punjab text book notes 9th chemistry

1. Chapter 2 Structure of atoms
ADNAN SOHAIL
M. Phil Chemistry
1. Do you know any element having no neutrons in its atoms?
Ans. Yes, the element having no neutrons in its atoms is hydrogen.
2. Who discovered an electron, a proton and a neutron?
Ans.
Electron was discovered by Sir William Crooks
Proton was discovered by Goldstein
Neutron was discovered by Chadwick
3. How does electron differ from a neutron?
Ans. An electron has a negative charge on it. It revolves around the nucleus of an atom
Neutron has no charge on it. It is present in the nucleus of an atom.
4. Explain, how anode rays are formed from the gas present in the discharge tube?
Ans. Electrons from cathode are bombarded on molecules of gas present in discharge tube.
These molecules are ionized and form positive ions. These positive ions travel towards
cathode in the form of rays. These rays are called anode rays or canal rays.
1. How was it proved that the whole mass of an atom is located at its centre?
Ans. Since most of the alpha particles in Rutherford’s experiment passed through the foil
undeflected, therefore most of the volume occupied by an atom is empty.
The complete rebounce of a few particles show that the nucleus is very dense and hard.
These observations show that the whole mass of an atom is located at its centre.
2. How was it shown that atomic nuclei are positively charged?
Ans. Rutherford bombarded alpha particles on gold foil. These are actually helium nuclei
(He2+
). The deflection of a few alpha particles proved that there is a ‘center of positive
charges’ in an atom, which is called nucleus of an atom.
3. Name the particles which determine the mass of an atom?
Ans. The mass of an atom is due to the particles present in the nucleus of an atom. These
particles are protons and neutrons.
4. What is the classical theory of radiation? How does it differ from quantum theory?
Ans. According to Classical Theory of radiation, moving charged particles i.e. electron in
an atom release or emit energy continuously.
According to Quantum Theory of Max Planck 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.
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2. Chapter 2 Structure of atoms
ADNAN SOHAIL
M. Phil Chemistry
5. How can you prove that angular momentum is quantized?
Ans.
Let angular momentum (mvr) of 1st orbit is mvr = nh/2π
By putting the values of h and π
1. How many the maximum number of electrons that can be accommodated in a p-
subshell?
Ans. Maximum number of electrons that can be accommodated in a p-subshell is six (6)
2. How many subshells are there in second shell?
Ans. second shell has two subshells i.e. s and p
3. Why does an electron first fill 2p orbital and then 3s orbital?
Ans. Electrons are accommodated in different shells according to increasing energy.
An electron first fill 2p orbital and then 3s orbital because 2p orbital is lower in energy than
3s orbital.
4. If both K and L shells of an atom are completely filled; how many total number of
electrons are present in them?
Ans. If both K and L shells of an atom are completely filled then there are 10 total electrons
present in them.
5. How many electrons can be accommodated in M shell?
Ans. 2n2
formula tells us that M shell can accommodate 18 electrons in it.
6. What is the electronic configuration of a hydrogen atom?
Ans. The electronic configuration of a hydrogen atom is 1s1
7. What is atomic number of phosphorus? Write down its electronic configuration?
Ans. Atomic number of phosphorus is 15. Its electronic configuration is 1s2
, 2s2
, 2p6
, 3s2
,
3p3
8. If an element has atomic number 13 and atomic mass 27; how many electrons are
there in each atom of the element?
Ans. The number of electrons present is this atom is also 13.
9. How many electrons will be in M shell of an atom having atomic number 15.
Ans. Electronic configuration of an element having atomic number 15 is:
K shell = 2, L shell = 8, M shell = 5
There are 5 electrons in its M shell
10. What is maximum capacity of a shell?
Ans. Maximum capacity of a shell is found by 2n2
formula.
Test yourself 2.3
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3. Chapter 2 Structure of atoms
ADNAN SOHAIL
M. Phil Chemistry
1. Why do the isotopes of an element have different atomic masses?
Ans: The isotopes of an element have different atomic masses because they have different
number of neutrons their nucleus.
2. How many neutrons are present in C-12 and C-13?
Ans. C-12 has 6 neutrons and C-13 has 7 neutrons. Number of neutrons in an atom is found
by the formula:
Number of neutrons, n = A – Z
For C-12, n = 12- 6 = 6
For C-13, n = 13- 6 = 7
3. Which of the isotopes of hydrogen contains greater number of neutrons?
Ans. There are three isotopes of hydrogen, protium, (1
1
H), deuterium (1
2
H) and tritium (1
3
H).
Tritium has greater number of neutrons (3) in its nucleus.
4. Give one example each of the use of radioactive isotope in medicine and
radiotherapy?
Ans. Isotopes of Iodine-131 are used for diagnosis of goiter in thyroid gland.
For cancer, affecting within the body, Co-60 is used because it emits strongly penetrating
gamma rays.
5. How is the goiter in thyroid gland detected?
Ans. The radioactive isotopes are used as tracers in medicine to diagnose the presence of
tumor in the human body. Iodine-131 is used for diagnosis of goiter in thyroid gland.
6. Define nuclear fission reaction?
Ans. A nuclear reaction in which a heavy nucleus splits into smaller parts spontaneously or
on impact with another particle. During this process large amount of energy is released.
7. When U-235 breaks up, it produces a large amount of energy. How is this energy
used?
Ans. When U-235 breaks up a large amount of energy is released which is used to convert
water into steam in boilers. The steam then drives the turbines to generate electricity.
8. How many neutrons are produced in the fission reaction of U-235?
Ans. Three neutrons are produced in the fission reaction of U-235.
9. U-235 fission produces two atoms of which elements?
Ans. The uranium nucleus breaks up to produce Barium-139 and Krypton-94.
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4. Chapter 2 Structure of atoms
ADNAN SOHAIL
M. Phil Chemistry
SHORT ANSWER QUESTIONS
1. What is the nature of charge on cathode rays?
Ans. The deflection of cathode rays in electric and magnetic field shows that these are
negatively charged rays.
2. Give five characteristics of cathode rays.
Cathode rays travel in straight line perpendicular to the direction of anode.
These rays travel from negative electrode to the positive electrode.
The deflection of cathode rays in electric and magnetic field shows that these are
negatively charged rays.
They can cause shadow of an opaque object placed in their path.
They raise temperature of the body on which they fall.
3. The atomic symbol of a phosphorus ion is given as 31
15P33-
?
• How many protons, electrons and neutrons are there in the ion?
• What is name of the ion?
• Draw the electronic configuration of the ion.
• Name the noble gas which has the same electronic configuration as the phosphorus ion has.
Ans.
There are 15 protons, 16 neutrons and 18 electrons in the ion.
Name of the ion is phosphide.
Electronic configuration is: K = 2, L = 8, M = 8
Noble gas which has same electronic configuration is Argon
4. Differentiate between shell and subshell with examples of each?
Ans. shells are the main energy levels that electrons can occupy. Shells are represented by
circles around the nucleus. They are represented by the alphabets K, L, M and so on.
A shell also consists of subshells the number of subshells in a shell is equal to its n value.
Each subshell is represented by a small alphabetical letter s, p, d, f .
5. An element has an atomic number 17. How many electrons are present in K, L and M
shells of the atom?
Ans. The electronic configuration of an element having atomic number 17 is:
K = 2, L = 8, M = 7
6. Write down the electronic configuration of Al3+
. How many electrons are present in its
outermost shell?
Ans.
The electronic configuration of Al3+
is : K = 2, L = 8
There are 8 electrons in its valence shell.
7. Magnesium has electronic configuration 2, 8, 2, How many electrons are in the
outermost shell? In which subshell of the outermost shell electrons are present? Why
magnesium tends to lose electrons?
Ans.
There are 2 electrons in outermost shell.
Its electronic configuration is 1s2
, 2s2
, 2p6
, 3s2
. Outer most electrons are present in
3s subshell.
To complete its octet, Magnesium metal loses its 2 outer most electrons and attains
noble gas electronic configuration to become stable.
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5. Chapter 2 Structure of atoms
ADNAN SOHAIL
M. Phil Chemistry
8. What will be the nature of charge on an atom when it loses an electron or when it
gains an electron?
Ans. An atom attains positive charge when it loses an electron and it attains negative charge
when it gains an electron.
9. For what purpose U-235 is used?
Ans. U-235 radioactive isotope is used to generate electricity by carrying out controlled
nuclear fission reactions in nuclear reactors.
10. A patient has goiter. How will it be detected?
Ans. Iodine-131 isotope is used for diagnosis of goiter in thyroid gland.
11. Give three properties of positive rays?
Ans.
These rays travel in straight lines in a direction opposite to the cathode rays.
Their deflection in electric and magnetic field proved that these are positively
charged.
The nature of canal rays depends upon the nature of gas, present in the discharge
tube.
12. What are the defects of Rutherford’s atomic model?
Ans.
1. 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.
2. If the electrons emit energy continuously, they should form a continuous spectrum but in
fact, line spectrum was observed.
13. As long as electron remains in an orbit, it does not emit or absorb energy. When
does it emit or absorb energy?
Ans. As long as electron remains in a particular orbit, it does not radiate or absorb energy.
The energy is emitted or absorbed only when an electron jumps from one orbit to another.
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