7. Periodic Classification of Element
Points To Remember
♦ Dobereiner's law of triads states that in a given set of three elements (triad) the atomic mass of the middle element is approximately equal to the average of the atomic masses of the other two elements.
♦ Newland's law of octaves states that if the elements are arranged in order of increasing atomic mass, the eighth element starting from a given element shows a repetition of the properties of the first element.
♦ According to Mendeleev's periodic law, the physical and chemical properties of elements are a periodic function of their atomic masses.
♦ Mendeleev's periodic table (i) helped correct the wrongly assigned values of atomic masses of some elements, and (ii) predicted the properties of some undiscovered elements.
♦ In Mendeleev's periodic table (i) isotopes had no place, and (ii) some similar elements are separated, while some dissimilar elements are placed in the same group.
♦ In Mendeleev's periodic table there are eight groups (I to VIII). Each group is divided into subgroups A and B, except group VIII. In the modern periodic table there are 18 groups.
♦ The vertical columns of the periodic table are called groups.
♦ The horizontal rows of the periodic table are called periods.
♦ Modern periodic law. The physical and chemical properties of elements are a periodic function of their atomic numbers.
♦ The modern periodic table (long form) is based on atomic number.
♦ There are seven periods in the periodic table. The first six periods consists of 2, 8, 8, 18, 18 and 32 elements respectively. The seventh periods is still incomplete.
♦ Elements in a group have similar properties.
♦ The regular occurrence of similar chemical properties of elements with increasing atomic number is known as chemical periodicity.
♦ A series of transition elements starting from actinium are called actinides or actinoids.
♦ A family of elements headed by helium in the periodic table constitutes the noble gases. They are unrecactive.
♦ An element that is intermediate between metals and nonmetals is known as a metalloid.
♦ The elements of group 1 are called alkali metals.
♦ The elements of group 2 are called alkaline earth metals.