Learning objective
Determine the numbers of protons, neutrons and electrons in atoms and ions from mass number, atomic number and charge.
Read the explanation, check the common trap, then practise with flashcards and questions.
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Topic
Atomic structure
Subtopic
Mass number, isotopes and mass spectrometry
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Understand this objective
Quick explanation
Determine the numbers of protons, neutrons and electrons in atoms and ions from mass number, atomic number and charge
- This point belongs to Atomic structure, especially Mass number, isotopes and mass spectrometry.
- You need to be able to determine the numbers of protons, neutrons and electrons in atoms and ions from mass number, atomic number and charge.
- The key ideas to know are ion, atomic number, and mass number.
- Use the linked flashcards and practice questions to check recall, then practise applying the idea in an exam-style answer.
Key concepts
Why it matters
This objective helps connect Mass number, isotopes and mass spectrometry to exam-style questions, flashcards, and revision notes for Atomic structure.
Quick student answer
What is the number of neutrons in the isotope of magnesium with mass number 27?
Direct answer
The isotope has 12 protons (atomic number of Mg) and a mass number of 27. Neutrons = 27 – 12 = 15.
Key terms
- Mass number: The sum of protons and neutrons in an atom’s nucleus, expressed as a whole number.
- Isotope: Atoms of the same element that have the same number of protons but different numbers of neutrons, giving different mass numbers.
Common trap
Confusing mass number with atomic mass: Remember that the mass number is an integer equal to the number of protons plus neutrons; atomic mass is a weighted average of isotopes and is not used for counting particles.
Related questions
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Flashcard prompts
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Revision notestopic notes
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Open revision notesRelated learning objectives
- Explain that scientific understanding of atomic structure has developed over time.
Fundamental particles
- State the relative charge and relative mass of protons, neutrons and electrons.
Fundamental particles
- Describe an atom as a nucleus containing protons and neutrons surrounded by electrons.
Fundamental particles
- Use particle properties to compare atoms, ions and isotopes.
Fundamental particles
- Explain the existence of isotopes using different neutron numbers.
Mass number, isotopes and mass spectrometry