1. Newlands proposed the law of octaves. Mendeleev suggested eight groups for elements in his table. How do you explain these observations in terms of modern periodic classification? (AS – 1) (4 Marks)
A: According to Newland’s law of octaves every eighth element starting from a given element resembles in its properties to that of the starting element. If we start at Lithium, then the eighth element is sodium and next coming eighth is potassium and so on. These elements show similar physical and chemical properties.
According to modern periodic table elements lithium, sodium, potassium …. etc. are all placed in the same group starting that they have similar physical and chemical properties.
Mendeleev’s periodic table has eight vertical columns. These are groups. There are seven horizontal rows. These are periods. According to Mendeleev’s table elements present in a group have similar properties. What is proposed by Mendeleev was supported in modern periodic table?
For example Mendeleev gave the general formula for the first group of elements as R2O and for second group of elements as RO, the same thing was followed in modern periodic table.
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2. What are the limitations of Mendeleev’s periodic table? How could the modern periodic table over come the limitations of Mendeleev’s table? (AS – 1) (4 Marks)
A: Limitations of Mendeleev’s periodic table
1. Anomalous pair of elements: Certain elements of highest atomic weights precede those with lower atomic weights. For example, tellurium (atomic weight 127.6) precedes iodine (atomic weight 126.9).
2. Dissimilar elements placed together: Elements with dissimilar properties were placed in same group as sub-group A and sub-group B. For example, alkali metal like Li, Na, K etc., of I A group have little resemblance with coinage metals like Cu, Ag, Au of I B group. Cl is of VII A group and ‘Mn’ is of VII B, but chlorine is a non metal, where as manganese is a metal.
Modern periodic table: Over coming the above limitations:
The periodic law is changed from atomic weight concept to atomic number concept in the modern periodic law.
a) In Mendeleev’s periodic table tellurium (Te) (atomic weight 127.6) precedes iodine (atomic weight 126.9).
b) In modern periodic table this placement is made justified because the atomic number of tellurium is 52 and that of iodine is 53.
c) So in the long form of periodic table these dissimilar elements changed their place in groups and periods.
a) In the long form of periodic table ‘Cl’ falls in 3rd period and VII A group and ‘Mn’ falls in 4th period and VII B group.
b) Metals and non-metals are separated here.