Atomic Structure
5 marks · EasyTwo atoms of lithium are shown below.
| Isotope | Mass number | Atomic number | Number of neutrons |
|---|---|---|---|
| 6 | 3 | ||
| 7 | 3 |
Complete the table by calculating the number of neutrons in each isotope.
Describe one way in which the isotopes of lithium are the same, and one way in which they are different.
State the number of electrons in a neutral atom of .
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) ^{6}\mathrm{Li}: 3 neutrons; ^{7}\mathrm{Li}: 4 neutrons · must show 6Li: 3 neutrons, 7Li: 4 neutrons
(b) Same: same number of protons / atomic number / electrons. Different: different mass number or different number of neutrons. · must show One correct similarity, One correct difference
(c) 3 · must show Number of electrons equals atomic number
Also accept: Alternative wording for how isotopes are same/different
Common mistakes: Getting number of neutrons incorrect; Mixing up electrons and neutrons; Describing chemical rather than nuclear similarity
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Marking points
A1 (a) · 1 mark · Correct number of neutrons for (3)
A2 (a) · 1 mark · Correct number of neutrons for (4)
B1 (b) · 1 mark · States a correct similarity (e.g., same number of protons, electrons, or same chemical properties)
B2 (b) · 1 mark · States a correct difference (e.g., different number of neutrons or mass number)
C1 (c) · 1 mark · Correct number of electrons for (3)
Worked solution
a) Number of neutrons = mass number - atomic number:
| Isotope | Mass number | Atomic number | Number of neutrons |
|---|---|---|---|
| 6 | 3 | 3 | |
| 7 | 3 | 4 |
b) The two isotopes have the same number of protons (atomic number = 3). They have different mass numbers because they have different numbers of neutrons.
c) Number of electrons in a neutral atom of = atomic number = 3.