The valence electron is the outermost electron of an atom
Atomic radius is the measure of the distance from the centre of the nucleus to the outer electron.
Ionisation energy is a measure of the amount of energy required to remove an outer electron from an atom creating a positively charged ion.
The Group 2 Alkaline earth metals are the 2nd group in the periodic table and are just before the transition metals. They are all similar in their color of silvery/white metals and react with water to form alkaline solutions. They are however not as reactive as the group 1 metals as they have 2 valence electrons instead of 1.
The atomic radius is a measure of the distance from the nucleus to the valence electron.
Shielding is the measure o the effect of inner sub shells of the S P D and F on their interference of the nuclear charge of the protons on the valence electron.
Ionic radius can be defined as the size of the radius of an ion from the nucleus to the valence electron. The ionic radius is difficult to measure due to the overlap of ions in a lattice structure.