A LevelChemistryCIERevision Notes1. Atomic Structure1.1 Particles in the Atom & Atomic RadiusDetermining Subatomic Structure

Syllabus Edition

First teaching 2023

First exams 2025

|

Determining Subatomic Structure (CIE A Level Chemistry)

Revision Note

Test yourself
Richard

Author

Richard

Last updated

Key Terms of Atomic Structure

  • The atomic number (or proton number) is the number of protons in the nucleus of an atom and has the symbol Z
    • The atomic number is also equal to the number of electrons present in a neutral atom of an element
    • E.g. the atomic number of lithium is 3, meaning that a neutral lithium atom has 3 protons and therefore, also has 3 electrons

  • The mass number (or nucleon number) is the total number of protons + neutrons in the nucleus of an atom, and has the  symbol A
  • The number of neutrons can be calculated by:

Number of neutrons = mass number - atomic number

    • Protons and neutrons are also called nucleons, because they are found in the nucleus

Examiner Tip

Atomic structure notation, IGCSE & GCSE AS & A Level Chemistry revision notes

 The mass (nucleon) and atomic (proton) number are given for each element in the Periodic Table

Determining the Subatomic Structure of Atoms & Ions

  • An atom is neutral and has no overall charge
  • Ions on the other hand are formed when atoms either gain or lose electrons, causing them to become charged
  • The number of subatomic particles in atoms and ions can be determined given their atomic (proton) number, mass (nucleon) number and charge

Protons

  • The atomic number of an atom and ion determines which element it is
  • Therefore, all atoms and ions of the same element have the same number of protons (atomic number) in the nucleus
    • E.g. lithium has an atomic number of 3 (three protons) whereas beryllium has atomic number of 4 (4 protons)

  • The number of protons equals the atomic (proton) number
  • The number of protons of an unknown element can be calculated by using its mass number and number of neutrons:

Mass number = number of protons + number of neutrons

Number of protons = mass number - number of neutrons

Worked example

Determine the number of protons in the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

  • The atomic number of a magnesium atom is 12 suggesting that the number of protons in the magnesium element is 12
  • Therefore, the number of protons in a Mg2+ ion is also 12

Answer 2:

  • The atomic number of a carbon atom is 6 suggesting that a carbon atom has 6 protons in its nucleus

Answer 3:

  • Use the formula to calculate the number of protons
    • Number of protons = mass number - number of neutrons
    • Number of protons = 63 - 34
    • Number of protons = 29
  • Element X is therefore copper

Electrons

  • An atom is neutral and therefore has the same number of protons and electrons
  • Ions have a different number of electrons to their atomic number depending on their charge
    • A positively charged ion has lost electrons and therefore has fewer electrons than protons
    • A negatively charged ion has gained electrons and therefore has more electrons than protons

Worked example

Determine the number of electrons in the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with mass number 63 and 34 neutrons

Answer 1:

  • The atomic number of a magnesium atom is 12 suggesting that the number of protons in the neutral magnesium atom is 12
  • However, the 2+ charge in the Mg2+ ion suggests it has lost two electrons
  • It only has 10 electrons left now

Answer 2:

  • The atomic number of a carbon atom is 6 suggesting that the neutral carbon atom has 6 electrons orbiting around the nucleus

Answer 3:

  • The number of protons of element X can be calculated by:
    • Number of protons = mass number - number of neutrons
    • Number of protons = 63 - 34
    • Number of protons = 29
  • The neutral atom of element X  therefore also has 29 electrons

Neutrons

  • The mass and atomic numbers can be used to find the number of neutrons in ions and atoms:

Number of neutrons = mass number (A) - number of protons (Z)

Worked example

Determine the number of neutrons in the following ions and atoms:

  1. Mg2+ ion
  2. Carbon atom
  3. An unknown atom of element X with an atomic number of 29 and a mass number of 63

Answer 1:

  • The atomic number of a magnesium atom is 12 and its mass number is 24
    • Number of neutrons = mass number (A) - number of protons (Z)
    • Number of neutrons = 24 - 12
    • Number of neutrons = 12
  • The Mg2+ ion has 12 neutrons in its nucleus

Answer 2:

  • The atomic number of a carbon atom is 6 and its mass number is 12
    • Number of neutrons = mass number (A) - number of protons (Z)
    • Number of neutrons = 12 - 6
    • Number of neutrons = 6
  • The carbon atom has 6 neutrons in its nucleus

Answer 3:

  • The atomic number of an element X atom is 29 and its mass number is 63
    • Number of neutrons = mass number (A) - number of protons (Z)
    • Number of neutrons = 63 - 29
    • Number of neutrons = 34
  • The neutral atom of element X has 34 neutrons in its nucleus

You've read 0 of your 10 free revision notes

Unlock more, it's free!

Join the 100,000+ Students that ❤️ Save My Exams

the (exam) results speak for themselves:

    Test yourself

    Did this page help you?

    Previous:Atomic Structure & Subatomic ParticlesNext:Variations in Atomic & Ionic Radius
    Richard

    Author: Richard

    Expertise: Chemistry

    Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.