Question 1 Medium
How many electrons are in a calcium ion, Ca²⁺? Calcium has atomic number 20.
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Answer: 18
- A calcium atom has 20 electrons.
- The ion has lost 2 electrons: 20 − 2 = 18.
Atomic structure, isotopes and models of the atom: GCSE Combined Science Chemistry practice questions
A second set of questions on atomic structure, isotopes and models of the atom. They are different from the ones on the topic page. Try each, then open the answer and compare your working.
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Question 2 Medium
Carbon-12 and carbon-14 are isotopes. How many more neutrons does carbon-14 have than carbon-12?
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Answer: 2
- Both have 6 protons.
- Neutrons: 14 − 6 = 8 and 12 − 6 = 6, a difference of 2.
Question 3 Medium
A sodium atom has atomic number 11 and mass number 23. How many electrons does it have?
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Answer: 11
- The atomic number is the number of protons.
- In a neutral atom, electrons equal protons.
Question 4 Medium
A sulfur atom has 16 protons. How many electrons are in a sulfide ion, S²⁻?
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Answer: 18
- A neutral atom has 16 electrons.
- The ion has gained 2 electrons.
- 16 + 2 = 18.
Question 5 Hard
Copper has two isotopes. 69% are copper-63 and 31% are copper-65. Calculate the relative atomic mass of copper to 3 significant figures.
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Answer: 63.6
- (69 × 63 + 31 × 65) ÷ 100.
- (4347 + 2015) ÷ 100 = 63.62.
- To 3 significant figures: 63.6.
Question 6 Hard
How many protons, neutrons and electrons are in the ion O²⁻? Oxygen-16 has atomic number 8. Give the answer as protons, neutrons, electrons.
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Answer: 8, 8, 10
- Protons: 8.
- Neutrons: 16 − 8 = 8.
- Electrons: 8 + 2 = 10.
Question 7 Hard
Chlorine has two isotopes, chlorine-35 and chlorine-37. The relative atomic mass is 35.5. Which isotope is more abundant?
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Answer: chlorine-35
- 35.5 is closer to 35 than to 37.
- So chlorine-35 is the more abundant.
Question 8 Hard
Explain why an atom has no overall charge.
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Answer: it has equal numbers of protons and electrons
- Protons are positive and electrons are negative.
- An atom has equal numbers of each, so the charges cancel.
Question 9 Medium
Which scientist described atoms as tiny solid spheres that cannot be divided?
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Answer: Dalton
- This was the earliest model.
- It was Dalton.
Quick checks
Short questions with options. Choose one before you open the answer.
Check 1
What is the relative charge of a proton?
Show answer
+1.
Check 2
What is the relative charge of an electron?
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−1.
Check 3
What is the relative charge of a neutron?
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0, so it has no charge.
Check 4
What is the relative mass of a proton and a neutron?
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1 for each.
Check 5
Where are the electrons in an atom?
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In shells around the nucleus.
Check 6
What is the approximate radius of an atom?
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About 0.1 nm, which is 1 × 10⁻¹⁰ m.
Check 7
What is the atomic number?
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The number of protons in an atom.
Check 8
What is the mass number?
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The number of protons plus the number of neutrons.
Check 9
How do you find the number of neutrons?
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Mass number − atomic number.
Check 10
Why is an atom neutral?
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It has equal numbers of protons and electrons.
Check 11
What are isotopes?
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Atoms of the same element with the same number of protons and different numbers of neutrons.
Check 12
Why do isotopes of an element react in the same way?
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They have the same number of electrons.
Check 13
What did the plum pudding model say?
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An atom is a ball of positive charge with electrons spread through it.
Check 14
What did most alpha particles do in the gold foil experiment?
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They went straight through.
Check 15
What did the deflection of a few alpha particles show?
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A small, dense, positive nucleus.
Check 16
Who discovered the neutron?
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James Chadwick.
Check 17
What is relative atomic mass?
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The mean mass of an atom of an element compared with carbon-12, allowing for the isotopes.
Check 18
What is the nucleus of an atom made of?
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Protons and neutrons.
Check 19
What is the relative mass of an electron?
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Very small, about 0.0005.
Check 20
What is the overall charge of the nucleus?
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Positive, because protons are positive and neutrons are neutral.
Check 21
What is an ion?
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An atom or group of atoms with a different number of electrons from protons.
Check 22
What happens to an atom when it becomes a positive ion?
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It loses electrons.
Check 23
Who discovered the electron?
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J J Thomson.
Check 24
Who suggested that electrons orbit in shells?
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Niels Bohr.
Check 25
Why is most of an atom empty space?
Show answer
The nucleus is about 10 000 times smaller than the whole atom.
Check 26
What was Dalton's model of the atom?
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Atoms were tiny solid spheres that could not be divided, with a different kind for each element.
Check 27
Why did Dalton's model have to change?
Show answer
The discovery of smaller particles inside atoms, such as the electron, did not fit it.
Keep going with Atomic structure, isotopes and models of the atom
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