Periodic Trends in Properties of Elements
According to the modern periodic law, properties of elements are repeated at certain intervals or periods when the elements are arranged in the order of increasing atomic number. In a period, the number of valence electrons increases with an increase in atomic number. There is repetition of similar electronic configurations in the outermost shells in a group. Therefore, in a group, properties are similar. In the long form of the periodic table, elements with the same number of outer electrons are in the same group. Moreover, there is a gradation in the properties of elements in a group. For example, all members of group 1 have one electron in the outermost shell. Properties of these elements are similar to each other. Similarly, F, Cl, Br and I have seven electrons in their outermost shells. They are chemically alike. Members of group 18 have similar electronic configurations with 8 electrons in their outermost shell (except He). All noble gases have similar properties.
We shall now discuss periodic trends in some of the properties of elements.
Density, Melting Point and Boiling Point
The properties like density, melting point and boiling point also show gradation, though not very regular, in a group of the periodic table.
In a period of representative elements, density increases across a period and reaches a maximum value somewhere in the middle. Melting points and boiling points increase, attain maximum values in the middle, thereafter, and begin to decline.