<Li> Van der Waals radius: half the distance between the nuclei of atoms of different molecules in a lattice of covalent molecules . </Li> <Li> Metallic radius: half the distance between two adjacent nuclei of atoms in a metallic lattice . </Li> <Li> Ionic radius: half the distance between two nuclei </Li> <P> The ionization potential is the minimum amount of energy required to remove one electron from each atom in a mole of atoms in the gaseous state . The first ionization energy is the energy required to remove the first electron, and generally the nth ionization energy is the energy required to remove the atom's nth electron, after the (n − 1) electrons before it has been removed . Trend-wise, ionization energy tends to increase while one progresses across a period because the greater number of protons (higher nuclear charge) attract the orbiting electrons more strongly, thereby increasing the energy required to remove one of the electrons . Ionization energy and ionization potentials are completely different . The potential is an intensive property and it is measured by "volt"; whereas the energy is an extensive property expressed by "eV" or "kJ / mole". </P>

Periodic properties and their variations in groups and periods