Long Form of the Periodic Table or Modern Periodic Table

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With developments on the structure of atoms, it was discovered that the atomic number (Z) is the important characteristic of the atom and not the atomic mass. This led to the development of the modern periodic law by Moseley in 1942. The modern periodic law states that "the physical and chemical properties of the elements are periodic function of their atomic numbers". Thus, when the elements were arranged in the order of their increasing atomic numbers, the elements of similar properties recur at regular intervals.

The long form of the periodic table or the modern periodic table is based on the general plan of the table as proposed by Mendeleev. This table is the most widely used periodic table of today.

Fig: 4.1- Modern Periodic Table

Structural Features of the Modern Periodic Table

This table consists of horizontal rows called as 'periods' and vertical columns called as 'groups'.

Periods

There are seven periods in the periodic table and each period starts with a different principal quantum number.

The first period corresponding to 'n' = 1 consists of only two elements hydrogen (1s¹) and helium (1s²). This is because the first energy shell has only one orbital (1s), which can accommodate only two electrons.

In the second period corresponding to 'n' = 2, there are four orbitals (one '2s' and three '2p') having a capacity of eight electrons and so contains eight elements. This period starts with lithium (Z = 3) with electron entering the '2s' orbital and ends with neon (Z = 10) where the second shell is complete (2s² 2p⁶).

In the third period corresponding to 'n' = 3, there are nine orbitals (one '2s' three '2p' and five '3d'). As '3d' orbitals are higher in energy, they are filled after the '4s' orbitals. This period involves filling of only four orbitals ('3s' and '3p') containing eight elements. It starts with sodium (Z = 11) with electron entering the '3s' orbital and ends with argon (Z = 18) where the third shell is partially complete (3s² 3p⁶).

The fourth period corresponding to 'n' = 4, consists of filling of one '4s' and three '4p' orbitals. The '4d' and '4f' orbital are higher in energy than the '5s' orbitals and are filled later. The five '3d' orbitals have energies in between '4s' and '4p' orbitals and so are filled accordingly. Totally nine orbitals are filled and therefore, there are eighteen elements in this period starting from potassium with electron entering the '4s' orbital (Z = 19) to krypton (Z = 36) where the third shell gets completed (4s² 3d¹⁰4p⁶).

In the fifth period there are 18 elements like the fourth period. It begins with rubidium (Z = 37) with the filling of '5s' orbital and ends with xenon (Z = 54) with the filling of '5p' orbital.

The sixth period contains 32 elements (Z=55 to 86) and the successive electrons enter into '6s', '4f', '5d' and '6p' orbitals in that order. It starts with caesium and ends with radon.

The seventh period, though expected to have 32 elements is incomplete and contains only 19 elements at present.

Number of elements in each period

There is a periodicity occurring at regular intervals of 2, 8, 8, 18, 18 and 32 and so the numbers 2, 8, 18 and 32 are called magic numbers. The first three periods are called short periods while the other three periods are called long periods.

Groups

The vertical column in the periodic table is called as group. There are 18 groups in the long form of the periodic table and they are numbered from 1 to 18 in the IUPAC system. In the old system of naming they are numbered as I to VIII with A and B groups. This convention is followed in many places.

Merits of the long form of the periodic table

• This classification is based on the most fundamental property of the elements - the atomic number, so it is more accurate.

• With the atomic number as the basis of this classification, the position of isotopes in one place is justified.

• The systematic grouping of elements into the four blocks of 's', 'p', 'd' and 'f ' has made the study of elements simpler.

• The electronic configuration determines the properties of the elements. The position of elements governed by this feature is useful in studying the properties of elements.

• The position of the elements, which were misfit on the basis of atomic mass is now justified on the basis of atomic number.

• The lanthanides and actinides have been placed separately due to their properties different from other groups.

• The whole table is easy to remember and reproduce in terms of electronic configuration and properties of the elements.

Demerits of the long form of the periodic table

Although the long form of the period table has been able to help in systematic studying the elements to a great extent, it has some minor defects:

• Hydrogen resembles both the alkali metals and halogens. But it has been placed with the alkalis.

• The lanthanides and actinides have not been placed in the main body of the table.