Structure of Atom – Class 9 Chemistry Important Questions

1. 1.  What are cathode rays?

Ans.:  The negatively charged rays moving from cathode to anode in the discharge tube experiment were named as cathode rays.

2. Compare the properties of electrons, protons and neutrons.

Ans:

 Electron Proton Neutron 1. Electrons are Negatively charged particles. 1. Protons are positively charged particles. 1. Neutrons do not carry any charge and are neutral. 2. Electrons revolve around the nucleus in well-defined orbits or discrete orbits. 2. Protons are present in the nucleus of all atoms. 2. Neutrons are present in the nucleus of all atoms, except hydrogen. 3. The mass of an electron is about 1/ 2000 times the mass of hydrogen atom. 3. The mass of a proton is taken as one unit and it is equal to the mass of neutron. 3. The mass of a neutron is taken as one unit and it is equal to that of proton. 4. An electron is represented as e–. 4. A proton is represented as p+. 4. A neutron is represented as n.

3. Helium atom has 2 electrons in its valence shell but its valency is not 2, Explain.

Ans.  Helium atom has 2 electrons in its outermost shell and its duplet is complete. Hence the valency is zero.

4.  Mention the main postulates of Thomson’s atomic model.

Ans.:  (a)  According to Thomson’s model, an atom consists of a positively charged sphere and the negatively charged electrons are embedded in it like the seeds in a water melon.

(b)  Thomson also proposed in his model that the negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.

5.  What Conclusions were drawn from Rutherford’s alpha scattering experiment?

Ans.:    Conclusions drawn from Rutherford’s alpha scattering experiment are:-

i) Most of the space inside an atom is empty. (As most of the α –particles passed straight through the gold foil.)

ii) The atom has a small nucleus having positive charge. (As some of the a-particles having positive charge were slightly deflected by small angles.)

iii)  The size of the nucleus is very small as compared to the size of the atom. (because very few α – particles appeared to rebound and most of the positive charge and mass of the atom is in the nucleus.)

6. 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?

(ii)        What is the charge on the atom?

Ans.:    (i)  Atomic number = No. of protons = 8.

(ii)  Since number of protons = Number of electrons ∴ the atom is neutral.

7. Which of the following are isoelectronic species:

$N{{a}^{+}},\,{{K}^{+}},M{{g}^{2+}},\,C{{a}^{2+}},\,{{S}^{2-}},\,Ar?$

Isoelectronic species are those which contain same number of electrons

$N{{a}^{+}},\,M{{g}^{2+}}$ are isoelectronic (contain 10 electrons)

${{K}^{+}},\,C{{a}^{2+}},{{S}^{2-}}$ and Ar are isolectronic(contain 18 electrons).

8. Write the main features of Rutherford’s model of an atom.

The main features of Rutherford’s model of an atom are:-

i) An atom has a positively charged nucleus at its centre and most of the mass of the atom is in the nucleus.

ii) The electrons revolve around the nucleus in different orbits, just as planets move around the sun.

iii)  The size of the nucleus is very small as compared to the size of the atom.

9. Describe Bohr’s model of atom.

Bohr put forward the following postulates about the model of an atom:

(i)   An atom has a small heavy positively charged nucleus at its centre and the electrons revolve around the nucleus in circular paths called orbits.

(ii)  Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.

(iii)  While revolving in discrete orbits the electrons do not radiate energy.

These orbits or shells are called energy levels. These orbits or shells are represented by the letters K, L, M, N, … or the numbers, n= 1, 2, 3, 4, …

Shells or energy levels in an atom

10. Atom ‘X’ has a mass number 209 and atomic number 82

Atom ‘Y’ has a mass number 209 and atomic number 83

(i) Write the number of protons present in X and Y

(ii) What is the usual name given to this pair of atoms, X and Y?

Ans.     (i) No of protons in X= 82

No of protons in Y=83

(ii) Isobars, since they have same mass number but different atomic numbers.

11. The relative abundance of two isotopes of an element having atomic masses 85u and 87u is 75% and 25% respectively. Find the average atomic mass of element.

Ans. Average atomic weight

$=\frac{85\times&space;75+87\times&space;25}{100}=85.5\,\,u$

12. The atom of an element has 2 electrons in the M-shell. What will be the atomic number of the element? Name the element.

Ans: As the atom has 2 electrons in the M-shell, this means that K and L shells are completely filled. As completely filled K-shell has 2 electrons and completely filled M-shell has 8 electrons, therefore, complete electronic configuration of the atom of the element will be:

K          L          M

2          8          2

$\therefore$  Total number of electrons = 2 + 8 + 2 = 12

As the atom is neutral, total number of protons = total number of electrons  = atomic number = 12. Hence, the element is magnesium.

13. Complete the following table.

 Atomic Number Mass Number Number of Neutrons Number of Protons Number of Electrons Name of the Atomic Species 9 – 10 – – – 16 32 – – – Sulphur – 24 – 12 – – – 2 – 1 – – – 1 0 1 1 –

Ans.

 Atomic Number Mass Number Number of Neutrons Number of Protons Number of Electrons Name of the Atomic Species 9 19 10 9 9 Fluorine 16 32 16 16 16 Sulphur 12 24 12 12 12 Magnesium 1 2 1 1 1 Deuterium 1 1 0 1 1 Protium

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

Ans.: Isotopes        –   $_{17}^{35}Cl$  $_{17}^{37}Cl$

Electronic configuration of both $_{17}^{35}Cl$  and   $_{17}^{37}Cl$  will be 2, 8, 7 as they have same atomic number.

Isobars            –  $_{20}^{40}Ca$    $_{18}^{40}Ar$

Electronic configuration of   $_{20}^{40}Ca$   is 2, 8, 8, 2

Electronic configuration of   $_{18}^{40}Ar$   is 2, 8, 8

15. Show diagramatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Since the atomic number of sodium atom is 11, it has 11 electrons. A positively charged sodium ion (Na+) is formed by the removal of one electron from a sodium atom. So, a sodium ion has 11–1 = 10 electrons in it. Thus, electronic distribution of sodium ion will be 2, 8. The atomic number of an element is equal to the number of protons in its atom. Since, sodium atom and sodium ion contain the same number of protons, therefore, the atomic number of both is 11.