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 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
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
- 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
Summary of atom and ion sizes
Trends in the ionic radii across a period and down a group
