WBBSE Class 9 Physical Science Solutions Chapter 4.1 Atomic Structure

Detailed explanations in West Bengal Board Class 9 Physical Science Book Solutions Chapter 4.1 Atomic Structure offer valuable context and analysis.

WBBSE Class 9 Physical Science Chapter 4.1 Question Answer – Atomic Structure

Very short answer type questions

Question 1.
Which atom has no neutron?
Answer:
Ordinary hydrogen atom has no neutron.

Question 2.
Which force binds the nucleons?
Answer:
The nuclear force, a short ranged attractive force binds the nucleons.

Question 3.
Which fundamental particle is responsible to produce isotopes?
Answer:
Neutron is responsible to produce isotopes, variation of it causes variation of mass number.

Question 4.
Name the element of the same mass number and atomic number?
Answer:
Ordinary hydrogen has the same mass number and atomic number, each being 1.

Question 5.
What are nucleons ?
Answer:
The constituents of the nucleous of an atom, proton and neutron are called nucleons.

Question 6.
Name the heaviest isotope of hydrogen.
Answer:
Tritium is the heaviest isotope of hydrogen.

Question 7.
State one difference between H and H⁺.
Answer:
H is an unstable hydrogen atom having no net electric charge. H⁺ is a positive hydrogen ion carrying unit positive charge.

Question 8.
What is deuteron and what is triton ?
Answer:
The nucleous of deuterium is called deuteron and that of tritium is called triton.

Question 9.
What is isotopic weight ?
Answer: The weight of an isotope of an element is known as isotopic weight.

Question 10.
Can there be a doubly ionised hydrogen ion ?
Answer:
Hydrogen has only one electron, so it can be ionised singly by losing the electron, so there cannot be a doubly ionised hydrogen ion.

Question 11.
What is the relation between atomic number and mass number ?
Answer:
The relation is A = Z + N, where A = mass number, Z = atomic number, N = number of neutrons.

Question 12.
Name two particies of an atom which are always equal in number.
Answer:
Proton and electron.

Question 13.
What is the magnitude of charge of a neutron ?
Answer:
Neutron has no charge.

Question 14.
Which part of an atom contains protons?
Answer:
Nucleus.

Question 15.
Why an atom always remains neutral inspite of the existence of electrons and protons which are charged particles ?
Answer:
As electrons and protons of an atom are always equal in number, the positive charges of the protons and negative chargy of the electrons cancell each other. As a result, an atom always remains neutral.

Question 16.
Who discovered neutron?
Answer:
J. Chadwick (1932) discovered neutron.

Question 17.
Who discovered nucleus of the atom ?
Answer:
Rutherford discovered nucleus of the atom.

Question 18.
What are electron-shells ?
Answer:
The electrons outside the nucleus of the atom rotate in definite number in certain specified circular paths around the nucleus. These con-centric circular paths of the rotating electrons are called electronshells.

Question 19.
Write the electronic configuration of chlorine.
Answer:
The electronic configuration of chlorine is :
K L M
2 8 7

Question 20.
What are ions ?
Answer:
Electrically charged atoms (or, group of atoms) are called ions.

Question 21.
How are ions formed from neutral atom ?
Answer:
The formation of ions from neutral atoms involves loss or gain of one or more than one electrons.

Question 22.
What are the fundamental particles that constitute an atom ?
Answer:
Electron, proton and neutron are the fundamental particles that constitute an atom.

Question 23.
Among the different fundamental particles, which one is positively charged and which one is negatively charged ?
Answer:
Proton is positively charged particle and electron is negatively charged particle.

Question 24.
What is cation ?
Answer:
Positively charged atom or radical is callled a cation.

Question 25.
What is anion?
Answer:
Negatively charged atom or radical is called an anion.

Question 26.
Among electron, proton and neutron, which one is the heaviest and which one is the lightest particle?
Answer:
Neutron is the heaviest particle and electron is the lightest particle.

Question 27.
Calculate the number of protons and neutrons in \({ }_{92}^{235} \mathrm{U}\).
Answer:
\({ }_{92}^{235} \mathrm{U}\) → protons : 92
neutrons : (235-92)=143

Question 28.
What is the relation between \({ }_{92}^{235} \mathrm{U}\) and \({ }_{92}^{238} \mathrm{U}\) ?
Answer:
These are two isotopes of uranium.

Question 29.
Give example where mass number and atomic numbers are equal.
Answer:
Mass number and atomic numbers are equal in those elements which do not have any isotope. e.g. Na, F etc.

Question 30.
Between K and K⁺ which one is more stable?
Answer:
K⁺is more stable than K as in the earlier case the electronic configuration is just like inert gas argon.

Question 31.
What is heavy water ?
Answer:
Oxide of deuterium (D₂O) is called heavy water.

Question 32.
What is the maximum capacity of L shell to accommodate electrons ?
Answer:
8 electrons.

Question 33.
What is the mass of an electron ?
Answer:
Mass of an electron is 9.11 × 10-28 g.

Question 34.
What is the charge of an electron ?
Answer:
Charge of an electron is 1.602 × 10^-19 coulomb or 4.8 × 10^-10 esu}

Question 35.
What is amu?
Answer: It is an unit used for measuring the atomic mass of an atom. 1 amu} = 1.66.3 × 10²⁴ g

Question 36.
What is the mass of a neutron ?
Answer:
Mass of a neutron is 1.675 × 10^-24 g.

Question 37.
What is the radius of an electron ?
Answer:
The radius of an electron is 2.8 × 10^-13 cm.

Question 38.
Which is the smaliest partices present in all atoms ?
Answer:
Electrons.

Question 39.
What is cathode ray ?
Answer:
When electric discharge passes from high by potential source through a gas at very low pressure (0.01 mm of Hg) in a glass tube provided with two metallic electrodes, a stream of invisible rays is emitted from the surface of the cathode which moves in straight line towards the anode with a high velocity. These rays are called cathode rays as they originate at the cathode.

Question 40.
Who discovered radio-activity ?
Answer:
Madam curie discovered radio-activity.

Short answer type questions

Question 1.
What are the postulates of Dalton’s atomic theory ?
Answer:
Postulates of Dalton’s atomic theory :
(i) Every element is composed of infinite number of very small indivisible particles. These smallest particles are known as ‘atoms’.
(ii) Atoms cannot be created or destroyed by a chemical reaction.
(iii) Atoms of different elements are different. But atoms of a definite element are same.

Question 2.
What are the fundamental particles? Why are they called ‘fundamental’ ?
Answer:
Fundamental particles: The sub-atomic particleselectron, proton and neutrons are known as fundamental particles.

Reason : Experimentally it is found that these particles are the primary components of all atoms of all elements, except ordinary hydrogen, the nucleus of which does not contain any neutron. That is why they are known as fundamental particles.

Question 3.
State some other sub-atomic particles other than electron, proton and neutron.
Answer:
Other sub-atomic particles are :

1. Positron
2. Antiproton
3. Messon
4. Neutrino
5. Antineutrino
6. V-particle
7. Deuteron etc.

Question 4.
What makes electrons move round the nucleus ?
Answer:
Explanation : The negatively charged electrons in each shell are attracted by Coulombian electrostatic force towards the positive nucleus that contains positively charged protons. To counter balance this inward force the electrons rotate about the nucleus, as a revolving body is always accompanied with an outward force.

Question 5.
What is nuclear force?
Answer:
Nuclear force: In the nucleus of an atom, the protons and neutrons are strongly held together by nuclear forces which result from the attraction between the constituent protons and neutrons. This force is so strong that it is very difficult to separate the nucleons.

Question 6.
Define atomic number. Atomic number is the fundamental property of an element – Explain.
Answer:
Atomic number : The atomic number of an atom of an element is the number of protons in its nucleus.
Atomic number is the fundamental property : Number of protons determine the nature of the element. Change in the number of protons changes its chemical properties. As a result, a new element is formed. So, atomic number of an element is its intrinsic property.

Question 7.
What do you mean by mass number of an element ?
Answer:
Mass number : The mass number of an atom of an element is the total number of protons and neutrons present in the núcleus of the atom.
Mass number of atom = Number of protons + Number of neutrons. Mass number is written on the upper left-hand side of the symbol of the element.

Question 8.
What is the relation between mass number and atomic number ?
Answer:
Relation between mass number and atomic number :
We know that,
Mass number of an atom = Number of protons + Number of neutrons

Suppose, number of protons in the nucleus of an atom = Z, number of neutrons = N, and mass number = A
Then we can write,
A = Z + N
As number of protons = atomic number
∴ Atomic number = mass number – number of neutrons

Question 9.
Define isotope.
Answer:
Isotope : The different atomic species of the same element, which have necessarily the same atomic number but different mass number are called isotopes.
e.g. Hydrogen has three isotopes :
\({ }_1^1 \mathrm{H}\) (Ordinary hydrogen) ; \({ }_1^2 \mathrm{H}\) (Deuterium) ; \({ }_1^3 \mathrm{H}\) (Tritium)

Question 10.
What is meant by \({ }_8^{16} \mathrm{O}\) ? Show its electronic configuration.
Answer:
\({ }_8^{16} \mathrm{O}\) is meant by : \({ }_8^{16} \mathrm{O}\) stands for an isotope of oxygen that has 8 protons and 8 neutrons in the nucleus and 8 electrons surrounding the nucleus.
The electronic configuration is :
K – shell has 2 electrons, L-shell has 6 electrons.

Question 11.
How do the physical and chemical properties differ in the isotopes of an element? Have they any common property?
Answer:
Difference between physical and chemical properties of isotopes of an element : Isotopes of an element have the same chemical property since their atomic number is same i.e. number of valence electrons is same. Physical property of the isotopes of an element slightly differ due to the increase of mass number which results to slight increase of density, melting point and boiling point.
Common property : As isotopes have same atomic number, so they have same chemical properties.

Question 12.
Define atomic weight of an element on the basis of carbon-12.
Answer:
Atomic weight of an element on the basis of carbon-12 : The relative weight of one atom of carbon 12_c is taken as the atomic weight of the element. Here, one-twelvth part of the weight of a carbon atom (C = 12) has been taken as the unit.

Question 13.
Distinguish between atomic number and mass number.
Answer:
Difference between atomic number and mass number:

Atomic Number	Mass Number
(i) It is equal to the number of protons present in the nucleus of an atom.	(i) It is equal to the sum of the numbers of protons and neutrons present in the nucleus of an atom.
(ii) From atomic number, number of valence electrons can be determined which in turn gives the idea of chemical combination of the atom.	(ii) Mass number gives an idea about the atomic mass of the element concerned but it does not give any idea about chemical activity of the element until the number of protons or the number of neutrons is indicated.

Question 14.
How is an atom transformed to an ion? What are cations and anions ?
Answer:
Ions : Electrically charged atoms or radicals are called ions. Cation : Positively charged atom or radical is known as cation. e.g. NH₄ ⁺, Ca²⁺, Na⁺etc.
Anion: Negatively charged atom or radical is called an anion. e.g. SO₄^-2, NO₃^–, Cl^–etc.

Question 15.
What are the differences between an atom and ion ?
Answer:
Differences between an atom and an ion:

Atom	Ion
(i) An atom is electrically neutral.	(i) An ion is electrically charged atom. It is formed when an atom either gains or loses one or more electrons.
(ii) An atom may or may not have free existence. e.g. Na atom reacts immediately when it comes in contact with water. 2 Na + 2 H2O = 2 NaOH + H2↑	(ii) An ion exists freely in solution.

Question 16.
What are the differences between atomic weight and actual weight of an atom ?
Answer:
Distinction between atomic weight and actual weight of an atom :

Atomic weight	Actual weight of an atom
(i) It is a number which represents how many times an atom of an element is heavier than 1/12 weight of carbon atom (C-12)	(i) It is a number which represents how many times an atom is heavier than one atomic mass unit (a.m.u.) which is equivalent to 1.6603 × 10-24 g.
(ii) It is a simple ratio and therefore has no unit.	(ii) It represents the actual weight of an atom and therefore has unit.

Question 17.
Mention two similarities between atomic structure and the structure of the solar system.
Answer:
Similarities between atomic structure and the structure of the solar system :

Atomic Structare	Solar system
(i) Nucleus is at the centre, electrons revolve round it.	(i) The sun is at the centre, planets revolve round it.
(ii) Mass of the nucleus is much greater than that of an electron moving round it.	(ii) Mass of the sun is much greater than that of any planet revolving round it.

Question 18.
Mention three dissimilarities betwoen atomic structure and the structure of the solar system.
Answer:
Dissimilarities between atomic structure and the structure of the Solar system :

Atomic Structure	Solar system
(i) More than one electron may be held in the same orbit.	(i) More than one planet never be held in the same orbit.
(ii) Force between the nucleus and electrons is electrostatic in nature.	(ii) Force between the sun and the planets is gravitational in nature.
(iii) In the atom, the protons and electrons are electrically charged bodies.	(iii) The sun and all other heavenly planets are uncharged.

Broad answer type questions

Question 1.
What conclusions are found from Rutherford’s experiment ?
Answer:
Conclusions from Rutherford’s experiment :
(i) The atom consists of an extremely small, centrally located region called the nucleus in which the whole mass and the entire positive charge carried by the protons remain confined.
(ii) The major portion of the space in an atom is empty.
(iii) Negatively charged electrons present in the atom remain outside the nucleus at relatively large distances and they revolve round the nucleus at some definite circular path known as orbit.

Question 2.
What is the fundamental difference between Rutherford’s model and Bohr’s model?
Answer:
Difference between Rutherford’s model and Bohr’s model : The fundamental difference between the two models is that Bohr’s model is based on the concept of quantisation of energy and angular momentum of electron. Electrons can move only is certain permitted orbits with definite amount of energy and angular momentum. Rutherford’s model does not give an idea about the permitted orbits.

Question 3.
What are the uses of Radio-isotopes ?
Answer:
Uses of Radio-isotopes :
(i) In medicine : Radio isotopes and proved to be very useful in medical diagnosis. Radio-isotope of iodine is used by the patients with thyroid disorder.
(ii) In agriculture : By introducing radioactive phosphorous fertiliser, it has been possible to enhance growth of the plant and crop.
(iii) In Industry : In research and process control radio-isotopes are used as a tracer.
(iv) Radio-carbon dating : With the help of radioactive isotope of carbon \({ }_6^{14} \mathrm{C}\) it has been possible to estimate the age of our earth which is about 4-5 billion years.

Question 4.
What is valence electron ? What is radioactivity ? Give the name of the natural radioactive elements and artificial radioactive elements.
Answer:
Valence Electron : Electron or electrons present in the outermost orbit of the electronic shell of an atom is known as valence electron.

Radioactivity : It is a neuclear phenomenon in which the elements having higher atomic number and mass number (n / p>1.54) disintegrate spontaneously emitting energy from the nucleus in the form of radioactive rays and this emission cannot be stopped by any physical or chemical process which means this emission is independent of external agencies and conditions.
Natural Radioactive element :
Radium (Ra), Thorium (Th), Uranium (U) etc.
Artifical Radioactive element :
Neptunium (Np), Eistenium (Es) etc.

Question 5.
‘Atomic number is the intrinsic property of an element’-explain.
Answer:
Explanation: Atomic number, the number of protons in the nucleus of an atom of an element is unique for an element. It cannot

change even during or after any chemical or physical change that the element undergoes. No two elements have the same atomic number, on the other hand, different elements have different atomic numbers. The atomic number of an element indicates the ability of chemical reactivity of the element. So, atomic number is the intrinsic property of the element.

Question 6.
Atomic weight or relative atomic mass of most of the elements is not a whole number. Why?
Answer:
Explanation : It is found that the atomic weights of most of the naturally occuring elements are fractional although their mass numbers (n+p) are whole numbers. This happens because, most of the natural element consist of a mixture of two or more of their isotopes in various proportions but almost in constant composition.

For example : The atomic weight of chlorine as found by chemical methods is 35.457 . Such fractional atomic weight is due to the fact that chlorine exists in nature as a mixture containing 75.4 % of the ligher \({ }_{17}^{35} \mathrm{Cl}\) and 24.6 % of the heavier \({ }_{17}^{37} \mathrm{Cl}\) isotopes. The composition of the mixture is always found to be constant.
∴ Atomic weight of chlorine
= \(\frac{75.4 × 35+24.6 × 37}{100}\) = 35.457

Numerical Problems

Working formula

(i) 1 gram-molecular weight of a substance contains molecules = 6.022 × 10²³
(ii) 1 gram-atom of an element contains atoms = 6.022 × 10²³
(iii) At NTP, the volume of 1 gram-molecular gas = 22.4 lit.
(iv) At NTP, 22.4 lit. volume of gas has molecules = 6.022 × 10²³
(v) Atomic number = Number of protons
(vi) Mass number = Number of protons + Number of neutrons
(vii) Maximum number of electron in an Bohr’s orbit = 2 n²(n = 1,2,3 … etc.)
(viii) Atomic weight = \(=\frac{\text { Weight of one atom of an element }}{\text { Weight of one atom of carbon }} \times 12\)
(ix) 1 mole = 1 gram-molecular weight of a substance.

Example 1: Calculate the number of moles in 50 g CaCO 3.
Answer:
Formula mass of CaCO₃ = 40+12+3 × 16 = 100
Gram formula mass of CaCO₃ = 100 g
Agin we know, gram formula mass of CaCO₃ = 1 mole ∴ 100 g CaCO₃ = 1 mole
∴ 50 g CaCO₃ = (\(\frac{1}{100}\) × 50) mole = 0.5 mole

Example 2 : Calculate the mass of a silver atom.
Answer:
Atomic mass of silver = 108
∴ Gram-atomic mass of silver = 108 g
Now, 6.022 × 10²³ atoms of silver = 1 mole of silver atoms = 108 g
∴ Mass of 6.022 × 10²³ atoms of silver = 108 g
∴ Mass of 1 atom of silver = \(\frac{108}{6.022 \times 10^{23}}\) = 17.93 × 10^-23 g

Example 3 : Calculate the mass of water in gram of 5 gram-molecular water.
Answer:
We know,
molecular weight of water = 2 × 1+16 = 18
1 gram-molecular mass of water = 18 g water
5 gram-molecular mass of water = (18 × 5) g = 90 g water

Example 4 : What is the volume of oxygen at NTP of 4 gram-molecular weight of Oxygen ?
Answer:
We know, at NTP,
1 gram-molecular weight of oxygen gas has volume 22.4 lit.
∴ 4 gram-molecular weight of oxygen gas has volume = (22.4 × 4) lit. = 89.6 lit.

Example 5 : Calculate the number of hydrogen molecules in 1 grammolecular weight of hydrogen and 1 gram of hydrogen.
Answer:
We know,
1 gram-molecular weight of hydrogen gas has molecule = 6.022 × 10²³.
Molecular weight of hydrogen gas = 2
∴ Gram-molecualr weight of hydrogen gas = 2 g.
∴ 2 g hydrogen has molecule = 6.022 × 10²³
1 g hydrogen has molecule = \(\frac{6.022 \times 10^{23}}{2}\)
= 3.0115 × 10²³

Example 6 : At NTP, 1.12 lit of a gas has weight 2.2 g. Calculate its molecular weight.
Answer:
At NTP 1.12 lit. of a gas weight = 2.2 g
At NTP, 22.4 lit. of a gas has weight = \(\frac{2.2}{1.12}\) × 2.2 .4 g
= 44 g
∴ The molecular weight of the gas = 44

Example 7 : What are the numbers of protons, neutrons, and electrons in \({ }_{28}^{65} \mathrm{Cu}\) atom ?
Answer:
We know from the symbol \({ }_{29}^{65} \mathrm{Cu}\), the atomic number of Cu atom is 29 and the mass number is 65 .
So, the number of protons in Cu atom is 29 ; the number of electrons in Cu atom is also 29.
∴ Number of neutrons = (65-29) = 36

Example 8: What is the mass number and what is the atomic number of \({ }_{92}^{235} \mathrm{X}\) ? What will be its symbol if the number of neutrons are three more?
Answer:
In \({ }_{92}^{235} \mathrm{X}\), the number of protons are 92 and the sum total of protons and neutrons are 235 .
So, the atomic number is 92 and mass number is 235 .
If the number of neutrons are three more, then the mass number will be (235+3) = 238.
In that case the symbol will be \({ }_{92}^{238} \mathrm{X}\).

Example 9: What is the relation between \({ }_{35}^{17} \mathrm{A}\), and \({ }_{37}^{17} \mathrm{B}\) ? Is there any difference in their chemical properties?
Answer:
\({ }_{35}^{17} \mathrm{A}\), and \({ }_{37}^{17} \mathrm{B}\), both have same atomic number but different mass numbers. So they are isotopes of each other. As the chemical property depends on number of protons, so they have same chemical properties.