Modern Periodic Law: Given by Henry Moosley in 1913. Properties of elements are a periodic function of their atomic number.

Atomic Number: denoted by Z and equals to the no. of protons in the nucleus of an atom.

• The modern periodic table contains 18 vertical columns known as groups and 7 horizontal rows known as periods.
• Elements in a group have the same number of valence electrons No. of the shells increases as we go down the group.
• Elements in a period have the same number of shells. Each period marks a new electronic shell getting filled.
• No. of elements placed in a particular period depends upon the fact that how electrons are filled into various shells.

The long form of the periodic table

• In the long form of the periodic table, the elements are arranged in groups and periods on the basis of the electronic configuration of elements.

Characteristic of the Long form of the Periodic Table.

Characteristics of the Groups :

• There are 18 groups in the long form of the periodic table.
• Group 1 is on the extreme left-hand side and group 18 on the extreme right-hand side of the periodic table.
• Groups 1, 2 and 13 to 17 contain normal elements. The normal elements are Sometimes called representative elements. In these elements, all the inner shells are completely filled with electrons, except the outermost shell which is incomplete.
• The elements in group 18 are known as noble gases or inert gases. They have 8 electrons in their valence shell, except helium, which has 2 electrons in the valence shell.
• The elements in group 3 to group 12 are called transition elements. In transition
• Elements, the outermost shell as well as the shell next to the outermost shell (penultimate shell) are incomplete.

Characteristics of the Periods:

• There are seven periods in all, such that each period has consecutive (or continuous) atomic numbers.
• The number of elements in a period corresponds to the maximum number of electrons which can be accommodated in one shell of the element.
• The number of the period to which an element belongs is given by the number of the outermost shell (quantum number).

Actinides

• They are inner transition elements from atomic number 89 (Actinium) to atomic number 103 (Lawrencium). They are kept outside the Periodic Table to mark their peculiar properties.
• On moving from left to right across a period, the number of electrons in the valence shell increases by one in groups 1, 2, 13, 14, 15, 16, 17 and 18.
• On moving from left to right across a period, the valency of elements with respect to hydrogen increases in group 1, 2, 13, and 14 and decreases in the groups 15, 16 and 17 till it is zero in group 18.
• On moving from left to right across a period, the valency of elements with respect to oxygen increases in groups 1, 2, 13, 14, 15, 16, 17, and then it is zero in group 18.
• On moving from left to right across a period, the atomic size of the elements decreases in groups 1, 2, 13, 14, 15, 16, 17 and then suddenly increases.
• On moving from left to right across a period, the metallic character of the elements gradually decreases and non-metallic character gradually increases, till in the 18th group it becomes a noble gas.
• On moving from left to right across a period, the chemical activity of the elements gradually decreases, then it starts increasing, such that the element in the last group is chemically inert.
• On moving from left to right across a period, the basic nature of the oxides gradually decreases and the acidic nature of the oxides gradually increases.
• As one moves down a group, the number of electrons in the valence shell in groups 1 and 2 are the same as the number of the group. The number of electrons in groups 13 to 18 is equal to the number of the group –10.
• All the elements in a group have the same valency which is equal to the number of electrons in the valence shell.
• The atomic size of the elements increases as one moves down the group.