has atomic number less by one unit compared to iodine. This atomic number concept forced the periodic law to be changed.

    The periodic law is changed from atomic weight concept to atomic number concept and now it is called the modern periodic law.

    We know that Mendeleeff’s periodic law is stated as “The properties of elements are the periodic functions of their atomic weights”. Now, let us try to understand modern periodic law.

    The modern periodic law may be stated as “the properties of the elements are periodic functions of their atomic numbers.”

    Based on the modern periodic law, the modern periodic table which is given here (page 132) is proposed. It is the extension of the original Mendeleeff’s periodic table known as short form of the table and this modern table is called the long form of the periodic table. It is given in fig (2). Atomic number of an element (Z) indicates not only the positive charges i.e., the protons in the nucleus of the atom of the element but also the number of electrons in the neutral atom of that element.

    The physical and chemical properties of atoms of the elements depend not on the number of protons but on the number of electrons and their arrangements (electronic configurations) in atoms. Therefore, the modern periodic law may be stated as “The physical and chemical properties of elements are the periodic functions of the electronic configurations of their atoms.”

The modern periodic table has eighteen vertical columns known as groups and seven horizontal rows known as periods.

    Let us see what decides the position of an element in modern periodic table.

    We can explain the classification of the elements in the modern periodic table in terms of certain electron arrangements which are periodically repeated. The elements with similar outer shell (valence shell) electronic configurations in their atoms are in the same column called group. Elements listed in a group down to it are in the order of their increasing principal quantum numbers.

    In the chapter ‘Structure of atom’ you have learnt that ‘s’ sub-shell with one orbital contains a maximum of two electrons. Each ‘p’ sub-shell contains 3 orbitals and accommodates a maximum of six electrons. The‘d’ sub-shell contains 5 orbitals and accommodates a maximum of 10 electrons and ‘f’ sub-shell contains 7 orbitals with 14 electrons maximum. Depending upon to which sub-shell the differentiating electron. i.e., the last coming electron enters in the atom of the given element, the elements are classified as ‘s’ ‘p’, ‘d’ and ‘f’ block elements.

    For example, sodium (Na) gets its new coming electron (differentiating