CIE AS Chemistry (9701) exams from 2022

Revision Notes

1.1.4 Atomic & Ionic Radius

Atomic & Ionic Radius

Atomic radius

  • The atomic radius of an element is a measure of the size of an atom
  • It is half the distance between the two nuclei of two covalently bonded atoms of the same type

Atomic Structure Atomic Radius Hydrogen Molecule, downloadable AS & A Level Chemistry revision notes

The atomic radius of a hydrogen atom is determined by halving the distance between the nuclei of two hydrogen atoms covalently bonded

  • Atomic radii show predictable patterns across the Periodic Table
    • They generally decrease across each Period
    • They generally increase down each Group
  • These trends can be explained by the electron shell theory
    • Atomic radii decrease as you move across a Period as the atomic number increases (increased positive nuclear charge) but at the same time extra electrons are added to the same principal quantum shell
    • The larger the nuclear charge, the greater the pull of the nuclei on the electrons which results in smaller atoms
    • Atomic radii increase moving down a Group as there is an increased number of shells going down the Group
    • The electrons in the inner shells repel the electrons in the outermost shells, shielding them from the positive nuclear charge
    • This weakens the pull of the nuclei on the electrons resulting in larger atoms

Atomic Structure Atomic Radius Trends, downloadable AS & A Level Chemistry revision notes

Trends in the atomic radii across a period and down a group

  • The diagram shows that the atomic radius increases sharply between the noble gas at the end of each period and the alkali metal at the beginning of the next period
  • This is because the alkali metals at the beginning of the next period have one extra principal quantum shell
    • This increases shielding of the outermost electrons and therefore increases the atomic radius

Ionic radius

  • The ionic radius of an element is a measure of the size of an ion
  • Ionic radii show predictable patterns
    • Ionic radii increase with increasing negative charge
    • Ionic radii decrease with increasing positive charge
  • These trends can also be explained by the electron shell theory
    • Ions with negative charges are formed by atoms accepting extra electrons while the nuclear charge remains the same
    • The outermost electrons are further away from the positively charged nucleus and are therefore held only weakly to the nucleus which increases the ionic radius
    • The greater the negative charge, the larger the ionic radius
    • Positively charged ions are formed by atoms losing electrons
    • The nuclear charge remains the same but there are now fewer electrons which undergo a greater electrostatic force of attraction to the nucleus which decreases the ionic radius
    • The greater the positive charger, the smaller the ionic radius

Atomic Structure Ionic Radius Trends, downloadable AS & A Level Chemistry revision notes

Trends in the ionic radii across a period and down a group

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.
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