_r) of chlorine. Show your working.
Draw a diagram to show the electronic configuration of a chlorine atom (atomic number 17), using shells and crosses (×) for electrons.
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) Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. · must show same atomic number, different number of neutrons
(b) Relative atomic mass = (75 × 35 + 25 × 37) / 100 = 35.5 · must show correct set up: (75×35 + 25×37)/100, correct answer: 35.5
(c) — · must show nucleus (labelled, optional), three shells drawn, 2 electrons in first shell (crosses), 8 electrons in second shell (crosses), 7 electrons in third shell (crosses)
Common mistakes: Mixing up abundance percentages
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Marking points
B1 (a) · 1 mark · Defines isotopes as atoms of the same element with the same atomic number, different numbers of neutrons.
M1 (b) · 1 mark · Sets up correct calculation: (75×35 + 25×37)/100.
M1 (b) · 1 mark · Multiplies abundances and isotopic masses correctly. (follow-through)
A1 (b) · 1 mark · Correctly calculates Ar as 35.5 (to 1 decimal place). (follow-through)
B1 (c) · 1 mark · Draws three shells (concentric circles).
B1 (c) · 1 mark · Places 2 electrons in the first shell, 8 in the second, 7 in the third, using × for electrons.
B1 (c) · 1 mark · Diagram is clear and follows correct layout of electron shells (accept dot or cross).
Worked solution
a) Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.
b) The relative atomic mass of chlorine is 35.5.
c) Diagram should show: first shell (2 ×), second shell (8 ×), third shell (7 ×) spaced around, using crosses.