CLASSIFICATION OF ELEMENTS

DAY 3
13.5.20
Good morning Boys,
Today we will start with classification of elements.  you have previuosly learned about newlands law of octaves, Mendeleevs law .
Learning Outcomes :

• Recall classification of elements
• Illustrate various law for classifying elements.

Dobereiner’s law of triads

Dobereiner classified the elements into groups of three elements 
(triads ) with similar properties in such a manner so that the atomic weight of the middle element was the arithmetic mean of the other two, e.g.,

Element	Li	NA	K
Atomic weight	7	23	39

Mean of atomic masses = (7 + 39) / 2 = 23

Similarly CI, Br, I; Ca, Sr, Ba are two more examples of such triads.

Limitations 
could not arrange all the elements known at that time into triads. He could identify only three such triads that have been mentioned.

• Newland’s Law of Octaves
•  when elements are arranged in order of increasing atomic masses the properties of eight elements are repetition of properties of first element.

LIMITATIONS-

1.  classification was successful up to  calcium only.

2.  inclusion  of Noble gases disturbed the law of octaves .

Lother Meyer’s Atomic Volume Curve (1869)

Meyer presented the classification of elements in the form of a curve between atomic volume and atomic masses and state that the properties of the elements are the periodic functions of their atomic volumes.

[Here, atomic volume = molecular mass / density

He concluded that the elements with similar properties occupy similar position in the curve.

Mendeleev’s classification of elements

Mendeleevs Periodic Table is based upon Mendeleevs periodic law which states ‘The physical and chemical properties of the elements are a periodic function of their atomic masses.”

This Periodic Table is divided into seven horizontal rows (periods) and eight vertical columns (groups). Zero group was added later on in the modified Mendeleefs Periodic Table.

Importance of Mendeleevs Periodic Table

Few important achievements of Periodic Table are

1 Systematic study of the elements.

2. Prediction of new elements and their properties. he left space for the elements yet to be discovered. e.g., he left spaces for Ga and Ge and named these elements as Eka-aluminium (Ga) and EKa-silicon (Ge) respectively

3. Atomic mass correction of doubtful elements on the basis of their expected positions and properties.

Defects in the Mendeleev’s Periodic Table

(i) Position of hydrogen Hydrogen has been placed in group IA (alkali metals). but it also resembles with halogens of group VIlA. Thus. its position in the Mendeleev’s Periodic Table is controversial.

(ii) Position of isotopes e.g., ¹H1 ²H1 ³H1 would occupy different positions.

(iii) Anomalous positions of some elements  element with higher atomic mass precedes the element with lower atomic mass. For example, AI (atomic weight = 39.9) precedes K (atomic weight = 39.1) and similarly Co (atomic weight. = 58.9) has been placed ahead of Ni (atomic weight = 58.7).

(iv) Position of Lanthanoids and actinoids Lanthanoids and actinoids were not placed in the main Periodic Table.

THATS ALL FOR THE DAY.
MARK YOUR ATTENDANCE ON THE FORM