Ionic Radius
Ionic radius can be defined as the distance between the nucleus of an ion and the point up to which the nucleus has influence on its electron cloud.
Points to Remember:
- An ion is an atom with an electrical charge.
- If an atom looses electrons, it will become positive. A positively charged ion is called a cation. As cations loose an energy level, electrons are more firmly attracted towards the nucleus that is why a reduction in ionic size takes place. Therefore cations are smaller than the original atom.
- If an atom gains electrons, it will become negative. A negatively charged ion is called a anion. Anions are bigger than the original atom due to increases repulsion of electrons which causes an expansion in electronic cloud.
Units used to measure ionic radius:
- Angstroms (Å): This the most common unit used. Equivalent to 1.0 x 10-10 meters.
- Nanometer (nm): Equivalent to 1.0 x 10-9 meters.
- Picometer (pm): Equivalent to 1.0 x 10-12 meters.
Ionic Radius Trends
Ionic radius trends follow the atomic radius trends.
Period Trends
As you move left-to-right across a period, in general (not including the noble gases), ionic radius decreases. As you go left-to-right across a period, electrons are added to the same energy level. At the same time, protons are being added to the nucleus increasing positive charge in the nucleus. Increased positive charge in the nucleus pulls the electrons closer to the nucleus resulting in a smaller ionic radius.
Group Trends
As you move down a group, in general, ionic radius increases. As you move down a group the number of electrons increases, thus increasesing the number of energy levels. The outermost orbital increases in size when you move down the group making the atom larger. Therefore, the ionic radius increases as the group and energy levels increase.
