Class 10 Science Periodic Classification of Elements Exam Notes

Class 10 Science Periodic Classification of Elements Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Introduction

One hundred and fifteen elements are known till date and many more may be known in future. All elements differ in their properties due to the presence of different kinds of atoms in them. Elements combine to form millions of compounds.

Important Terms and Concepts :

1.Need of Classification : It is difficult to study each and every element individually and to know its properties and uses. Therefore, they have been classified into groups on the basis of their similarities in properties.

2.Classification : Classification means grouping of elements on the basis of similarities in properties, e.g., All of you belong to class X because you resemble in you properties, e.g., All of you have passed class IX, all of you are in age group of 13-15 years.

3.Basis of Classification : Classification is done on the basis of similarities in properties so that the systematic study could be made about them.

Early Attempts of classification

Lavoisier's Classification :

Lavoisier classified elements into metals and nonmetals.This classification was based on certain distinctive physical properties such as hardness, malleability and luster. On the basis of these properties, sodium and lead were classed together as belonging to the group of metals.

Limitations

(1) Hardness, malleability and luster were found to be the only common properties of sodium and lead, otherwise the two elements were entirely different.

(2) In such a classification there was no place for elements with properties resembling those of metals as well as nonmetals.

 There fore, Lavoisier's classification was found to be inadequate.  

Dobereiner's Classification :

Law of triads In 1817,German chemist Johann Dobereginer classified elements having similar chemical properties into groups of three. These groups were called triads.He proposed a law known as Dobereiner's law of triads. According to this law, when elements are arranged in the order of increasing atomic mass in a triad, the atomic mass of the middle element was found to be approximately equal to the arithmetic mean of the atomic masses of the other two elements.

The classification of elements into triads was very successful in predicting the atomic mass and properties of the middle element. Further, this classification showed that there exists some relationship between the properties of elements and their atomic masses. This paved the way for future attempts at classification of elements.

Limitation : All the elements could not be grouped into triads.

Newlands' Classification :

Law of octaves In 1864, John Newlands, and English chemist, showed that when elements are arranged in the order of their increasing atomic masses, the eighth element, starting from a given element, was a kind of repetition of the first one, like the eighth note in an octave of music, i.e., sa re ga ma pa dha ni sa, where the first and the eighth note are same.
A part of Newlands' classification is given below where the figures under the symbols show the atomic masses

Starting from lithium (Li) the eight element is sodium (Na). The eight element starting from sodium is potassium. The properties of lithium, sodium and potassium are similar. The properties of beryllium, magnesium and calcium are similar too.

Limitation :

(i) This law worked well for lighter elements (up to calcium), but it could not be applied to heavier ones (elements of higher atomic masses) because starting from calcium every eight element was found to have properties different from those of the first element.

(ii) Newlands emphatically said that only 56 elements do exist in nature and no more element is likely to be discovered in future. But this concept was later on found to be untrue with the discovery of many new elements which defined the law of octaves.

(iii) In arranging elements in the form of a table, Newlands clubbed two elements together at the same place and in the same column. Not only this, he also placed some dissimilar elements in the same column. For example, cobalt (Co) and nickel (Ni) were clubbed together in the column of fluorine (F), chlorine (Cl) and bromine (Br) (under sa/do). We know that cobalt and nickel have properties entirely different from those of fluorine, chlorine and bromine. It is also known that cobalt and nickel have properties similar to those of iron. But iron (Fe) was placed in a column (under ni/ti) different from the column of cobalt and nickel.
However, this law support to the idea that the properties of elements depend upon the atomic masses. It also showed that the properties of elements are repeated after a certain interval, i.e., the properties of elements are periodic in nature.

Mendeleev's periodic law and periodic table

While working systematically on the physical and chemical properties of elements, Dmitri Invanovich Mendeleev noticed that properties of elements varied regularly with the atomic mass. He arranged the 63 elements then known in a table on the basis of similarities in properties. It was found that most of the elements occupied places in the table in order of their increasing atomic masses. In 1869, Mendeleeve formulated a law, now known as the periodic law. The law is stated as follows.

The properties of elements are periodic functions of their atomic masses. This means, if the elements are arranged in order of increasing atomic masses then those with similar properties are repeated at regular intervals.

On the basis of the periodic law, Mendeleev presented his classification in the form of a table, now known as Mendeleev's periodic table. A simplified version of this periodic table is given below. In this table, copper, silver and gold find places in groups I as well as VIII.

This table consists of vertical columns called groups and horizontal rows called periods. There are only eight groups in the table. Mendeleev left some vacant places (shown by question marks) for the yet undiscovered elements. Noble gases were not discovered then. So, he did not provide any place for them in his periodic table.
Mandeleev's idea was remarkable in that he used a fundamental atomic property (atomic mass) as the basis of classification. While classifying elements he laid special emphasis on tow factors.

1. Similar elements were grouped together.
2. Elements were arranged in order of increasing atomic masses.

Modified version of Mendeleev's Periodic Table :

The elements which were undiscovered and for whom Mendeleev had left vacant places were discovered later. Some of these are scandium (Sc), gallium (Ga) and germanium (Ge). These elements were accommodated in their proper places in the table. The elements helium (He), neon (Ne), argon (Ar), Krypton (Kr), Xenon (Xe) and radon (Rn) became known only towards the end of the nineteenth century.
These elements, called noble gases, were placed in the table as a separate group, called 0 group. The periodic table had to be modified then. The modified version of the table is shown below.

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