Or
Write a short note on electron, Proton and Neutron?
Ans: An atom consists of Electron, Proton and Neutron
Electron: Electron was discovered by J.J Thomson. Electron has negative charge and has very low mass.
The mass of electron is 1/1840 U and its
- 19
charge is 1.6 × 10 C.
Proton: Proton was discovered by E. Goldstein. Proton is positively charged and has high mass.
The mass of proton is 1840 times than
-19
electron and its charge is 1.6 × 10 C.
Neutron: Neutron was discovered by Chadwick. Neuton has no charge and its mass is equall to proton.
Ans: When J.J Thomson gave his model of atom, only electrons and protons were discovered.
Thomson suggested that atom is a positive sphere where in negative charge is embedded.
His model can be compared with Pudding or watermelon.
Q. Elaborate the Rutherfords Model of Atom?
Or
How was the nucleus of atom discovered?
This experiment led to the Discovery of Nucleus and Rutherfords model of atom.
In this experiment, he hit a thin gold foil with α Particles. The results were as:
i. Most α Particles passed straight.
ii. Some particles were deflected.
iii. Very few bounced back.
Based on it, he gave a model of atom called Rutherfords model of atom. It suggested:
i. Most space in atom is empty.
ii. The positive charge takes small space.
iii. Positive charge is concentrated in small volume.
He concluded that atom has small positive nucleus and electons revolve around it in orbits.
Q. List the drawbacks of Rutherford's model of atom?
Ans: Rutherford said electrons revolve around nucleus in orbits. However in a circular orbit, a charged particle will loose energy due to acceleration and fall into nucleus.
Q. Explain the Bohr's Model of Atom?
Ans: Neils Bohr gave Bohrs model of atom.
Bohr said the atom contains Protons (+), Electrons (-) and Neutrons. The atom is overall neutral.
Protons and neutrons occupy small space in nucleus and electrons revolve around it in shells or energy levels.
These shells were named as K, L, M, N and each shell has fixed energy.
If electron jumps to lower shell, it looses energy and if it jumps to higher shell, it gains energy.
There are fixed number of electrons that
2
Q. What is meant by Valancy?
What are valance electrons?
Define valency by taking examples of silicon and oxygen.
Ans: Valency means the number of electrons that exist in outer most shell and such electrons are called valance electrons.
Depending on Valency , an atom gains, looses or shares electrons.
E.g. Valency of sodium is 1, Chlorine is 7.
Ans: Atomic number is defined as number of protons in an atom. It is also given as number of electrons in a stable atom.
It is denoted by "Z" and is unique to each element.
E.g, Atomic number of Carbon (C) is 6.
Q. What is meant by Mass Number?
Ans: Mass number is defined as number of protons plus neutrons in an atom.
It is denoted by "A" and gives atomic mass of an element.
E.g, Mass number of Carbon (C) is 12.
Q. What are Isotopes? Give their uses?
Ans: Isotopes are different forms of same element that have different mass number.
They have different physical properties but same chemical properties.
Examples: Hydrogen has three isotopes i.e
1 2 3
Proteum H1 , Dutreum H1 , Tritreum H1
Uses:
Isotopes of Uranium is used in nuclear reactors.
Isotopes of Cobalt, Arsenic are used in cancer treatment.
Isotopes of Iodine is used in Goitre treatment.
Q. What are Isobars?
Ans: Isobars are different elements which which have same mass number.
They have same number of protons plus neutrons.
40 40
Example: Calcium , C 20 , Argon, Ar 18 are Isobars.
Q. What are canal rays?
Ans: Canal rays are the beam of Positive charged ions that
were first obtained by E. Goldstein during Anode ray experiment.
These rays are made up of Protons.
Q. If an atom contains one electron and one proton, will it
carry any charge or not?
Ans: If an atom contains one electron and one proton, it
will not carry any charge and will be neutral.
This is because the charge of electron is equal and opposite
to proton.
Q. 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-particle scattering experiment is carried out
using a foil of a metal other than gold, the results will be same as that of Rutherford’s
experiment. This is because all the atoms have same structure.
Rutherford choose gold foil because it more malleable than
other metals and can be made into very thin sheets.
Q. Helium atom has an atomic mass of 4 u and two protons in
its nucleus. How many neutrons does it have?
Ans: Helium which has the atomic mass of 4 U and 2 protons
in its nucleus will have 2 neutrons in its nucleus.
Q. Write the distribution of electrons in carbon and sodium
atoms.
Ans: Carbon: The Carbon (C) has the atomic Number of 6. The
electron distribution will be as follows:
K Shell: 2 electrons
L Shell: 4 electrons
Sodium: The Sodium (Na) has the atomic number of 11. The
electron distribution will be as follows:
K Shell: 2 electrons
L Shell: 8 electrons
M Shell: 1 electron
Q. If K and L shells of an atom are full, then what would be
the total number of electrons in the atom?
Ans: If both K and L shells of an atom are full, then the
total number of electrons in the atom are 10.
The atom will be Neon (Ne)
Q. How will you find the valency of chlorine, Sulphur and
magnesium?
Ans: Valency means the number of electrons present in the
outermost shell or the number of electrons required to complete the outermost
shell of an Atom.
Chlorine: Chlorine has the atomic number of 17. Its
electronic configuration is 2,8,7. It needs 1 electron to complete its
outermost shell. So, its Valency is 1.
Sulphur: Sulphur has the atomic number of 16. Its electronic
configuration is 2,8,6. It needs 2 electrons to complete its outermost shell.
So, its Valency is 2.
Magnesium: Magnesium has the atomic number of 12. Its
electronic configuration is 2,8,2. It has only 2 electrons in its outermost
shell. So, its Valency is 2.
1. 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): The Atomic number is the number of Protons or the number
of electrons in a neutral atom. The atomic number of an element with 8
electrons and 8 protons is 8
(II): Since the number of electrons is equal to number of protons,
so the atom will be neutral.
1. For the symbol H, D and T tabulate three sub-atomic
particles found in each of them.
Ans: H, D and T are the tree isotopes of Hydrogen atom which
have same atomic number but different mass number.
|
Symbol |
Protons |
Neutrons |
Electrons |
|
H |
1 |
0 |
1 |
|
D |
1 |
1 |
1 |
|
T |
1 |
2 |
1 |
Q. With the help of Table 4.1, find out the mass number of
oxygen and Sulphur atom.
Ans: Mass number is the sum of Number of Protons and
neutrons in an atom.
The mass number of oxygen is 16 and the mass number of
sulphur is 32.
Q. Write the electronic configuration of any one pair of
isotopes and isobars.
Ans: Isotopes are different forms of same element that have different mass number. E.g. Chlorine-35, Chlorine-37. The electronic configuration will be same in both i.e. 2,8,7
Isobars are different elements which have same mass number
e.g. Calcium and Argon both have mass number of 40.
The electronic configuration of Calcium is 2,8,8,2
The electronic configuration of Argon is 2,8,8
Q. What are the limitations of J.J. Thomson’s model of the
atom?
Ans: The J.J Thomson gave plum pudding model of atom. This
model could not explain the Rutherford’s α-particle scattering experiment
results.
The model also failed to explain stability of atom with
negative charge embedded in the positive charge.
Q. Compare all the proposed models of an atom given in this
chapter.
Ans:
|
Thomson's Model |
Rutherford’s
Model |
Bohr's Model |
|
Electrons
exist inside sphere of Neutrons like a plum Pudding |
Neutrons
exist in nucleus which is Positively charged and electrons move around it |
Electrons
exist in specific energy levels called shells. Electrons can jump from one
shell to another |
Q. Summarise the rules for writing of distribution of
electrons in various shells for the first eighteen elements.
Ans: The rules for distribution of electrons in various
shells for first eighteen elements can be summarized as follows:
i. The maximum number of electrons that can present in a
shell is given by 2n^2 where ‘n’ is the number of the shell.
ii. Electron will first fill inner shells then the outer
shells.
Q. Na+ has completely filled K and L shells. Explain.
Ans: Na+ is the
cation of sodium which has lost one electron. In Na+ ion, only 10 electrons are
present instead of 11.
The electronic configuration of Na+ is 2,8 which shows it
has completely filled K and L shells.
Q. If bromine atom with atomic number 35 is available in
the form of two isotopes, with atomic mass 79 (49.7%) and 81 (50.3%), calculate
the average atomic mass of bromine atom.
Ans: Given,
Percentage of Isotope 1(Mass number 79) = 49.7%
Percentage of Isotope 2(Mass number 81) = 50.3%
We know,
Average atomic mass =
[Atomic mass of Isotope 1 x Percentage +Atomic mass of isotope
2 x Percentage] / 100
Average atomic mass of Bromine = [79 x 49.7 + 81 x 50.3] /
100
Average atomic mass of Bromine = 80.006 U
Q. The average atomic mass of a sample of an element X is
16.2 u. What are the percentages of isotopes with atomic mass 16 and 18 of the
sample X?
Given,
Average atomic mass = 16.2 U
Percentage of Isotope of atomic mass 16 = X %
Percentage of isotope
of atomic mass 18 = (100 – X) %
We know that;
Average atomic mass = Atomic mass 1 x Percentage + Atomic
mass 2 x Percentage / 100
16.2 = 16 x X/100 +
18 x 100 – X/100
16.2 = 16X
+1800 -18X (LCM)
100
16.2 = 1800 – 2X /100
1620 = 1800 – 2X
2X = 1800 – 1620
X = 180/2
X = 90
Therefore,
Percentage of isotope with atomic mass 16 = X =90%
Percentage of isotope with atomic mass 18 = 100 – X = 100
-90 = 10%
Q. If Z = 3, what would be the valency of the element?
Also, name the element.
Ans: If Z (Atomic number) of an Element is 3, Its valency is
1 and the element is Lithium.
Q. 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?
Ans:
i) The Mass number of X = No. of protons + No. of Neutrons
= 6 + 6
= 12 U
ii) The Mass number of Y = No of protons + No of Neutrons
= 6 + 8
= 14 U
Relation between two species:
Both of them are the isotopes of Carbon with same atomic
number of 6 but different mass number.
Q. For the following statements, write T for True and F for
False.
(a) J.J. Thomson proposed that the nucleus of an atom
contains only nucleons. (False)
(b) A neutron is formed by an electron and a proton
combining together. Therefore, it is neutral. (False)
(c) The mass of an electron is about 1/2000 times that of
proton. (True)
(d) An isotope of iodine is used for making tincture iodine,
which is used as a medicine. (True)
Q. Rutherford’s alpha-particle scattering experiment was
responsible for the discovery of
(a) Atomic Nucleus (b) Electron (c) Proton (d) Neutron
Ans: (a) Atomic
Nucleus
Q. Isotopes of an element have
(a) the same physical properties (b) different chemical
properties
(c) different number of neutrons (d) different atomic
numbers.
Ans: (c) different
number of neutrons
Q. Number of valence electrons in Cl– ion are:
(a) 16 (b) 8 (c) 17 (d) 18
Ans: (b) 8
Q. Which one 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
Ans: (d) 2,8,1
Q. Complete the following table.
Ans:
.png "Structure of Atom Class 9th")
