4CH1

Atomic Structure

Principles of Chemistry · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable
Medium
Stable9%

Subatomic particles and electronic configuration appear in every exam series.

  • (Z): the number of in the nucleus of an atom
    • Each element has its own unique atomic number — no two elements share one
    • In a neutral atom, the number of electrons also equals Z
  • (A): the total number of protons + in the nucleus

Counting subatomic particles

  • Number of protons = Z

  • Number of electrons = Z (for a neutral atom)

  • Number of neutrons = A − Z

  • Example. An atom of beryllium has atomic number 4 and mass number 9, so it has:

    • 4 protons
    • 4 electrons
    • 9 − 4 = 5 neutrons

Reading the periodic table

  • Both Z and A are given alongside every element on the periodic table
  • The larger of the two numbers is the mass number
  • The smaller is the atomic number
Lithium symbol with the mass number 7 written above and the atomic number 3 written below the element symbol
Source: Atoms: Definitions & Structure by Save My Exams
Exam tip

Atomic number vs mass number

Defining atomic number or mass number is a recurring 1-marker, so you need to know: atomic (proton) number = the number of protons in the nucleus; mass number = protons + neutrons. Define them in the nucleus only — "number of protons and electrons" is not credited.