Classification of elements and periodic table

Modern periodic law and modern periodic table: In 1913, Henry Moseley discovered that elements are classified on the basis of atomic numbers in the modern periodic law. It states that “The physical and chemical properties of the elements are periodic function of their atomic numbers”.

The modern periodic law is superior to Mendeleev periodic law due to the following reasons:

1. Modern periodic law eliminates many defects present in the Mendeleev periodic law.
2. Modern periodic law explains the cause of periodicity in the properties of elements in the periodic table.

Main features of modern periodic table

1. There are 7 horizontal rows which are called periods and indicated by 1, 2, 3, 4, 5, 6 and 7.

2.There are 18 vertical columns which are called groups. They are:
i. IA, IIA, IIIA, IVA, VA, VIA, VIIA
ii.IB, IIB, IIIB, IVB, VB, VIB, VIIB
iii. VIII, it consists of three vertical columns
iv. zero groups, it consists of inert gases.

3. Lanthanides and actinides are not included in the main body of the periodic table. They are placed at the bottom of the table.

Defect of periodic table

1. The position of hydrogen is not properly settled. This problem remains unsolved.
2. It failed to accommodate the lanthanides and actinides in the mainframe of the periodic table.
3. Group VIII consist of 3 columns without any proper justification
4. The location of helium in the p-block element is not fully justified as its electronic configuration justify it to be included in the s-block.

Advantages of modern periodic table

i. It explains the cause of periodicity in the properties of elements. When elements are arranged in the increasing order of atomic numbers, the properties of elements changes and after a regular interval the elements having similar properties reappears. This is called periodicity in the properties of elements. This periodicity in properties arises due to periodicity in the outer electronic configuration of elements.

ii. It eliminates many defects in Mendeleev’s periodic table since the atomic number is the basis for classification, isotopes do not need separate positions. Similarly, the position of Ar before K and Ni before Co justifies that the former has a lower atomic number than later.

iii. Separation of metals and non-metals has been achieved: The left position includes metals, the right portion includes non-metals and the middle portion includes transition elements.

iv. Elements have been divided into four distinct blocks: s, p, d and f-block.

Defect of periodic table

1. The position of hydrogen is not properly settled. This problem remains unsolved.
2. It failed to accommodate the lanthanides and actinides in the main frame of the periodic table.
3. Group VIII consist of 3 columns without any proper justification
4. The location of helium in the p-block element is not fully justified as its electronic configuration justify it to be included in s-block.

Types of elements in periodic table

On the basis of electronic configuration of incompletely filled orbitals, elements are classified into four types:

1. S-block elements

The elements in which the last electron enters the s-orbital are called s-block elements. The elements of IA (alkali metals) and IIA (alkaline earth metals) belong to this block. The elements of s-block are collectively known as normal or representative elements.

General characteristics of S block elements

• Their general outer electronic configuration is ns^1-2.
• They are highly reactive soft metals.
• They possess a low melting point and boiling point.
• They are highly electropositive having low ionization energy.
• They form ionic compounds and show oxidation state of +1 and +2.
• They are strong reducing agents.

2. p-block elements

The elements in which the last electron enters the p-orbital of the outermost energy level are called p-block elements. The elements of group 13(IIIA) to group 17(VIIA) and group 18 (zero) of the periodic table belong to this group. They are situated at the extreme right of the periodic table.

General characteristics of p block elements

• Their general outer electronic configuration is ns² np^1-6.
• They include almost all non-metals including noble gases, metalloids and some metals.
• They mostly form covalent compounds.
• They show a variable oxidation state.
• The elements of an s orbital and p orbital except noble gases are called a normal or representative element.

3. d-block elements

The elements in which the last electron enters the d-orbitals of their penultimate energy level are called d-block elements. They are also called transition elements because they exhibit transitional behaviour between s and p-block elements. The elements of group IIIB, IVB, VB, VIB, VIIB, VIII, IB and IIB belong to this block.

General characteristics of d block elements

• Their general outer electronic configuration is (n-1)d^1-10 ns^1-2.
• They are hard metals with high melting and boiling points.
• They form both ionic and covalent compounds.
• They show variable oxidation state.
• They form coloured compounds and complex salts.

4. f-block elements

The elements in which the last electron enters the f-orbital of their antipenultimate energy level are called f-block elements. The elements of f-block are also called inner transition elements and are classified into two series (4f and 5f) each containing 14 elements. The elements of the 4f series are called lanthanide and the 5f series are called actinides.

General characteristics of f-block elements

• Their general outer electronic configuration is (n-2)f^1-14 (n-1)d^1-10 ns².
• They are heavy metals with high melting points.
• They show variable oxidation state.
• They form coloured compounds.
• They have tendency to form complex.
• All actinides are radioactive in nature.