Why definition of an atom given by Dalton is no longer valid?

20g silver nitrate solution is added to 20g of sodium chloride solution. What change in mass do you expect after the reaction and why?

In a given sample of ammonia, 9g hydrogen and 42g nitrogen are present. In another sample, 5g hydrogen is present along with nitrogen. Calculate the amount of nitrogen in the second sample.

Why is it necessary to use the symbol for the elements?

What is wrong with the following symbols? Give the correct symbol in each case.

(i) Sodium (So)

(ii) Hydrogen (Hg)

(iii) Copper (Co)

(iv) Sulphur (S)

(v) Calcium (CA)