Question

Enlist merits and demerits of Mandeleev’s periodic table.

Answer 1

Periodic table: It is defined as the table in which all the elements are arranged in a systematic order i.e. the elements having the same properties are categorized in the same group. This is the modern periodic table.

Complete step by step solution:
Let us first talk about Mandeleeve’s periodic table.
Mandeleev arranged the elements according to the increasing order of their atomic mass. He was the first scientist who proposed the idea of periodic table. He left spaces for those elements which was not discovered.
Merits of mandeleeve’s periodic table:
Correction of atomic mass: for example the mass of indium, gold and platinum was supposed wrong but by the mendeleev's periodic table their mass was corrected.
Prediction of new elements: he left some gaps in which he thought that there should be some element.
Noble gases were discovered too late but can be fitted in mendeleeve’s periodic table.
But the model was rejected because:
i) He was not able to locate the hydrogen in the table.
ii) He was not able to locate the isotopes in the table (i.e. the elements which have the same atomic numbers but different atomic mass).
iii) Chemically the same elements were not together.

Additional information:
Now let us first talk about modern periodic tables.
Periodic table is a table made to place the elements in a systematic order (i.e. according to the increasing atomic number). There are two components in a periodic table i.e. periods (horizontal lines) and groups (vertical lines).
Periods: The horizontal lines in the periodic table are known as periods. There are a total seven horizontal i.e. seven periods in a periodic table. And they are named as period one, period two and so on. In period first there are a total two elements i.e. hydrogen and helium. In periods $2$ and $3$ there are eight elements in each. In periods $4$ and $5$ there are $18$ elements in each and in periods $6$ and $7$ there are $32$ elements in each period. Hence are total $112$ elements. Elements of the same period have different properties.
Groups: The vertical lines in the periodic table are known as groups. There are a total eighteen groups in the periodic table. And they are named as groups $1,2,3,$ and so on. The elements of the same group have almost the same chemical and physical properties. As we down in a group then its size increases and also volume of the atom increases as the number of electrons and protons increases. But the increase in mass is greater than that of increase in volume so the density which is defined as mass per unit volume will increase generally. But if increase in volume is greater than that of increase in mass then density will decrease in that case.

Note: Isotopes: Those elements which have the same atomic number but different atomic mass. For example: $C - 12$ and $C - 13$. They both have the same atomic number i.e. $6$ but different atomic mass i.e. in $C - 12$ mass is $12$ and in $C - 13$ mass is $13$.
Isobars: Those elements which have the same atomic mass but different atomic numbers. For example: argon and calcium. The atomic number of argon is $18$ and that of calcium is $20$ but the atomic mass of both the elements is the same i.e. $40$.