Principles of Chemistry
7 marks · EasyThis question is about isotopes and interpreting diagrams of electron configurations.
The diagram below shows the electronic configuration of atom X: First shell: 2 electrons Second shell: 8 electrons Third shell: 7 electrons State the number of electrons in each shell for atom X.
What is the total number of electrons in atom X?
Using your answer to part (b), state the group and period of atom X in the Periodic Table.
Two isotopes of atom X have mass numbers 35 and 37. Explain how these isotopes are similar and how they are different.
If a sample of atom X contains 75% of the isotope with mass number 35 and 25% of the isotope with mass number 37, which isotope is more abundant? Give a reason.
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) First shell: 2, second shell: 8, third shell: 7. · must show 2, 8, 7
(b) 17 · must show 17
(c) Group 7, Period 3 · must show group 7, period 3
(d) Both isotopes have the same number of protons and electrons (17 each), so the same chemical properties. They differ in the number of neutrons: one has 18, the other has 20. · must show same number of protons/electrons, different number of neutrons
(e) The isotope with mass number 35 is more abundant, because 75% is greater than 25%. · must show isotope 35, more abundant
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Marking points
B1 (a) · 1 mark · States correct number of electrons in each shell: 2, 8, 7.
A1 (b) · 1 mark · Adds up to total of 17 electrons.
B1 (c) · 1 mark · Identifies group 7 (7 outer electrons), or allows correct answer from context.
B1 (c) · 1 mark · Identifies period 3 (3 shells).
B1 (d) · 1 mark · Explains similarity: same number of protons/electrons.
B1 (d) · 1 mark · Explains difference: different number of neutrons.
B1 (e) · 1 mark · Identifies isotope 35 as most abundant and gives correct reasoning from data.
Worked solution
a) First shell: 2, second shell: 8, third shell: 7.
b) Total electrons = 2 + 8 + 7 = 17.
c) 7 electrons in outer shell → Group 7; three shells used → Period 3.
d) Both have same number of protons and electrons, so the same chemical properties. They differ in number of neutrons: one (mass number 35) has 18 neutrons (35-17), one (mass number 37) has 20 neutrons (37-17).
e) The isotope with mass number 35 is more abundant because 75% > 25%.