CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.3 Determining Subatomic Structure

Determining the Subatomic Structure of Atoms & Ions

  • An atom is neutral and has no overall charge
  • Ions on the other hand have either gained or lost 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
    • Eg. 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

Atomic Structure Worked example - Determine the number of protons, downloadable AS & A Level Chemistry revision notes

Answer

  • 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

Atomic Structure Worked example - Determine the number of electrons, downloadable AS & A Level Chemistry revision notes

Answer

  • 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 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

Atomic Structure Worked example - Determine the number of neutrons, downloadable AS & A Level Chemistry revision notes

Answer

  • 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

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
Close

Join Save My Exams

Download all our Revision Notes as PDFs

Try a Free Sample of our revision notes as a printable PDF.

Join Now
Already a member?
Go to Top