CLASS-9
ATOMS AND MOLECULES
STRUCTURE OF AN ATOM

1. If an element M has atomic mass 27 and atomic number 13.How many electrons and neutrons does it contain? What valency should be shown by M?

2. WRITE THE ELECTRONIC CONFIGURATION  OF AN ATOM WITH ATOMIC NUMBER 6.

3.What are the limitations of Rutherford's Model of an atom?

4.Calculate the number of aluminium atoms present in 0.05g of aluminium oxide(Al2O3).(Al=27,O=16)

5.What are polyatomic ions? Give examples?

6.Write the chemical formulae of the following:
(a)        Magnesium chloride
(b)        Calcium oxide
(c)        Copper nitrate
(d)       Aluminium chloride
(e)        Calcium carbonate

7.What is the mass of−−
(a) 1 mole of nitrogen atoms?
(b) 4 moles of aluminium atoms (Atomic mass of aluminium = 27)?
(c) 10 moles of sodium sulphite (Na₂SO₃)?

 8.Describe Bohr’s model of the atom.

 9.Compare all the proposed models of an atom given in this chapter.

Thomson’s model	Rutherford’s model	Bohr’s model
An atom consists of a positively charged sphere with electrons embedded in it.	An atom consists of a positively charged particles concentrated at the centre known as the nucleus. The size of the nucleus is very small as compared to the size of the atom. The electrons revolve around the nucleus in well-defined orbits.	There are only certain orbits known as discrete orbits inside the atom in which electrons revolve around the nucleus. Electrons do not radiate energy while revolving.

10.Write the distribution of electrons in carbon and sodium atoms?

The total number of electrons in a carbon atom is 6. The distribution of electrons in carbon atom is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 4 electrons
Or, we can write the distribution of electrons in a carbon atom as 2, 4.
The total number of electrons in a sodium atom is 11. The distribution of electrons in sodium atom is given by:
First orbit or K-shell = 2 electrons
Second orbit or L-shell = 8 electrons
Third orbit or M-shell = 1 electron

11.If K and L shells of an atom are full, then what would be the total number of electrons in the atom?

 The maximum number of electrons that can occupy K and L-shells of an atom are 2 and 8 respectively. Therefore, if K and L-shells of an atom are full, then the total number of electrons in the atom would be 10 

12 What do you think would be the observation if the α-particle scattering experiment is carried out using a foil of a metal other than gold?

If the α-scattering experiment is carried out using a foil of a metal rather than gold, there would be no change in the observation. In the α-scattering experiment, a gold foil was taken because gold is malleable and a thin foil of gold can be easily made. It is difficult to make such foils from other meta

12.Complete the following table

.

13.Write the electronic configuration of any one pair of isotopes and isobars.

14.How will you find the valency of chlorine, sulphur and magnesium?

If the number of electrons in the outermost shell of the atom of an element is less than or equal to 4, then the valency of the element is equal to the number of electrons in the outermost shell. On the other hand, if the number of electrons in the outermost shell of the atom of an element is greater than 4, then the valency of that element is determined by subtracting the number of electrons in the outermost shell from 8.

The distribution of electrons in chlorine, sulphur, and magnesium atoms are 2, 8, 7;    2, 8, 6 and        2, 8, 2 respectively.

Therefore, the number of electrons in the outer most shell of chlorine, sulphur, and magnesium atoms are 7, 6, and 2 respectively.

Thus, the valency of chlorine = 8 −7 = 1

The valency of sulphur = 8 − 6 = 2

The valency of magnesium = 2

15.If number of electrons in an atom is 8 and number of protons is also 8, then (i) what is the atomic number of the atom and (ii) what is the charge on the atom?

(i) The atomic number is equal to the number of protons. Therefore, the atomic number of the atom is 8.

(ii) Since the number of both electrons and protons is equal, therefore, the charge on the atom is 0.

16.Composition of the nuclei of two atomic species X and Y are given as under

                 X                Y

Protons =   6              6

Neutrons = 6               8

Give the mass numbers of X and Y. What is the relation between the two species?Mass number of X = Number of protons + Number of neutrons

= 6 + 6

= 12

Mass number of Y = Number of protons + Number of neutrons

= 6 + 8

= 14

These two atomic species X and Y have the same atomic number, but different mass numbers. Hence, they are isotopes.