NIOS Chemistry Lesson 3 Periodic Table and Periodicity in Properties Part 1

Lesson 3 Periodic Table and Periodicity in Properties

Early Attempts

The Atomic Weight of the atomic weight of the middle element was almost the arithmetical mean of the other two elements in that group. This was proposed by J.W. Dobereiner in 1817.

For example

 Element Lithium Sodium Potassium Atomic Weight 6.94 22.99 39.10 Mean Atomic Weight ----- 23.02 -----
• Such a group of three elements was called a triad by J.W. Dobereiner.

• In 1865, J.A.R. Newlands proposed that if the chemical elements are arranged according to increasing atomic weight, those with similar physical and chemical properties occur after each interval of seven elements.

• Periodicity-Reoccurrence of properties after regular intervals is known as Periodicity.

Mendeleev’S Periodic Table

• Mendeleev arranged the elements in horizontal rows (known as periods) in the increasing order of their atomic masses.

• The elements were arranged in vertical column called groups according to their resemblance in properties.

• The Periodic table consisted of 8 groups and 7 periods.

• He left gaps for the undiscovered elements as only 63 elements were known at that time.

Modern Approach

• The periodic table based on the atomic number is known as Modern Periodic Table.

• All the elements were arranged by Moseley according to increasing atomic number.

• He proposed that the properties of elements are periodic function of their atomic numbers.

• The Modern Periodic Law proposed that the properties of elements are the periodic function of their atomic numbers.

• The modern periodic law also stated that the physical and chemical properties of the elements are the periodic function of the atomic numbers.

Long Form of the Periodic Table

• The electronic configuration of the elements matched the physical and chemical properties of the elements.

• An atom loses electron(s) from or gains electron(s) in the outermost shell of an atom during a chemical reaction.

• The outer most shell is involved in the sharing of an electron or electrons by an atom with other atom or atoms. The electrons in the outermost shell of an atom largely determine the chemical properties of the elements.

• The elements with similar outer electronic configuration should possess similar physical and chemical properties. Also, it should be placed together for an easy and systematic study.

Structural Features of the Long Form of Periodic Table

• There are 18 vertical columns called GROUPS and the same are numbered from 1 to 18 with each GROUP having a unique configuration.

• There are seven horizontal rows called PERIODS and these periods have been numbered from 1 to 7.

• Out of the total 115 elements known today 90 are naturally occurring and others are made through nuclear transformations or are synthesized artificially.

• There are only two elements in the first period (very short period). Second and third periods consist of only eight elements each (short periods). Fourth and fifth periods consist of 18 elements each (long periods). Sixth period consists of 32 elements (long period). Seventh period is yet incomplete and more and more elements are likely to be added as the scientific research advances.

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