Class 9 Science Chapter 4 Important Questions

Class 9 Science Chapter 4 Important Questions of Structure of the Atom. All the questions as well as answers are taken from NCERT Books only. Class 9 Science Chapter 4 Extra Question answers are useful for the school terminal tests and final exams also.

Once read NCERT Book and then practice with these questions to get good marks and solve your doubts.

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Class 9 Science Chapter 4 Important Extra Questions for 2020-2021

Class: 9Science
Contents:Important Extra Questions with Answers
Chapter: 4Structure of the Atom

9th Science Chapter 4 Extra Question Answers for 2020-2021

Class 9 Science Chapter 4 Extra Question answers are given below with complete explanation. These important extra questions are taken from NCERT Textbooks 2020-2021 only. This is the collection of frequently asked questions in different Public Schools, Kendriya Vidhyalaya and Government School examinations.

Class 9 Science Chapter 4 Important Extra Questions Set – 1

J. Chadwick discovered a subatomic particle which has no charge and has mass nearly equal to that of a proton. Name the particle and give its location in the atom.

The particle is neutron and it is present in the nucleus of the atom.

Which subatomic particle is absent in an ordinary hydrogen atom?

Neutron.

Is it possible for the atom of an element to have one electron, one proton and no neutral? If so name the element.

Yes it is true for hydrogen atom which is represented as H(1, 1).

Electron attributes negatives charge, protons attributes positive charge. An atom has both but why there is no change.

The positive and negative charges of protons and electrons are equal in magnitude. So atom as a whole is electrically neutral.

Write the electronic configuration of an element whose atomic number is 12.

K, L, M = 2, 8, 2

Size of Nucleus as compared to the size of Atom

Rutherford observed that when a particles were bounded on a very thin foil they bounced back. But the number of a particle bouncing back got doubled when he doubled the thickness of gold foil. Then he concluded that the area of nucleus is very small in comparison to the total area of the atom.




Class 9 Science Chapter 4 Important Extra Questions Set – 2

What do you understand by the ground state of an atom?

The state of an atom where all the electrons in the atom are in their lowest energy levels is called the ground state.

What is the maximum number of electrons which can be accommodated in N shell?

N shell can accommodate maximum 32 electron.

What will be the valency of an atom if it contains 3 protons and 14 neutrons?

The valency of the atom will be one.

One electron is present in the outer most shell of the atom of an element X. What would be the nature and value of charge on the ion formed if these electrons is removed from the outermost shell?

The charge would be +1.

In the atom of an element X, 6 electrons are present in the outermost shell. If it acquires noble gas configuration by accepting requisite number of electrons, then what would be the charge on the ion so formed?

-2.

Give two important applications of radioactive isotopes.

Answer:

    • (i) An isotopes of carbon -12, C14, is used in carbon dating.
    • (ii) U235 is used in the nuclear reactors to generate electricity.

Class 9 Science Chapter 4 Important Extra Questions Set – 3

Which isotopes of hydrogen is present in heavy water?

Among the three isotopes of hydrogen, deuterium (2/1H) is found in heavy water.

Chemical formula of a metal sulphate is MSO4. What will be the formula of its chloride?

MCl2

Valency of an element X is 3. Write the chemical formula of its oxide.

X2O3

Will Cl-35 and Cl-37 have different valences? Justify your answer.

No, Cl-35 and Cl-37 are isotopes of an element.

The atomic number of calcium and argon are 20 and 18 respectively, but the mass number of both these elements is 40. What is the name given to such a pairs of elements?

Isobars

Characteristic of the Canal Rays
    • The canal rays are deflected by the magnetic fields in a direction opposite to that of the cathode rays.
    • They consist of positively charged particles.



Class 9 Science Chapter 4 Important Extra Questions Set – 4

Write the electronic configuration of a positively charged sodium ion (Na+). Atomic number of Sodium 11.

Number of electrons in Na atom = Atomic number 11
Number of electrons in Na+ ion = 11 – 1 = 10
Electronic configuration of Na+ ion: 2, 8

The electronic configuration of phosphorous is 2, 8, 5. Give the electronic configuration of P3 ion.

Electronic configuration of P = 2, 8, 5
P atom gains 3e- to form P3
Therefore, P3 has configuration = 2, 8, (5 + 3) = 2, 8, 8

The atomic number of Al and Cl are 13 and 17 respectively. What will be the number of electrons in Al3+ and Cl-?

Atomic number of Al = Number of electrons = 13
Number of electrons in Al3+ = 13 – 3 = 10
Atomic number of chlorine = Number of electrons = 17
Number of electrons in in Cl- = 17 + 1 = 18

Define valence electrons. Which electrons of an atom are involved in the chemical bond formation with other atom?

The electrons present in the outermost shell of an atom or ion are known as valence electron. In a chemical bond formation only valence electrons of an atom take part.

Helium atom has 2 electrons in its valence shell but its valency is not equal? Explain.

Helium atom has 2 electrons in its valence shell and its outer shell is complete. Hence, the valency zero.

Why do helium, neon and argon have a zero valency?

Answer: Helium has two electrons in its energy shell, while argon and neon have 8 electrons in their valence shells. As these have maximum number of electrons in their valence shells, they do not have any tendency to combine with other elements. Hence they have a valency equal to zero.

Class 9 Science Chapter 4 Important Extra Questions Set – 5

Why do isotopes show similar chemical properties?

Isotopes have same atomic number and thus same number of electrons. Therefore, they have the same electronic configuration which provides them similar chemical properties.

An element Z forms the following compound when it reacts with it reacts with hydrogen, chlorine, oxygen and phosphorous. ZH3, ZCl3, Z2O3 and ZP (a) What is the valency of element Z? (b) Element Z is metal or non-metal?

(a) The valency of Z is 3.
(b) Element Z is a metal because it is electropositive and is reacting with non-metals.

In the gold foil experience of Geiger and Marsden, that paved the way for Rutherford’s model of an atom 1.00% of a particle were found to deflect at angles > 50 0. If one mole of a particles were bombarded on the gold foil, compute the number of a particles that would deflect at angles less than 500.

% of a particles deflected more than 500 = 1% of a particle.
% of a particles deflected less than 500 = 100 – 1 = 99%
Number of particles that deflected at an angle less than 500
= 99/100× 6.022 × 10^23
= 596.178/1000 × 10^23
= 5.96 × 10^23

Predict the valency of the following elements: (i) A (atomic number 5), (ii) B (atomic number 12), (iii) C (atomic number 14), (iv) D (atomic number 17).

(i) Valency of element A = 8 – 5 = 3
(ii) Valency of element B = 12 – 10 = 2
(iii) Valency of element C = 14 – 10 = 4
(iv) Valency of element D = 18 – 17 = 1

An element X contains 6 electrons in M shell as valence electrons: (a) What is the atomic number of X? (b) Identify whether X is a metal or non-metal.

(a) If X contains 6 electrons in M shell as valence electrons then the electronic configuration of X is K = 2, L = 8, M = 6
Therefore, atomic number = 16
(b) X is a non-metal.

Difference between the models of an atom proposed by J.J Thomson and Ernest Rutherford.




J .J Thomson Model of AtomRutherford’s Model
1. Positive charge forms a kernel.1. Nucleus (positive charge) is in the center.
2. Electrons present throughout the atom.2. Electrons revolve in orbits.
3. No space is empty.3. Most of the space is empty.

Class 9 Science Chapter 4 Important Extra Questions Set – 6

What are canal rays?

E. Gold stein discovered new radiations in a gas discharge in 1886. These rays were positively charged radiations which are called canal rays.

Explain Thomson’s model of an atom?

Thomson proposed that the model of an atom is similar with Christmas pudding. The electrons in a sphere of positive charge were like currents in a spherical Christmas pudding. He also compared the model of an atom with a watermelon in which the seeds are like electrons and the red part of watermelon is like the positive charge in the atom.

Write the drawbacks of Thomson’s model of an atom or write the limitation Thomson’s model of an atom.

Thomson’s said that the atom is like a charge sphere and the electrons are scattered in the sphere but we know that the electrons revolve in the sphere but we know that the electrons revolve in different shells around the nucleus and the atom is divided into different shells. It is not like a positive charge.

Write the results of Rutherford’s on alpha particles scattering experiment?

The result of alpha particles scattering experiment are:
1. Most of the fast moving alpha particles passed straight through the gold foil.
2. Some of alpha particles were deflected by the foil by small angles.
3. Surprisingly one out of every 12000 particles appeared to rebound.

Explain Rutherford model of an atom?

The explanation of Ruther ford model of an atom:
1. There is a positively charged center in an atom called the nucleus.
2. The mass of an atom residue in the nucleus.
3. The electrons revolve around the nucleus in well-defined orbit.
4. The size of the nucleus is very small and compared to the size of the atom.

Properties of Electron, Proton, Neutron
ElectronProtonNeutron
1. They are negatively charged particles.1. Protons are positively charged particles.1. They are neutral.
2. It’s charges is -1.2. They have charge +1 coulomb.2. They have mass equal to that of proton.
3. It’s mass 2000 times less than proton.3. They have mass 2000 times more than electron.3. They are present in the nucleus of an atom.
4. They revolve in different shells around the nucleus.4. They are present in the nucleus of an atom.4. They are more in number than electron or proton in some atom.

Class 9 Science Chapter 4 Important Extra Questions Set – 7

Write the drawbacks of rather fords model of an atom?

The drawbacks of rather ford model of an atom are:
1. Any particle in a circular orbit would undergo acceleration. During acceleration the charged particle radiate energy.
2. If the charged particles would radiate energy than the electron will fall in the nucleus and the atom will become unstable but we know that atom are stable.

Explain Bohr’s model of an atom?

1. Only certain special orbits known as discrete orbit of electrons are allowed inside the atom.
2. While revolving discrete orbit the electrons do not radiate energy.
3. The orbits or shells are called energy levels and these are represented by the letter K, L, M, N.

Define valency? How can we find the valency of an element?

The combining capacity of an element is known as valency. The valency of an element is depends upon valence electron which is found in the outermost shell of an element.
For example, the atomic number of sodium is 11.
Electronic configuration = 2, 8, 1
No. of valence electron = 1
The valency of sodium is 1.

What is atomic number and mass number? Define them.

Atomic number= The total number of protons found in nucleus of an atom is called atomic number. It is represented by Z.
Atomic number = No of proton = No of electron
Mass number = The sum of the total number of protons and neutrons present in the nucleus of an atom is called mass number.
Mass number = Number of proton + Number of neutron

Explain the following a similar reasoning, Rutherford concluded from the α-particle scattering experiment?

(i) Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
(ii) Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
(iii) A very small fraction of α-particles were deflected by 1800, indicating that all the positive charge and mass of the gold atom were concentrated in a very small volume within the atom.

What are isotopes?

Answer: In nature, a number of atoms of some elements have been identified, which have the same atomic number but different mass numbers. For example, take the case of hydrogen atom, it has three atomic species, namely protium H-1, deuterium H-2 or D and tritium H-3 or T. The atomic number of each one is 1, but the mass number is 1, 2 and 3, respectively. Other such examples are (i) carbon, C-12 and C-14, (ii) chlorine, Cl-35 and Cl-37, etc.



Class 9 Science Chapter 4 Important Extra Questions Set – 8

Explain some application which follows in isotopes?

Since the chemical properties of all the isotopes of an element are the same, normally we are not concerned about taking a mixture. But some isotopes have special properties which find them useful in various fields. Some of them are:
(i) An isotope of uranium is used as a fuel in nuclear reactors.
(ii) An isotope of cobalt is used in the treatment of cancer.
(iii) An isotope of iodine is used in the treatment of goiter.

What is the gold foil experiment? Name the scientist who performed this experiment. Write the conclusions and shorter comings of Ruther Fords model of atom.

In 1911, Rutherford performed the gold foil experiment. He bombarded a stream of α-partciles on a gold foil, a thin sheet which was 0.00006 cm thick in an evacuated chamber. An α-particle is a positively charged helium ion (He2+).
In this famous experiment the following observations were made.
(i) Most of the α-particles passed straight through the foil without any deflection. This concluded that most of the space inside of an atom is empty.
(ii) A few α-particles were deflected through small angle and few through larger angles. This happened due to positive charge on α-particles and core (nucleus) of the atom. The heavy positively charged ‘core’ was named as nucleus.
(iii) The number of α-particles which bounced back was very small. This concluded that the volume of the nucleus is very small in comparison to the total volume of the atom.
On the basis of gold foil experiment Rutherford concluded that an atom consists of nucleus which has positive charge and it is surrounded with electrons which are moving around the nucleus. The number of electrons and protons are equal and the entire mass of the atom is concentrated at its nucleus.
Drawbacks in the Rutherford’s model
(i) According to classical electro-magnet theory, a moving charged particles, such as an electron under the influence of attractive force loses energy continuously in the form of radiations. As a result of this electron should lose energy and therefore, should move in even smaller orbits ultimately falling into the nucleus. But the collapse does not occur. There is no explanation for this behavior.
(ii) Rutherford did not specify the number of orbits and the number of electrons in each orbit.

In what way is the Rutherford’s atomic model different from that of Thomson’s atomic model?

Rutherford proposed a model in which electrons revolve around the nucleus in well-defined orbits. There is a positively charged center in an atom called the nucleus. He also proposed that the size of the nucleus is very small as compared to the size of the atom and nearly all the mass of an atom is centered in the nucleus. Whereas, Thomson proposed the model of an atom to be similar to a Christmas pudding. The electrons are studded like currants in a positively charged sphere like Christmas pudding and the mass of the atom was supposed to be uniformly disturbed.

What are the postulates of Bohr’s model of an atom?

The postulates put forth by Neil’s Bohr’s about the model of an atom:
(a) Only certain special orbits known as discrete orbits of electrons are allowed inside the atoms.
(b) While revolving in discrete orbits the electrons do not radiate energy. These orbits are called energy levels. Energy in an atom are shown by circles.
These orbits are represented by the letter K, L, M, N…. or the number n = 1, 2, 3, 4….

The atomic structure of an atom of an element X. Write the following information about the element X. (a) Atomic number of X (b) Atomic mass of X (c) Valence electron. (d) Valence of X (e) X should be metal or non- metal.

(a) Atomic number = Number of protons = 8
(b) Atomic mass
= Number of protons + Number of neutron
= 8 + 10 = 18
(c) Valence electron = 6
(d) Valency of X = 8 – 6 = 2
X should be non-metal because there are six valence electrons hence it will take two more electrons to complete its outermost shell.

Frequently Asked Question

(i) State the method of determining the valency of an element if its atomic number is given.
(ii) determine the valency of the following elements, the atomic numbers of which are given parenthesis.
Chlorine (17), sulphur (16), Aluminium (13)



Answer

(i) The number of electrons gained, lost or shared to make the octet of electrons (in the outermost shell) gives us directly the combining capacity of the element, that is the valency.
(ii) Elements Atomic No. Electronic configuration Valency.
Chlorine 17 2, 8, 7 -1
Sulphur 16 2, 8, 6 -2
Aluminium 13 2, 8, 3 +3