AQA GCSE PHYSICS · PAPER 1 · FOUNDATION & HIGHER
Mass number, atomic number and isotopes
Atomic structure · Lesson 2 of 10
Teacher copy - includes the notes for whoever is teaching from it.
Warm-up
Answer each one, then check.
1. How many protons does carbon have?
6
2. What is an element?
A substance with only one type of atom
3. What is an ion?
A charged atom
4. What does the atomic number tell you?
The number of protons
5. What is a neutron?
A neutral particle in the nucleus
Learning Objectives
1. Define atomic number and mass number.
2. Work out the numbers of protons, neutrons and electrons in an atom.
3. Explain what isotopes are and relate differences in isotope symbols to identities, charges and masses.
4. Explain how atoms become positive ions.
Atomic Number And Mass Number
The atomic number is the number of protons in an atom. The mass number is the total number of protons and neutrons.
Isotopes of an element have the same number of protons but different numbers of neutrons.
Nuclear Symbols and Isotopes
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Same number of protons, different numbers of neutrons. |
The nuclear symbol for carbon-14 with mass number and atomic number labelled, and a table of the isotopes of carbon.
Counting Particles
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An atom of ¹⁴₆C is neutral. How many protons, neutrons and electrons does it have? |
1. Protons
Atomic number = 6
2. Neutrons
Mass number − atomic number = 14 − 6 = 8
3. Electrons
Same as protons in a neutral atom = 6
Answer: 6 protons, 8 neutrons and 6 electrons.
Isotopes
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Compare the atoms ³⁵₁₇Cl and ³⁷₁₇Cl. |
1. Protons
Both have 17
2. Electrons
Both have 17 if neutral
3. Neutrons
35 − 17 = 18 and 37 − 17 = 20
Answer: They are isotopes: the same number of protons and electrons but different numbers of neutrons (2 more in chlorine-37).
Ions
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A sodium atom ²³₁₁Na loses one electron. Describe the ion formed. |
1. Protons
Still 11
2. Electrons
Now 10
3. Charge
One more proton than electrons: +1
Answer: A positive ion, Na⁺, with 11 protons, 12 neutrons and 10 electrons.
Key Ideas
Learn these.
▸ Element. Determined by the number of protons.
▸ Isotopes. Same element, so same chemical properties, but different masses.
▸ Charge. Ions have an unequal number of protons and electrons.
▸ Symbols. The mass number is the top number; the atomic number is the bottom number.
Key Terms
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Atomic number The number of protons in an atom. |
Mass number The total number of protons and neutrons. |
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Isotopes Atoms of the same element with different numbers of neutrons. |
Ion An atom that has lost or gained electrons. |
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Element A substance made of atoms with the same number of protons. |
Nuclide A particular type of nucleus, such as carbon-14. |
Your Task: Fill the Table
10 minutes
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For ⁷₃Li and ⁶₃Li, state the number of protons, neutrons and electrons in each neutral atom. Explain why they are isotopes. 1. Use atomic number for protons. 2. Mass number minus atomic number for neutrons. |
A good answer shows: Li-7: 3 protons, 4 neutrons, 3 electrons. Li-6: 3 protons, 3 neutrons, 3 electrons. They have the same number of protons but different numbers of neutrons.
Note: Ask which one is heavier.
Can I...?
☐ Define atomic number.
☐ Define mass number.
☐ Work out neutrons.
☐ Read a nuclear symbol.
☐ Define isotopes.
☐ Compare two isotopes.
☐ Explain positive ions.
☐ Say why isotopes react the same.
Summary
✓ Atomic number = protons.
✓ Mass number = protons + neutrons.
✓ Isotopes: same protons, different neutrons.
✓ A positive ion has lost electrons.
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EXAM FOCUS An atom has 6 protons and 8 neutrons. State its mass number. (1 mark) Add protons and neutrons. |
Exam Practice: Mass number, atomic number and isotopes
Answer all questions. Use the mark allocation as a guide to how much to write. · 14 minutes
▸ Question 1 · 2 marks · Calculate. An atom of carbon-14 is represented as ¹⁴₆C. State the number of protons and the number of neutrons in the atom.
▸ Question 2 · 2 marks · Explain. Explain what is meant by isotopes.
▸ Question 3 · 3 marks · Compare. Chlorine has two isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl. State one similarity and one difference between atoms of these isotopes.
▸ Question 4 · 3 marks · Explain. A lithium atom ⁷₃Li becomes a positive ion. Explain how this happens and give the number of electrons in the ion if the charge is +1.
▸ Question 5 · 3 marks · Calculate. An atom of uranium-235 has 92 protons. Calculate the number of neutrons and the number of electrons in a neutral atom of uranium-235.
Question 1 · 2 marks · Calculate
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“An atom of carbon-14 is represented as ¹⁴₆C. State the number of protons and the number of neutrons in the atom.” |
HOW TO ANSWER IT Command word: Calculate. Worth 2 marks, so plan before writing.
Question 1 · mark scheme
2 marks available. Award a mark for each point made.
▸ 6 protons. 1 mark
▸ 8 neutrons. 1 mark
▸ Model answer. 6 protons; 14 − 6 = 8 neutrons.
Question 2 · 2 marks · Explain
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“Explain what is meant by isotopes.” |
HOW TO ANSWER IT Command word: Explain. Worth 2 marks, so plan before writing.
Question 2 · mark scheme
2 marks available. Award a mark for each point made.
▸ Same number of protons or same element. 1 mark
▸ Different number of neutrons. 1 mark
▸ Model answer. Atoms of the same element (same number of protons) that have different numbers of neutrons.
Question 3 · 3 marks · Compare
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“Chlorine has two isotopes: ³⁵₁₇Cl and ³⁷₁₇Cl. State one similarity and one difference between atoms of these isotopes.” |
HOW TO ANSWER IT Command word: Compare. Worth 3 marks, so plan before writing.
Question 3 · mark scheme
3 marks available. Award a mark for each point made.
▸ Similarity. 1 mark
▸ Difference in neutrons. 1 mark
▸ With numbers or different mass. 1 mark
▸ Model answer. Similarity: same number of protons (17) or electrons. Difference: chlorine-37 has 2 more neutrons (20 rather than 18), so a larger mass number.
Question 4 · 3 marks · Explain
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“A lithium atom ⁷₃Li becomes a positive ion. Explain how this happens and give the number of electrons in the ion if the charge is +1.” |
HOW TO ANSWER IT Command word: Explain. Worth 3 marks, so plan before writing.
Question 4 · mark scheme
3 marks available. Award a mark for each point made.
▸ Loses an electron. 1 mark
▸ 3 protons and 2 electrons. 1 mark
▸ More protons than electrons gives positive charge. 1 mark
▸ Model answer. The atom loses an outer electron. It has 3 protons and 2 electrons, so the charge is +1.
Question 5 · 3 marks · Calculate
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“An atom of uranium-235 has 92 protons. Calculate the number of neutrons and the number of electrons in a neutral atom of uranium-235.” |
HOW TO ANSWER IT Command word: Calculate. Worth 3 marks, so plan before writing.
Question 5 · mark scheme
3 marks available. Award a mark for each point made.
▸ Neutrons = mass number − atomic number. 1 mark
▸ 143. 1 mark
▸ 92 electrons. 1 mark
▸ Model answer. Neutrons: 235 − 92 = 143. Electrons: 92.