IB DP Chemistry: SL复习笔记3.1.2 Periodic Trends: Physical - 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 the distance between the nucleus of an atom and the outermost electron shell
  • It can be quite hard to determine exactly where the boundary of an atom lies, so a variety of approches are taken such as half the mean distance between two adjacent atoms
  • This will vary depending on the type of structure and bonding, but it gives a comparative value for atoms


The atomic radius of an atom is the typical distance between the nucleus and the outermost electron shell

  • 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 shellsshielding them from the positive nuclear charge
    • This weakens the pull of the nuclei on the electrons resulting in larger atoms


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
    • The trend down a group is the same as atomic radius - it increases as the number of shells increases
    • The trend across a period is not so straightforward as it depends on whether it is positive or negative ions being considered
    • 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 extra electrons experience repulsion with the other valence electrons 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 towards the nucleus which decreases the ionic radius
    • The greater the positive charge, the smaller the ionic radius


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

Worked Example

Which option shows atoms in order of decreasing atomic radius?

A. N > C > Be > Mg

B. Mg > N > C > Be

C. Be > C > N > Mg

D. Mg > Be > C > N


Option D is the correct answer

    • First, you need to identify that Be, C and N are all in period 2, but Mg is in period 3, so Mg will have the biggest radius.
    • Secondly, atomic radius decreases across the period so Be, C and N decrease in that order as they belong to groups 2, 14 and 15, respectively