Classification of Elements - Part V

5. Modern periodic table
As more and more elements and their isotopes were discovered, Mendeleev’s periodic table that gave a chart of all the elements, was found to be inconsistent in many ways. Henry Moseley, in 1913, found the reason for the inconsistencies. He performed X-ray experiments on the elements and found that each element has an integral positive charge, the atomic number (Z) or the number of protons. Moseley revised the periodic table and made a bold change that removed all inconsistencies. He suggested that instead of arranging elements in the ascending order of their atomic masses, they should be arranged in the ascending order of their atomic numbers. The number of electrons in an atom is equal to the atomic number Z. Thus by making this change, Moseley put the emphasis on electronic configuration of the elements. Also it must be remembered that all physical and chemical properties come about because of the arrangements of electrons.

The modern statement of the periodic law is that the properties of the elements are a periodic function of their atomic numbers.

The modern periodic table is a very neat representation of all elements. The chart is easy to read and the arrangement is so accurate, that if you know properties of a few elements, you will be able to make a close guess of other elements close to them.  The table also eliminated anomalies in Mendeleev’s periodic table of elements.

Removal of anomalies in Mendeleev’s periodic table
1. The position of isotopes is taken care of when the elements are arranged in the ascending order of their atomic numbers. The isotopes will occur at the same position as its original element since the electronic configuration of the element and its isotopes is identical[1]. Let us go back to the example of the isotope of carbon, ¹⁴C. This would have to be accommodated now along with carbon (¹²C) itself and not with nitrogen.

2. The anomaly regarding e few elements such as tellurium (Te) – iodine (I) and argon (A) – potassium (K) were solved elegantly. Although Te had a higher atomic mass, Mendeleev was forced to place it ahead of I which had a lower atomic mass. But if one sees their atomic numbers, Te is 52 and I is 53. Thus Te will naturally come before I when atomic numbers are considered. Similarly for the pair of Ar-K.

Thus the modern periodic table of elements removed all anomalies of the Mendeleev’s periodic table, by simple considering the atomic numbers of elements.

Neils Bohr prepared the present version of periodic table. Figure below gives the modern periodic table or what is called as the long form of the periodic table.

The features of the periodic table are:

1. It consists of 7 horizontal periods. The lengths of the periods increase with the order of the period. Elements in a period have consecutive atomic numbers.

• The 1^st period is the shortest period.  It consists of just two elements H and He.
• The 2^nd and the 3^rd periods have 8 elements each and are called short periods.
• The 4^th and the 5^th periods are long periods and have 18 elements each.
• The 6^th period has 32 elements. The period has a 15 element series called Lanthanide series, separated from the table. The lanthanide series are rare-earth elements that show similar properties.
• The 7^th period contains all the rest of the elements. It is incomplete. This period also has a 15 element series called the Actinide series, separated from the table. The actinide series have a separate identity and contains uranium and most of the known transuranic elements.

2. The vertical columns are called groups. There are 18 groups in the periodic table. Elements in a group do not have consecutive atomic numbers.

• The groups are divided into A and B groups.
• Group 1A to VIII A has all the normal elements.
• Group 1B to VIII B holds all the transition metal elements.
• The other two groups are the lanthanide and the actinide series. They are also known as inner transition elements.

3. The modern periodic table is approximately divided into metals and non-metals. The most metallic elements such as alkalis are on the left-hand side. The non-metals are on the right hand side. The inert gases or the noble gases with their completely filled electronic shells are placed on the extreme right hand side. The transition metals, which are a bridge between highly metallic alkali elements and the non-metals, lie in the centre of the table. Lanthanide and actinide series (or the inner transition elements), which have metal like behavior, are kept separately as their outermost electronic configurations differ from the transition metal elements.

 A c[1] A chart of isotopes is needed in nuclear physics. This chart follows the principle of  the periodic table of elements but N (neutron number) versus Z (proton number) is plotted.  

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