The electron configuration is the numerical layout of electrons in the orbitals around the atom. Can be done simply for example lithium has 3 electrons and has 2 in the first shell (as this is the maximum) and 1 in the second shell giving it the configuration 2,1.
Electrons exist in shells around the nucleus and have different configurations and layouts based on the element in the periodic table. Electrons are also very important in bonding.
Electrons are essential components of an element and decide its reactivity and bonding. They have their own unique properties known as quantum numbers which makes them all unique, existing in a number of sub orbitals around a nucleus.
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.