(a) The modern periodic table has been evolved through the early attempts of Dobereiner, Newland and Mendeleev. List one advantage and one limitation of all the three attempts.
(b) Name the scientist who first of all showed that atomic number of an element is a more fundamental property than its atomic mass.
Answer
(a)
Dobereiner’s method of classification of elements:
- Advantage: This was the first time that someone tried to classify elements based on their chemical properties .He was able to arrange elements in groups of three such that the atomic mass of the middle element was the mean of the atomic masses of first and third elements.This encourages others to classify elements.
- Disadvantage: He was just able to classify 9 elements in 3 triads .
Newland’s law of octave:
- Advantage: He was able to find a recurring pattern in the properties of elements .In his system of classification, the properties of the eight elements were similar to the properties of the first element.
- Disadvantage: Newland’s law of octaves was only valid for lighter elements i.e. only till calcium. Also, some elements like Co and Ni which have very different properties were grouped together.
Mendeleev’s Periodic Table:
- Advantage: He was able to classify all the known elements and also left gaps for the undiscovered elements and predicted their properties. The elements were classified on the basis of their atomic number.
- Disadvantage: The position of H could not be decided. There was an irregular increase in the atomic mass and it could also not be decided if the isotopes should be placed in the same slot or not.
(b)
Henry Mosely was the scientist who showed that periodic properties of elements are a function of their atomic number rather than their atomic mass.
