Inside the atom Cambridge IGCSE Chemistry (9–1) revision
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In plain words
An atom has a tiny, heavy centre called the nucleus, made of protons and neutrons, with electrons arranged in shells around it. The number of protons is the atom's identity: 6 protons is always carbon. The electrons, especially the outer ones, decide how it behaves.
Three things to know
- The nucleus contains protons (charge +1, mass 1) and neutrons (charge 0, mass 1). Electrons (charge −1, almost no mass) are in shells around it.
- Proton (atomic) number = number of protons. In an atom the number of electrons is the same. Mass (nucleon) number = protons + neutrons.
- Shells fill in the order 2, 8, 8. The number of outer electrons is the group number; the number of shells is the period.
Worked example
Give the electronic configuration of sulfur, which has proton number 16.
- An atom of sulfur has 16 electrons.
- Fill the shells in order: 2 in the first, 8 in the second, leaving 6.
- 2,8,6.
Tips and tricks
- Bottom number = protons (and electrons, in an atom). Top number = protons + neutrons. Neutrons = top − bottom.
- The shells fill 2, then 8, then 8. The last number in the configuration is the group.
It lands in your notebook with its questions as flashcards.
Inside the atom: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What is the relative charge of a proton?
Protons are positive; electrons are −1 and neutrons 0.
Which particle has almost no mass?
An electron has about 1/1840 of the mass of a proton.
How many electrons are there in an atom with proton number 12?
An atom has as many electrons as protons.
What is the electronic configuration of an atom with 13 electrons?
The first shell holds 2, the second 8, and 3 are left over.
What decides which element an atom is?
Every atom of an element has the same proton number.
What does the number of outer-shell electrons tell you?
Elements in the same group have the same number of outer electrons.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Inside the atom
Cambridge IGCSE Chemistry (9–1) 0971 · 9 marks · papermunch.org
Name ______________________________ Date ______________
A sodium atom is written ²³₁₁Na. State the number of protons, neutrons and electrons.[3]
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11 protons, 12 neutrons (23 − 11), 11 electrons.
Write the electronic configuration of chlorine (proton number 17) and state its group.[2]
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2,8,7. Group 7 (VII).
Explain why an atom has no overall charge.[2]
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It has equal numbers of positive protons and negative electrons, so the charges cancel.
An atom has the electronic configuration 2,8,3. State its group and its period.[2]
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Group 3 (III), Period 3.
Answers: Inside the atom
- 1. 11 protons, 12 neutrons (23 − 11), 11 electrons.
- 2. 2,8,7. Group 7 (VII).
- 3. It has equal numbers of positive protons and negative electrons, so the charges cancel.
- 4. Group 3 (III), Period 3.