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 16 exam-style questions and 25 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. 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.

📚 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?

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.

Atomic Number and Mass Number