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## Saturday, December 8, 2012

### Structure of Atom: IX NCERT Solution

Page 47
1. What are canal rays?

Ans: Canal rays are positively charged radiations that can pass through perforated cathode plate. These rays consist of positively charged particles known as protons

2. If an atom contains one electron and one proton, will it carry any charge or not?
Ans: An atom containing one electron and one proton will not carry any charge they make an atom neutral.

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3. On the basis of Thomson’s model of an atom, explain how the atom is neutral as a whole.

Ans:  According to Thomson’s model of the atom, an atom consists of equal number of negatively and positively charged particles. The negatively charged particles are embedded in the positively charged sphere and balance their charges. Thus, they make an atom neutral.

4. On the basis of Rutherford’s model of an atom, which subatomic particle is present in the nucleus of an atom?

Ans: protons (positively-charged particles)

5. Draw a sketch of Bohr’s model of an atom with three shells.
Ans:
6. 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?

Ans: 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 but It is difficult to make such foils with other metals.

7. Helium atom has an atomic mass of 4 u and two protons in its nucleus. How many neutrons does it have?

Ans: No of neutron = Atomic mass – atomic number = 4-2=2

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

Ans: Atomic number of carbon is 6. The distribution of electrons in a carbon atom as 2(k), 4(L).
Atomic number of carbon is 11. The distribution of electrons in a carbon atom as 2(k), 8(L).1(M)

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

Ans: K + L = 2+8 = 10 electrons
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10. How will you find the valency of chlorine, sulphur and magnesium?

Ans: 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.

(i) Valency of chlorine = 8 – Number of electrons in the valence shell
= 8 – 7 = 1
(ii) Valency of sulphur = 8 – Number of electrons in the valence shell
= 8 – 6 = 2
(iii) Valency of magnesium = Number of electrons in the valence shell= 2

11. 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?

Ans:  (i) Atomic number = number of protons=  8
(ii) The charge on the atom is zero, because total number of positive charges due to the protons is equal to total number of negative charges due to electrons

12. With the help of the given table, find the mass number of oxygen and sulphur atom.

Element       Symbol        No. of Protons       No. of Neutrons     No. of Electrons
Oxygen       O                 8                           8                          8
Sulphur        S                 16                         16                         16
Ans: The mass number of oxygen   =  No. of Protons + No. of Neutrons
=8+8=16 u
The mass number of oxygen           = No. of Protons + No. of Neutrons
=16+ 16= 32 u
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13. For the symbol H, D and T, tabulate three subatomic particles found in each of them.

Ans: These are isotopes of Hydrogen
Element                         Symbol       Protons      Neutrons     Electrons
Hydrogen      (1H1)         H                1                 0                 1
Deuterium     (2H1)        D                1                 1                 1
Tritium          (3H1)       T                 1                 2                 1

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

Ans: Isotopes of chlorine are   35Cl17           and 37 Cl17
Since isotopes of elements have atomic number so,  there is no change in Electronic configuration = 2,8,7
Isotopes of carbon  are   12C6      ,   14 C6  and  16 C6
Electronic configuration= 2,4
40Ca20     and 40Ar18  are a pair of isobars having atomic number 20 and 18
The electronic configuration of   40Ca20   is   2, 8, 8, 2.
The electronic configuration of   40Ar18    is   2, 8, 8.
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15. Compare the properties of electrons, protons and neutrons.

 Electron Proton Neutron (i) present outside the nucleus of an atom. present in the nucleus of an atom. Present in the nucleus of an atom. (ii) Negatively charged. Positively charged. No charge (iii) The mass is considered to negligible. The mass of a proton is approximately 2000 times as the mass of an electron. The mass of neutron is nearly equal to the mass of a proton.

16. What are the limitations of J.J. Thomson’s model of the atom?

Ans: J.J. Thomson’s fail to explain how electrons are arranged in positively charged sphere of an atom.

17. What are the limitations of Rutherford’s model of an atom?

Ans: Rutherford fail to explain how electron revolving round nucleus in well define orbit undergo acceleration without losing or gaining energy.
If an electrically charged particle revolves around a circular path, then it always radiates out energy, gradually move towards the nucleus in a spiral path, till fall into the nucleus. If this happen atoms may be unstable. This is not true.

18. Describe Bohr’s model of the atom.

Ans: Bohr’s model of the atom
(i) Electrons are allowed inside the atom in special orbit or shell he called the energy levels   K, L, m and N
(ii) While revolving in these discrete orbits, the electrons do not radiate energy.
(iii) Each energy level has a fixed amount of energy. The orbit nearest to the nucleus has minimum energy, whereas the orbit farthest from the nucleus has highest energy.
(iv) There is no change in the energy of electrons as long as they revolve in the same orbit and remain stable. But if an electron gains energy, it jumps to a higher energy level and if it loses energy it falls to a lower energy level.
(v) The mass of the atom is almost entirely due to the nucleus, as electrons have negligible mass

19. Compare all the proposed models of an atom given in this chapter
Ans:
 Thomson’s Model Rutherford’s Model Bohr’s Model An atom is positively charged sphere and electrons are embedded in it. An atom has positively charged centre and electrons revolve around it An atom has positively charged centre and electrons revolve around it in special energy level.

20.i f bromine atom is available in the form of, say, two isotopes 7935 Br (49.7%) and 8135 Br (50.3%), calculate the average atomic mass of bromine atom.

Ans:  the average atomic mass of bromine atom = 49.7% of 79 + 50.3% of 81= 39.263+40.743 = 80.006 u = 80 u,

21. Summaries the rules for writing distribution of electrons in various shells for the first eighteen elements.

Ans. (i) The maximum number of electrons revolving around the nucleus of an atom in different  shells is given by the formula 2n2, where n is the number of the shell as counted from the nucleus of an atom. So,
Maximum number of electrons in shell number 1 = 2n2 = 2(1) 2 = 2
Maximum number of electrons in shell number 2 = 2n2 = 2(2) 2 = 8
Maximum number of electrons in shell number 3 = 2n2 = 2(3) 2 = 18
(ii) The outermost shell will not have more than 8 electrons. For example, if there are 9 electrons after filling the first two shells then in the third shell there will be 8 electrons and one electron goes to the fourth cell.

22. Define valency by taking examples of silicon and oxygen.

Ans. The number of electrons given or received or shared by an atom of an element, to gain electronic configuration of the nearest noble gas is called its valency.
The valency is equal to the number of electrons in the outermost shell (valence shell) of an atom, provided it has 1 to 3 electrons in it. Such atoms are generally metals.
The valency is equal to 8 – number of electrons in valence shell of an atom, provided it has 4 to 8 electrons in the valence shell.
Silicon has electronic configuration 2(K), 8(L), 4(M). As silicon has four electrons in its valence shell, therefore, its valency is (8 – 4) = 4.
Oxygen has electronic configuration 2 (K), 6(L). As oxygen has six electrons in its valence shell, therefore, its valency is (8 – 6) = 2

23. Explain with examples (i) atomic number (ii) mass number (iii) isotopes and (iv) isobars.  Give any two uses of isotopes.

Ans. (i) Atomic number : The number of protons in the nucleus of an atom is called its atomic number.
Example : Chlorine has 17 protons in its nucleus, therefore, its atomic number is 17.
(ii) Mass number : The number of protons and neutrons present in the nucleus of an atom is known as its mass number.
Example : Sodium has 11 protons and 12 neutrons in its nucleus, therefore, its mass number is (11 + 12) = 23.
(iii) Isotopes : Atoms of the same element having the same atomic number but different mass numbers are called isotopes.
Example : Chlorine has two isotopes 3517Cl and 3717 Cl , in which atomic number is 17 for both, but mass numbers are 35 and 37.
(iv) Isobars : Atoms of different elements having the same mass number, but different atomic numbers are known as isobars.
Example : Calcium and argon atoms have the same mass number 40, but different atomic numbers 20 and 18 respectively.

Uses of Isotopes

1. Isotope of uranium 23592U is used as a nuclear fuel in atomic reactors.
2. Cobalt – 60, an isotope of cobalt is used in the treatment of cancer.
3.Isotopes of iodine is used to treat goitor

24. Na+ has completely filled K and L shells. Explain.

Ans. Na loose 1 electron to form sodium ion Na+ . So, Na+ ion has one electron less than Na atom i.e., it has 10 electrons.  Therefore, Electronic configuration of Na+ = 2,8  completely filled k(2) and(8) l shells

25. If bromine atom is available in the form of, say, two isotopes 7935 Br (49.7%) and 8135 Br (50.3%), calculate the average atomic mass of bromine atom.

Ans:  the average atomic mass of bromine atom = 49.7% of 79 + 50.3% of 81= 39.263+40.743 = 80.006 u = 80 u,

26. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes 16 8 X and 18X in the sample?
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