Download chemistry (chapter 2)

CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
1) What is an atom? Write definitions of different scientists about atom.
The name ‘Atom’ was derived from the Latin word ‘Atomos’ meaning indivisible. Atom is
made up of subatomic particles like electron, proton and neutron.
Democritus: Matter is composed of many indivisible particles called atoms.
John Dalton: All matter is made up of very small indivisible particles called atoms.
2) Write any four postulates of Dalton’s atomic theory.
i) All matters are composed of atoms.
ii) An atom is the smallest, fundamental particle. It is hard and dense sphere.
iii) Atoms cannot be created, divided into smaller particles or destroyed.
iv) In a chemical reaction, atoms are separated, combined, or rearranged.
3) What is Thomson’s plum pudding theory?
Thomson put forth his “plum pudding” theory. He postulated that atoms were solid
structures of positively charge with tiny negative particles struck inside. It is like Plums
in the pudding.
4) How cathode rays are discovered.
In 1895 William Crooks performed experiments by passing electric current through
gasses at low pressure in a discharge tube. When electric current was passed at low
pressure of about 10-4 shiny rays were emitted from the cathode. These rays were
called cathode rays.
5) Why the rays produced in discharge tube are called cathode rays?
These rays were called cathode rays as these were originated from the cathode and
move towards anode.
6) What is discharge tube?
A gas discharge tube is a hard glass tube fitted with two metallic cathode and anode at
the two ends. A gas is taken in the discharge tube and connected to high voltage
battery.
7) Write two properties of cathode rays.
i) These rays travel in a straight lines perpendicular to the cathode surface.
ii) They are deflected towards positive plate in an electric field. It shows that they are
negatively charged.
8) How canal (positive) rays are produced?
In 1886 Goldstein observed that in addition to cathode rays, others rays were also
produced at the same time in the discharge tube. These rays were travelling in the
opposite direction to cathode rays. He found that these rays passed through canals
(holes) of cathode. He called these rays as canal rays.
9) Write two properties of canal rays.
i) These rays travel in a straight line in a direction opposite to the cathode rays.
ii) Their deflection in an electric field and magnetic field show that these were positively
charged.
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CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
10) What is origin of canal rays?
These rays do not originate from anode. These are produced when the cathode rays
collide with the residual gas molecules present in the discharge tube and ionize them as
follows.
M + e-  M+ + 2e11) How Chadwick discovered neutron?
In1932 He bombarded alpha particles obtained from a polonium source on a Beryllium
target. He observed some highly penetrating radiations were produced. These
radiations were called neutrons.
12) Write any three properties of neutron.
i) They carry no charge. They are neutral.
ii) They are highly penetrating particles.
iii) Mass of neutron is nearly equal to the mass of proton.
13) Why Rutherford performed gold foil experiment?
He performed gold foil experiment in order to understand how positive and negative
charges were distributed in an atom.
14) Describe Rutherford’s atomic model experiment.
He bombarded alpha particles on a 0.00004 cm thick gold foil. Alpha particles are
emitted by radioactive elements like radium or polonium. These are actually helium
nuclei. He observed the effect of alpha particles on a photographic plate or screen
coated with zinc sulphide. He proved that the ‘Plum Pudding’ model of the atom was not
correct.
15) Write any two observations of Rutherford’s atomic model.
i) Almost all the alpha particles passed straight through the gold foil un-deflected.
ii) Out of 20000 particles, only a few were deflected at fairly large angles and very few
bounced back on hitting the gold foil.
16) Write four results of Rutherford’s experiment.
i) The complete re-bounce of a few particles show that the nucleus is very dense and
hard.
ii) Since a few particles are deflected it shows that the size of the nucleus is very small
as compared to the total volume of an atom.
iii) The electron revolves around the nucleus.
iv) An atom is a whole is neutral, therefore the number of electrons in an atom is equal
to the number of protons.
17) Define nucleons.
Except electrons, all other fundamental particles lie within a nucleus and are known as
nucleons.
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CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
18) Write defect’s of Rutherford’s atomic model.
i) According to classical theory of radiation, electrons being the charged particles should
release or emit energy continuously and they should ultimately fall into the nucleus.
ii) If the electron emits energy continuously, they should form a continuous spectrum but
in fact, line spectrum was observed.
19) Describe shortly Bohr’s Atomic theory. OR Define Energy level.
In 1913, Neil Bohr presented his atomic model. The Quantum theory of Max Planck was
the foundation for this model. According to Bohr’s model, revolving electron in an atom
does not absorb or emit energy continuously. The energy of a revolving electron is
“quantized” as it revolves only in orbits of fixed energy called energy levels.
20) What is difference between Rutherford and Bohr’s Atomic theory.
Rutherford’s Atomic theory
Bohr’s Atomic Theory
i) It was based upon classical theory
i) It was based upon quantum theory.
ii) Electron revolves around the nucleus
ii) Electrons revolve around the nucleus
in orbits of fixed energy.
iii) No idea about Orbits was introduced
iii) Orbits had angular momentum.
iv) Atoms should produce continuous
iv) Atoms should produce line spectrum.
spectrum.
v) Atoms should collapse.
v) Atoms should exist.
21) Define Electronic configuration.
The distribution of electrons in various sub shells is called electronic configuration.
Principle: In filling the sub-shells electron always enter in lower energy sub-shells first.
22) Define isotopes.
Isotopes were discovered in 1913 by English chemist F. Soddy.
“Two or more atoms of an element having the same atomic number, number of protons,
chemical properties but different mass number, number of neutrons, physical properties
are called isotopes.”
23) Define isotopes of Hydrogen.
Hydrogen has three isotopes. These are protium (1H1), deuterium (2H1) and tritium (3H1)
each having the same number of protons and electrons but different number of
neutrons.
24) Define isotopes of Carbon.
Carbon has three isotopes Carbon-12, Carbon-13, Carbon-14. Carbon-14 is a
radioactive. Carbon-12 is present in abundance 98.9% while Carbon-13 and Carbon-14
are both present only 1.1% in nature. All have the same number of protons and
electrons but different number of neutrons.
25) Define isotopes of Chlorine.
There are two isotopes of chlorine. Chlorine-35 and Chlorine-37. The abundance of
chlorine-35 is 75.53% and chlorine-37 is 24.4%.
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CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
26) Define isotopes of Uranium?
There are three isotopes of uranium-234, uranium-235, uranium-238. The uranium-238
is found in nature nearly 99% pure.
27) Define radioactive isotopes.
Isotopes which are produced artificially are called radioactive isotopes.
28) In which fields isotopes are used.
Isotopes are used in the following fields.
i) Radiotherapy.
ii)Tracer for Diagnosis and Medicine.
iii) Archaeological and Geological uses.
iv) Chemical reaction and structure Determination.
v) Application in Power Generation.
Exercise Questions
1) What is the nature of charge on cathode rays?
The nature of charge on cathode rays is negative.
2) Give five characteristics of cathode rays.
i) These rays travel in a straight lines perpendicular to the cathode surface.
ii) They are deflected towards positive plate in an electric field. It shows that they are
negatively charged.
iii) They cast a sharp shadow of an opaque object placed in their path.
iv) They raise the temperature of the body they fall.
v) Light is produced when these rays strike the sides of the discharge tube.
3) The atomic symbol of phosphorus ion is given as 31P15-3.
a) How many protons, electrons and neutrons are there in the ion?
b) What is the name of the ion?
c) Draw the electronic configuration of the ion.
d) Name the noble gas which has the same electronic configuration.
a) Number of protons = 15
Number of electrons = 15+3=18
Number of neutrons = 31-15 = 16
b) Phosphide ion.
c) Electronic configuration = 1s2, 2s2, 2p6, 3s2, 3p6.
d) Argon
4) Differentiate between shell and sub-shell with example of each.
Shell: “The circular path of an electron around the nucleus is called shell” or “Principle
energy level is called shell”
Example: K-shell, M-shell
Sub-shell: “Each shell consists of sub-shells”
The number of sub-shells present in a shell is equal to the shell number.
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CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
Example: s sub-shell, p sub-shell
5) An element has an atomic number 15. How many electrons are present in K, L
and M shell of the atom?
Atomic number of element = 15
Number of electrons = 15
Electronic configuration K = 2, L = 8, M = 5 electrons, 1s2 2s2 2p6 3s2 3p3
6) Write down the electronic configuration of Al3+ . How many electrons are
present in the outermost shell?
Atomic number of Al =13
Number of electrons of Al3+ =10
Electronic Configuration of Al3+ ion = 1s2 2s2 2p6
7) Magnesium has electronic configuration 2,8,2
a) Number of electrons in the outermost shell = 2
b) The outer most shell electrons are present in 3s
c) Magnesium tends to lose electrons of the outermost shell in order to acquire stable
electronic configuration of neon.
8) What will be the nature of charge on an atom when it loses an electron or when
it gains an electron?
When an atom loses an electron the charge on it will be +1, when an atom gains an
electron the charge on it will be -1.
9) For what purpose U-235 used?
U-235 is used to get large amount of energy which is released on its nuclear fission
reaction.
10) A patient has goiter. How it will be detected?
Iodine-131 is used for diagnosis of goiter in thyroid gland. Thyroid gland preferentially
absorbs iodine. Iodine 131 is injected into the thyroid gland to test the activity of the
thyroid. Inside the thyroid gland the radiation destroy cancer cells more rapidly than
normal cells.
11) Give three properties of positive rays.
i) These rays travel in a straight line in a direction opposite to the cathode rays.
ii) Their deflection in an electric field and magnetic field show that these were positively
charged.
iii) The mass of these positive particles was found almost the same as that of an atom
of the gas used for their production.
12) What are the defects of Rutherford’s atomic model?
i) According to classical theory of radiation, electrons being the charged particles should
release or emit energy continuously and they should ultimately fall into the nucleus.
ii) If the electron emits energy continuously, they should form a continuous spectrum but
in fact, line spectrum was observed.
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CHEMISTRY
SHORT QUESTIONS
CHAPTER 2
13) As long as electron remains in an orbit, it does not emit or absorb energy.
When does it emit or absorb energy.
Electrons emit energy when it jumps from a higher energy orbit to lower energy orbit. An
electron absorbs energy when it jumps from a lower energy orbit to a higher energy
orbit
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