ICSE Class 9 Chemistry Chapter 08 The Periodic Table

Chapter 8: The Periodic Table

Latest Syllabus - Scope of Syllabus - The Periodic Table

The Periodic Table

Dobereiner's Triads, Newland's Law of Octaves, Mendeleev's contributions; Modern Periodic Law, The Modern Periodic Table (groups and periods).

General idea of Dobereiner's triads, Newland's Law of Octaves, Mendeleev's periodic law, Discovery of Atomic number and its use as a basis for Modern Periodic law. Modern Periodic Table (groups 1 to 18 and periods 1 to 7)

NOTE: According to the recommendation of International Union of Pure and Applied Chemistry (IUPAC), the GROUPS are numbered from 1 to 18 replacing the older notation of GROUPS IA., VIA, VIII, IB... VIIB and 0. However, for the examination both notations will be accepted.

A. Introduction

Elements - are pure substances made up of one kind of atoms and cannot be broken down into simpler substances. There are today about - 118 elements discovered so far.

Elements had to be properly classified - and the best classification is the one which puts together those elements which show - maximum resemblance and separates the others.

Basis of Classification of Elements

Early Chemists - Attempted to classify elements on the basis of their - properties, nature, character, valency etc.

Dobereiner [1815] - Arranged the elements in increasing order of their atomic weights - into groups of three called - 'triads'.

Newland [1864] - Arranged the elements in increasing order of their atomic weights - and suggested that in a given series - 'every eighth element - was a repetition of the first'.

Mendeleeff [1869] - Arranged the elements in increasing order of their atomic weights - in the form of a table called periodic table and observed that - 'Elements with similar properties - appear at regular intervals'. This is called 'periodicity of properties of elements'. He stated that - 'Properties of elements are periodic functions of their atomic weights'.

Moseley [1912] - Arranged the elements in increasing order of their atomic numbers - and found that atomic number is a more fundamental characteristic of an element than its - atomic weight. He stated that - 'Properties of elements are periodic functions of their - atomic numbers'.

Teacher's Note

The evolution of the periodic table is like how we organize our kitchen - we first try grouping by size, then by color, then by function, until we find the most logical system that tells us where everything belongs and why items are related.

B. Classification of Elements

Classification - By Early Chemists

Basis of Classification of Elements

On basis of - properties, valencies and metallic and non-metallic nature of elements.

The above method of classification was discarded since elements behave - both as metals and non-metals, show metallic and non-metallic nature and - have variable valency.

Atomic weight - and not properties were considered for further classification.

I. Dobereiner's Triads

Basis of Classification of Elements

Dobereiner arranged elements in - increasing order of atomic weights - in groups of three called 'triads'.

Dobereiner's Law of Triads

'Chemically analogous elements arranged in - increasing order of their atomic weights formed - well marked groups of three called 'triads' in which the - atomic weight of the middle element was found to be - generally the average (arithmetic mean) of the atomic weights of the other two elements'.

The atomic weight of Sodium is the - Average mean of Lithium and Potassium

Method was a disordered collection of elements - not holding true for all elements.

Properties of elements grouped into triads - were found not to be similar.

II. Newland's Law of Octaves

Basis of Classification of Elements

Newland arranged elements in - increasing order of atomic weights - in series of eight.

Newland's Law of Octaves

'Elements when arranged in - increasing order of their atomic weights show - resemblance in physical and chemical properties between the - eighth element and the first element (similar to the eighth and first notes on a musical scale)'.

The eighth element from Li is Na and from F is Cl - which show similarity in properties.

Method showed odd grouping and failed - to leave spaces for undiscovered elements.

Method of classification showed meaning with light metals e.g. Li and Na, - but failed with heavy metals beyond K.

Teacher's Note

Newland's approach is like noticing that a musical scale repeats every octave - similarly, every eighth element shows similar properties, but this pattern breaks down for heavier elements just as musical harmony becomes more complex.

C. Mendeleeff's Contributions - To the Periodic Table

Mendeleeff's Contributions

Basis of Classification of Elements

Mendeleeff arranged elements in - increasing order of atomic weights - in the form of a table called the 'periodic table'.

- This arrangement had elements with - similar properties - appearing at regular intervals.

- This phenomenon is called - 'periodicity of properties' and the - properties which repeat themselves at regular intervals are called - 'periodic properties'.

Periodic properties include- atomic radius, ionisation potential, electron affinity, etc.

Mendeleeff's Periodic Law

'The physical and chemical properties of elements are - periodic functions of their - atomic weights'.

Mendeleeff's Original Periodic Table of Elements - (first three periods shown above)

Mendeleeff's Contributions

- Elements were arranged - In increasing order of atomic weights - in horizontal rows called 'periods' and vertical columns called 'groups'.

- Elements which are similar with respect to their chemical properties - are grouped together and have atomic weights of nearly the same value.

- Elements in the same group - had the same 'valency' and similar chemical properties.

- Based on the periodicity of properties - a number of gaps were left in the table - for undiscovered elements i.e. elements now discovered e.g. Scandium, Gallium and - Germanium originally called eka-boron, eka-aluminium and eka-silicon respectively

- The properties of the undiscovered elements - left in the vacant gaps was predicted.

- Incorrect atomic weights of some of the arranged elements - were corrected with the knowledge of the atomic weights of the adjacent elements.

Defects in Mendeleeff's Periodic Table

- Anomalous pairs of elements - Certain pairs of elements having higher atomic weights have been given positions before the elements having lower atomic weights. This defect disappears if elements were arranged according to their - atomic numbers. e.g. Co (at. wt. 58.9, at. no. 27) was placed before Ni (at. wt. 58.6, at. no. 28).

- Position of rare earths and actinides - Could be justified only if arranged according to their - atomic numbers.

- Position of isotopes - Isotopes had to be placed in same position according to - atomic numbers

Teacher's Note

Mendeleeff's genius was predicting undiscovered elements like gaps in a crossword puzzle - just as we can guess a missing letter based on surrounding words, he predicted element properties based on their neighbors in the table.

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