In this unit, you have studied the development of the Periodic Law and the Periodic Table. Mendeleev's Periodic Table was based on the atomic masses. Modern Periodic Table arranges the element in the order of their atomic numbers in seven horizontal rows (periods) and eighteen vertical columns (groups or families). Atomic numbers in a period are consecutive, whereas in a group they increase in a patern. Elements of the same group have similar valence shell electronic configuration, and therefore exhibit similar chemical properties. However, the elements of the same period have incrementally increasing number of electrons from left to right and therefore, have different valencies. Four types of elements can be recognised in the periodic table on the basis of their electronic configurations. These are s-block, p-block, d-block, f-block elements. Hydrogen with one electron in the 1s orbital occupies a unique position in the periodic table. Metals comprise more than 78% of the known elements. Non-metals which are located at the top of the periodic table, are less than 20 in number. Elements which lie at the border between metals and non metals are called metalloids or semi-metals. Metalloids tend to have lower electrical conductivity than metals, yet often higher than nonmetals. They tend to form chemical bonds similarly to nonmetals, but may dissolve in metallic alloys without covalent or ionic bonding. Metalloid additives can improve properties of metallic alloys, sometimes paradoxically to their own apparent properties. Periodic Trends in properties of elements are observed in atomic sizes, ionisation enthalpies, electron gain enthalpies, electronegativity and valence.

The atomic size or radius increases with the increase in the number of electronic shells. Atomic radius is proportional to the number of electronic shells.From left to right across a period of elements, electronegativity increases. If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one.

Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.Conceptually, ionization energy is the opposite of electronegativity. The lower this energy is, the more readily the atom becomes a cation.Elements on the left side of the periodic table have low ionization energies because of their willingness to lose electrons and become cations. Thus, ionization energy increases from left to right on the periodic table.

If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one.