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47
1. What are canal rays?
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.
5. Draw a sketch of Bohr’s model of an atom with three shells.
12. With the help of the given table, find the mass number of oxygen and sulphur atom.
14. Write the electronic configuration of any one pair of isotopes and isobars.
Ans: Isotopes of chlorine are 35Cl17 and 37 Cl17
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: Bohr’s model of the atom
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.
22. Define valency by taking examples of silicon and oxygen.
Chapter 9th class Chapter : Structure of Atoms
9th Chemistry Term 02 (8 post by CBSE adda)
Structure of Atom: IX NCERT Solution View
Empirical formula and molecular formula View
CBSE Board Question banks View
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 188 X in the sample?
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 188 X in the sample?
For further study material Visit :
Chapter 9th class Chapter : Structure of Atoms
9th Chemistry Term 02 (8 post by CBSE adda)
Structure of Atom: IX NCERT Solution View
Empirical formula and molecular formula View
CBSE Board Question banks View