Periodicity refers to the recurring trends seen among the elements of the periodic table. By arranging the elements in order of increasing atomic number trends can be easily interpreted. It was actually because of periodicity which made the completion of the modern periodic table possible – as it could help predict the properties and atomic mass of the missing elements.
The main periodic trends are ionization energies, atomic radius, electronegativity and electron affinity. Ionization energy refers to the amount of energy required to move one valence electron of an element in the gaseous state. Although atomic radius can’t be measured directly, it can be inferred by halving the distance between the centers of two atoms engaged in a covalent bond. Electronegativity is the measure of an atom's ability to form a bond; while electron affinity is a measure of an atoms attraction towards a particular electron.
With the arrangement of the periodic table in rows and columns based on the number of main electron shells and number of valence electrons, respectively, these trends can be observed based on these electron characteristics. If an electron is further away from the nucleus then then the ionization energy is small as it is easier to remove; the atomic radius is large as the electron is far from the nucleus’ the electronegativity would depend on the number of protons; while the electron affinity would also be low as the attraction is not very strong. Thus, understanding periodicity is extremely important for predicting the properties of elements.