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 shells, shielding 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
Answer:
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