In the years after Mendeleev, chemists made many discoveries that brought about changes in the Periodic Table. Consequently in 1913, an English physicist, Henry Mosley performed experiments that led to the discovery of a new property of elements. This fundamental property of the elements is known as atomic number. No two elements can have the same atomic number. Therefore, atomic number of an element is a better basis for the classification of elements than atomic mass.

In the Modern Periodic Table, the elements are classified in periods and groups.

Periods: The horizontal rows of elements in the Periodic Table are called periods. There are seven periods in the modern Periodic Table. Each period contains a certain numbers of elements.

Period numbers are represented by Arabic numerals, 1 to 7.

Groups: The vertical column of the elements in the Periodic Table are known as Groups or Families. In the modern Periodic Table, there are 18 vertical columns or groups.

The “A” groups are designated with IA to VIIIA and referred to as the main groups or representative elements.

Electronic configuration and arrangement of elements

Electronic configuration of the elements helps us to explain the repetition of properties. The position of elements in the Periodic Table can also be predicted from their electronic configurations.

The period number of an element corresponds to the number of shells in its atom. For example, if an element has 3 shells (K, L and M) then it belongs to the third period.

The number of shells is equal to the period number to which the element belongs. For example, if an element has two shells, it is found in period 2, an element with three shells belongs to the third period, and so on.

Relationship between the number of shells and period number

• The group number of an element is equal to the number of its valence electrons. For example, if the valence electron of an element is 1, it belongs to groups IA, if it has 2 valence electrons, the element is found in group IIA, and so on.
• All elements in the same group have the same number of valence electrons. For example, Group IA elements, have 1 valence electron, Group IIA elements have 2 valence electrons, etc.

Electrons found in the outer most shell of an atom are called valence electrons.

Classification of the elements

Elements are classified as metals, non-metals and metalloids.

• Metallic elements are found to the left and center of the Periodic Table.
• Non-metallic elements are found to the right of the Periodic Table.

All Group IA and IIA elements except hydrogen are metals.

All transition elements are also metals.

There are a few number of elements that show both metallic and non-metallic properties. These are called semi-metals or metalloids.

Elements that are found near the border line are metalloids.

Main group elements

These are elements having valence electrons entering the s or p orbital. The group in which they occur are usually designated as the “A” groups. These elements are also called the s-block and the p-block elements. s-block elements are elements in which the last electron enters the s-orbital. p-block elements are elements in which the last electron occupies the p-orbital.

The transition elements

The transition elements are characterized by:

• Metallic nature such as metallic luster, thermal and electrical conductivity
• Variable valences
• Ability to form colored compounds
• Ability to form complex compounds

The rare earth metals

f-block elements or rare earth elements are those metals whereby the last electron goes to f-orbital. They are also called inner transition elements. There are two rows of these elements placed separately outside the main table. The first series of elements (4f) is known as the lanthanides and the second series (5f) is called actinides.