Online Notes on Chapter=5 ( Periodic Classificatoin of Elements) Part 1

This is Online Notes on Chapter=5 ( Periodic Classificatoin of Elements)  Part 1 for preparation of CBSE BOARD Examination, NTSE etc.

• Matter around us is present in the form of elements, compounds and

mixtures.

• Elements are substances containing atoms of only one type. E.g., Na, Mg,

Au, etc.

• There are 118 elements known to us. All these have different

properties.

=> Need for Periodic Classification:

• To make the study of these elements easy, these elements have been

divided into few groups in such a way that elements in the same group

have similar properties. 

Now study of a large number of elements is reduced to a few groups of elements.

• Dobereiner’s Traids : When elements were arranged in the order of

increasing atomic masses, groups of three elements (known as traids),

having similar chemical properties are obtained.

The atomic mass of the middle element of the triad was roughly the average

of the atomic masses of the other two elements.

=> Newland’s Law of Octaves :

Newland arranged the then known elements in the order of increasing

atomic masses and found that the properties of every 8th element is

similar to that of the 1st element.

He compared this to the octaves found in music and called it the ‘Law of

Octaves’.

For example, the properties of lithium (Li) and sodium (Na) were found to

be the same.

Newland’s Octave:

=> Limitations :

• It was applicable upto calcium (for lighter elements only).

• Properties of new discovered elements did not fit into the law of octave.

• To fit elements into his table, Newlands put even two elements together

in one slot and that too in the column of unlike elements having very

different properties.

=> Mendeleev’s Periodic Table : 

When elements are arranged in the order of increasing atomic masses, the element with similar properties occur at regular intervals. 

The properties of elements are a periodic function of their atomic

masses.

Mendeleev’s periodic table is based on the chemical properties of

elements. It contains 7 periods (horizontal rows) and 8 groups (vertical columns).