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Atomic Number and Mass Number

Part of Atomic Structure — GCSE Physics

This key facts covers Atomic Number and Mass Number within Atomic Structure for GCSE Physics. Revise Atomic Structure in Atomic Structure for GCSE Physics with 13 exam-style questions and 25 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 4 of 12 in this topic. Use this key facts to connect the idea to the wider topic before moving on to questions and flashcards.

Topic position

Section 4 of 12

Practice

13 questions

Recall

25 flashcards

📚 Atomic Number and Mass Number

Atomic Number (Z):

  • Number of PROTONS in the nucleus
  • Defines which ELEMENT it is (all carbon atoms have 6 protons)
  • In a neutral atom: protons = electrons

Mass Number (A):

  • Total number of PROTONS + NEUTRONS
  • Also called nucleon number
  • Neutrons = Mass number − Atomic number

Notation:

Mass number (A) ──→ ²³Na ←── Element symbol Atomic number (Z) ──→ ₁₁ This sodium atom has: • 11 protons (atomic number) • 11 electrons (neutral atom) • 23 − 11 = 12 neutrons

Quick Check: An atom of chlorine has atomic number 17 and mass number 35. How many neutrons does it have?

Keep building this topic

Read this section alongside the surrounding pages in Atomic Structure. That gives you the full topic sequence instead of a single isolated revision point.

Structure of the AtomIsotopesMostly Empty SpaceInside the Atom

Practice Questions for Atomic Structure

What does the atomic number of an element tell you?

  • A. The number of neutrons in the nucleus
  • B. The total number of particles in the nucleus
  • C. The number of protons in the nucleus
  • D. The mass of one atom in grams
1 markfoundation

Describe the structure of an atom. Include the location and charge of the three main subatomic particles.

2 marksstandard

Quick Recall Flashcards

Why are atoms electrically neutral?
Atoms are electrically neutral because the number of protons (positive charges) equals the number of electrons (negative charges). The positive and negative charges cancel out.
What are isotopes?
Isotopes are atoms of the same element (same number of protons) with different numbers of neutrons. This means they have the same atomic number but different mass numbers.

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Structure of the Atom

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Isotopes