Variations in Atomic & Ionic Radius (Cambridge International AS Chemistry)

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

Diagram showing the atomic radius of hydrogen

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

    • This 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

Summary of atomic radii

Atomic radius increases down a group and decreases across a period

• 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

Negative ions

• 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
  • For ions with the same electron arrangement, the greater the negative charge, the larger the ionic radius (for example O2- 126 pm > F- 119 pm)

Positive ions

• 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
• For ions with the same electron arrangement, the greater the positive charge, the smaller the ionic radius (for example Na+ 116 pm > Mg2+ 86 pm > Al3+ 68 pm)

Summary of atom and ion sizes

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