As we all know, chemistry is a branch of science that deals with atoms, molecules, elements, matters of composition etc. The only thing everyone is familiar with is the periodic table. From lower classes to higher studies, the periodic table is something that must be learned thoroughly. It is nothing but a one-paper document comprising half of the chemistry concepts, and it has been an essential landmark in the history of chemistry.

Classification of elements and periodicity in property is a chapter from NCERT class 11 chemistry textbook. You can refer to the class 11 chemistry notes for further information, as well as the rest of the chapters. In this article, we will be discussing some main concepts mentioned in this chapter.

Development of modern periodic table

 Dobereiner’s Triads

John Dobereiner was a German chemist who classified the elements with similar properties in three groups and named triads. His concept was that the atomic weight of the middle element was equal to the average of the atomic weights of the first and third elements.

Triad	Atomic Masses
Lithium	6.94
Sodium	22.99
Potassium	39.1

Limitations

• He failed to arrange all the elements known at that time into triads. So a limited number of elements were placed and named as triads.

Newlands’ Law of Octaves

John Newland is an English chemist who arranged the elements in the order of increasing atomic mass. The concept was that every eight elements have properties similar to the first, which he named as the law of octaves. For instance, sodium, the eighth element from lithium, has similar properties to that of lithium, the first element and similar observations have been made for Be and Mg, B and Al and so on.

Limitations

• He failed to implement the law of octaves after the element calcium.
• After discovering noble gas elements at a later stage, their inclusion in the table interrupted the entire arrangement.

Mendeleev’s Periodic Table

Mendeleev, a Russian chemist in 1869, states that the physical and chemical properties of the elements are the periodic functions of their atomic masses. Mendeleev arranged the elements known at that time in order of increasing atomic masses, and this arrangement was called the periodic table.

• Mendeleev’s original table consists of eight vertical columns (called groups) & six horizontal rows (called periods). 
• Roman numerals indicated the group number.
• The zero group contains elements belonging to inert gases or noble gases, and elements present have zero valencies.
• The first half of the elements were placed in the upper left corner of the table, whereas the second half was in the lower right corner of each box.

Merits

• His classification condensed the study of about 90 elements (only 63 – 65 elements were known at that time, but he left a requirement for many more) to the study of only eight groups of elements.
• Some vacant spaces were left by him in his periodic table for the undiscovered elements and could even predict the properties of undiscovered elements. For example:  eka – boron = scandium.
• For certain elements, he corrected the atomic masses with the help of their expected positions and properties.

Demerits

• The position of hydrogen was not proper and controversial
• Separate classes were not provided for isotopes
• Anomalous positions of lanthanides and actinides in the periodic table.
• The similar elements were placed under different groups

Modern periodic law

The present form of the periodic table or long-form periodic table is based on the modern periodic law, which states that the elements have been arranged in order of increasing atomic numbers.

• The 18 vertical columns of the long form of the periodic table are called groups.
• Some groups have familiar names. 
• Group 1 elements are called alkali metals.
• Group 2 elements are called alkaline earth metals
• Elements of group 15 are called the Nitrogen family
• Chalcogens were named for group 16 elements
• Halogens are the common name for group 17 elements
• Elements of group 18 are called noble gases.
• Elements of group 11 are called coinage metals.
• The seven horizontal rows or columns of the long forms of the periodic tables are called periods.

Classification of the Elements

1. s-block elements: Group 1 & 2 elements considered as s-block. The general electronic configuration is [inert gas] ns¹⁻².
2. p-block elements: Group 13 to 18 elements are said to be p-block. General electronic configuration is [inert gas] ns²np¹⁻⁶
3. d-block elements: Group 3 to 12 elements are under d-block. General electronic configuration is [inert gas] (n−1)d¹⁻¹⁰ns¹⁻²
4. f-block elements: General electronic configuration is (n−2)f¹⁻¹⁴(n−1)d⁰⁻¹ns².

The central concept of classification has been explained in this article. For further references, you can refer to the classification of elements and periodicity in properties class 11 notes from Myncert.com. Classification and the trends are not limited to a particular area. Hope this article gives you a brief overview of the periodic table discovery and its classification.