AtomicAndIonicRadii

The atomic radius of an atom refers to the distance between an atom's nucleus and its valence electrons

Moving from left to right across a period, the atomic radius decreases
- The nucleus of the atom gains protons moving from left to right, increasing the positive charge of the nucleus and increasing the attractive force of the nucleus upon the electrons
The atomic radius increases moving down a group
- Once again protons are added moving down a group, but so are new energy shells of electrons. The new energy shells provide shielding, allowing the valence electrons to experience only a minimal amount of the protons' positive charge

The ionic radius is the effective distance from the nucleus of the ion to its outer shell of electrons

When one or more electrons is added to an atom, the radius increases
- This/these extra electrons increase the forces of repulsion between the electrons, pushing them apart
When one or more electrons is removed from an atom, the radius decreases
- This is because there are less electrons thus less repulsive forces, so the protons are able to attract the remaining electrons closer to the nucleus
Within a group of elements, the ionic radius generally decreases like the atomic radius
In a period, however, there is no clear pattern