The atom and the nucleus Cambridge IGCSE Co-ordinated Sciences (Double Award) revision
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In plain words
An atom is mostly empty space. Nearly all of its mass is packed into a tiny positive nucleus made of protons and neutrons, with the electrons far out around it. The number of protons decides which element it is. Atoms of one element with different numbers of neutrons are called isotopes.
Three things to know
- An atom has a tiny, positive nucleus of protons and neutrons, with electrons around it. Almost all the mass is in the nucleus.
- Proton number (Z) is the number of protons. Nucleon number (A) is protons plus neutrons. Isotopes have the same number of protons but different numbers of neutrons.
- Rutherford's alpha-scattering experiment showed this: most alpha particles went straight through gold foil, but a few bounced back.
Worked example
How many neutrons are there in an atom of ²³⁵₉₂U?
- Nucleon number (top) = protons + neutrons = 235. Proton number (bottom) = 92.
- Neutrons = 235 − 92.
- = 143.
Tips and tricks
- Top number: protons + neutrons. Bottom number: protons. Neutrons = top − bottom.
- For the alpha-scattering experiment, pair each observation with what it showed: most went straight through → the atom is mostly empty space; a few bounced back → a tiny, dense, positive nucleus.
It lands in your notebook with its questions as flashcards.
The atom and the nucleus: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
Which particles are found in the nucleus?
Electrons are outside the nucleus.
An atom has 11 protons and 12 neutrons. What is its nucleon number?
Nucleon number = protons + neutrons.
What do isotopes of an element have in common?
The proton number decides which element it is.
What did the alpha-scattering experiment show about the nucleus?
A few positive alpha particles were strongly repelled by something tiny and dense.
What is the charge on a neutron?
Neutrons are neutral.
A neutral atom has 17 protons. How many electrons does it have?
In a neutral atom the charges balance.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 10 marks. Write your answers on paper, then check them.
The atom and the nucleus
Cambridge IGCSE Co-ordinated Sciences (Double Award) 0654 · 10 marks · papermunch.org
Name ______________________________ Date ______________
A nucleus of carbon is written ¹⁴₆C. State the number of protons, neutrons and electrons in a neutral atom.[3]
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6 protons, 8 neutrons (14 − 6), 6 electrons.
State what is meant by isotopes.[2]
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Atoms of the same element (same number of protons) with different numbers of neutrons.
In the alpha-scattering experiment most alpha particles passed straight through the gold foil but a few were deflected through large angles. State what each observation shows.[3]
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Most of the atom is empty space. The mass and positive charge are concentrated in a very small nucleus.
An atom loses two electrons. State the charge on the ion formed and explain why.[2]
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2+. It now has two more protons than electrons.
Answers: The atom and the nucleus
- 1. 6 protons, 8 neutrons (14 − 6), 6 electrons.
- 2. Atoms of the same element (same number of protons) with different numbers of neutrons.
- 3. Most of the atom is empty space. The mass and positive charge are concentrated in a very small nucleus.
- 4. 2+. It now has two more protons than electrons.