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Structure of the atom
Answer:
Q1. Compare the properties of electrons , protons
and neutrons.
Answer:
Mass Charge Symbol
Electrons
- 9.108x10-31Kg
-1.602x10-19C 0-1e
Protons
- 1.676 x 10 -27
Kg 1.602x 10 -19 01e
Neutron
- 1.676 x 10
-27 Kg no charge 10n
Q2. What are the limitations of J.J. Thomson's
model of the atom?
Answer:
J.J. Thompson model explained the neutrality of
the atom. According to the model both the charges are scattered equally in an
atom giving overall neutrality of the atom.
However, there were some limitations of the
model, few of them are:
1. Failed to explain the results obtained by the
Rutherford's Sacttering experiments.
2. Could not explained the arrangement of
electrons and protons within an atom.
Q3. What are the limitations of Rutherford's
model of the atom?
Answer:
The major drawback of the Rutherford's model
could not explained the stability of the atom.
According to the Rutherford's model, electron are
revolving in a circular path. When charge particles move in a circular orbits
then give out radiations and hence would loose energy. Due to this the
electrons should have come closer and closer to the nucleus and would utimately
fall in the nucleus.
But atom are stable and rutherford's model could
not explained this phenomenon.
Q4. Describe Bohr's model of the atom.
Answer:
Bohr's model of atom was proposed by Neil Bohr in
1913. Basic postulates of Neil Bohr's model are:
1. An atom consists of a small positively charged
nucleus surrounded by negatively charge electrons mloving in definite circular
orbits. These circular orbits are called as energy shells.
2. An electron revolving in a particular orbit
has a fixed amount of energy.
3. The orbits are number as 1,2,3...or K,L,M..
starting from the nucleus. The electrons in an atom starts first fill K shell,
then shell and so on.
4. The electrons when move from one shell to
another ,they either loses or gain a certain fix amount of energy.
5. The number of electrons in the outer orbits
determines the properties of the element.
Q5. Compare all the proposed models of atom given
in this chapter.
Answer:
Q6. Summarize the rules for writing of
distrubution of electrons in various shells for the first eighteen elements.
Answer:
1. The maximum number of electrons that a shell
can have is 2n2., Where n is the quantum number of the shell. for
example
K shell -
2(1)2 = 2 electrons
L shell - 2(2)2 = 8 electrons
M shell - 2(3)2 = 18 electrons
N shell - 2(4)2 = 32 electrons
2. The number of electrons in the outermost shell
can only be 8. The penultimate shell coulc have a maximum of 18 electrons. Not
for K and L shells.
3. First inner shells should be filled then the
electrons in the given shells should be filled first.
Q7. Define valency by taking examples of silicon
and oxygen.
Answer:
The combining capacity of an atom of an element
is called as its valency. Or the number of electrons an atom can gain, loose or
share to get the nearest noble gas configuration is called valency of that
element.
Silicon -
Atomic number = 14
Elements in K shell = 2
Elements in L shell = 8
Elements in M shell = 4
Number of electrons required to gain noble gas
configuration = 8-4 = 4
Thus valency is 4
Oxygen -
Atomic number = 8
Elements in K shell = 2
Elements in L shell = 6
Number of electrons required to gain noble gas
configuration = 8-2 = 2
Thus valency is 2
Q8. (a)
Explain with examples
I. atomic number
II. mass number
III. isotopes
IV. isobars
Answer : (a)
I. Atomic number - The number of protons in the nucleus of the
atom is the atomic number of the element.
Example:
Atomic number of the hydrogen is 1 and silicon is 14
Example:
Atomic number of the hydrogen is 1 and silicon is 14
II. Mass number - The sum of the number of protons and the number of neutrons
in the nucleus of the atom is the mass number of the atom.
Example
Mass number of carbon is 12. That means carbon has 6 protons and 6 neutrons
Example
Mass number of carbon is 12. That means carbon has 6 protons and 6 neutrons
III. Isotopes - Isotopes are the atoms of the same elements with the same atomic
number but different atomic masses.
example
Protium and Deuterium are two isotopes of hydrogen having mass 1 and 2 units respectively.
example
Protium and Deuterium are two isotopes of hydrogen having mass 1 and 2 units respectively.
IV. Isobars - Isobars are the atoms of those elements that have the same atomic
mass numbers but different atomic numbers are called Isobars.
Example:
Example:
V. Calcium and Argon both have mass numbers of 40 but have atomic numbers of
20 and 18 respectively.
(b) Give any two uses if isotopes.
Answer:
1. Treatment of cancer - Cobalt isotope
2. As a fuel in nuclear reactor. Example -
uranium
Q9. Na+ has completely filled K and L shells. Explain.
Answer:
Atomic number of sodium is 11
Therefore,
Electrons in shell K = 2
Electrons in shell L = 8
Electrons in shell m = 1
Na+ lost 1 electron. There is no M
shell.
Therefore, the remaining shells K and L are completely filled.
Q10. If Bromine atom is available in the form of
say, two isotopes, 3579Br (49.7%)and 3581Br(50.3%).
calculate the average atomic mass of bromine.
Answer:
The average weight is = 79(49.7/100) + 81(50.3/100)
= 79x0.497 + 81x0.503
=
39.263 + 40.743
= 80.006u
Q11. The average atomic mass of a sample of an
element X is 16.2u. What are the percentage of isotopes 816X
and 818X in the sample.
Answer:
Let the percentage of isotope 816X be n %
Then the percentage of isotope 818X will be (100 - n)%
Therefore,
16.2 = 16(n/100) + 18(100-n)/100
=>
1620 = 16n + 1800 - 18n
=> 1620
= -2n + 1800
=> -2n = -180
=> n = 90%
Hence, the
percentage of isotope 816X is 90%
and the percentage of isotope 818X is 100 -90 = 10
Q12. If z=3, what would be the valency of the
element? Also, name the element.
Answer:
Given atomic number of the element is 3
Therefore,electrons in the K shell = 2
Electrons in the L shell = 1
There are 1 electron in the outermost shell is 1
and hence the valency is 1.
Q13. Composition of the nuclei of two atomic
species X and Y are given as under
X
Y
Protons =
6 6
Neutorns= 6 8
Give the mass numbers of X and Y. What is the
relation between the two species.
Answer:
Number of protons for X is 6 and the number of
neutrons is 6. Therefore the atomic mass of X is 6 +6 = 12 au
Number of protons for Y is 6 and the number of
neutrons are 8. therefore the atomic number is 6 + 8 = 14au
Given that both the X and Y have same number of
protons, then their atomic number are the same and hence they are isotopes.
Q14. For the following statements write T for
True and f for false.
a. J.J. Thompson proposed that the nucleus of an atom contains only
nucleons. FALSE
b. A neutron is formed by an electron and proton combining together. Therefore
it is neutral. FALSE
c. The mass of an electron is about 1/2000 times that of a proton. FALSE
d. Isotopes of iodine is used for making tincture iodine which is used as
medicine. TRUE
Q15. Rutherford's aplha-particle scattering
experiment was responsible for the discovery of
a. atomic nucleus
b. electron
c. proton
d. neutron
Answer:
(a) Atomic nucleus
Q16. Isotopes of an element have
a. the same physical properties
b. different chemical properties
c. different number of neutrons
d. different atomic numbers
Answer:
(c) different numbers of neutrons
Q17. Number of valence electrons in Cl-
ion are:
a. 16
b. 8
c. 17
d. 18
Answer:
(b) 8
Q18. Which of the following is a correct
electronic configuration of sodium?
a. 2,8
b. 8,2,1
c. 2,1,8
d. 2,8,1
Answer:
(d) 2,8,1
Q19. 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
|
-
|
-
|
-
|
1
|
-
|
1
|
-
|
-
|
-
|
20
|
0
|
10
|
10
|
-
|
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
|
Hydrogen
|
10
|
20
|
0
|
10
|
10
|
Neon
|
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