(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.
(c) State Modern periodic law.
Answers
Answer :
a) 1. Dobereiner’s method of classification:
♦ Advantage: This was the first time that metals were grouped on the basis of their similar chemical properties. Three Dobereiner’s triad could be found which had special characteristics.
♣ When arranged in increasing atomic masses, the atomic mass of the middle element was equal to the average of the atomic masses of the other two elements.
For example: Li – 6.9 u
Na – 23 u
K – 39.1 u
♠ This encouraged others to classify elements on the basis of chemical properties and atomic masses.
♣ Disadvantage: He was able to only place nine such elements in three triads and therefore this method wasn’t efficient enough for a study.
2. Newland’s law of octaves:
♦ Advantage: Newland was able to place 56 elements known at the time in his classification system. This system also, for the first time, allowed period recurrence of properties to be studied.
♦ Disadvantage: His classification system worked only for the lighter elements and the properties matched only for elements till Calcium. He also placed elements like Cobalt and Nickel in the same slot.
3. Mendeleev’s classification:
♦ Advantage: In his classification, Mendeleev arranged elements on the basis of increasing atomic masses and similar formula for hydrides and oxides. He also left gaps in his classification table for undiscovered elements.
♦ Disadvantage: Mendeleev placed elements with greater atomic mass prior to ones having lesser atomic mass for the similarity of properties. However he couldn’t justify his reason for doing so.
♦ Henry Moseley showed that the atomic number of an element is a more fundamental property than its atomic mass.
♦ The modern periodic law states that the properties of elements are the periodic function of their atomic number.
Answer:
a) 1. Dobereiner’s method of classification:
♦ Advantage: This was the first time that metals were grouped on the basis of their similar chemical properties. Three Dobereiner’s triad could be found which had special characteristics.
♣ When arranged in increasing atomic masses, the atomic mass of the middle element was equal to the average of the atomic masses of the other two elements.
For example: Li – 6.9 u
Na – 23 u
K – 39.1 u
Explanation: