1. State the Law of Triads of elements.

It states that the atomic mass of the middle element of a triad is roughly the arithmetic mean of the other two elements. In 1829, Dobereiner proposed the Law of Triads.

2. Who propounded the Law of Triads? Give an example of a triad.

J.W. Dobereiner in 1817 propounded the Law of Triads. Lithium, sodium and potassium constitute a triad and have similar physical and chemical properties.

3. What is the achievement of classification of elements into triads?  

The basic achievement of classification on the basis of triads is that it recognised for the first time, the relationship between the atomic masses and properties of elements.

4. State the Law of Octaves for the classification of elements.

An English scientist called John Newlands put forward his Law of Octaves in 1864. When the elements are arranged in the order of their increasing atomic masses, the properties of the eighth element (starting from a given element) are a repetition of the properties of the first element.  For example, starting at Li, Be is the second element, B is the third and Na is the eighth element.

5. Who stated the Law of Octaves? Why was it named as Law of Octaves?

Newland stated the Law of Octaves. He found that when elements are arranged in the order of their increasing atomic masses, the elements with similar properties reoccurred each time, after every seventh element, like the musical notes in octaves.

6. State Mendeleev’s Periodic Law.

It states that the physical and chemical properties of all the elements are the periodic function of their atomic masses. Mendeleev arranged the known elements according to increasing order of their atomic masses because, according to him fundamental property of an element was atomic mass. He arranged these elements into eight   groups which were further divided into subgroups. Mendeleev’s periodic table was very precise and provided the basis of modern periodic classification.

7. How many groups and periods were there in the original Mendeleev’s Periodic table?

There were eight groups and seven periods in the original Mendeleev’s Periodic table. Each group was further subdivided into subgroups.

8. How many groups in the original Mendeleev’s periodic table, are further subdivided in sub groups ‘a’ and  ‘b’?

A group or family is a vertical column in the periodic table. Groups usually have more significant periodic trends than periods and blocks. Groups one to seven are subdivided into subgroup ‘a’ and subgroup ‘b’.

9. How many vertical columns are there in the original Mendeleev’s periodic table?

There are 15 vertical columns in the original Mendeleev’s periodic table.

10. Why was there no zero group in the original Mendeleev’s periodic table?

The elements of the zero group were not discovered at the time when Mendeleev made the periodic table and hence there was no zero group in his periodic table.