AP 9th Class Physical Science 4th Lesson Important Questions Structure of the Atom

These AP 9th Class Physics Important Questions 4th Lesson Structure of the Atom will help students prepare well for the exams.

AP 9th Class Physical Science 4th Lesson Structure of the Atom Important Questions

Class 9 Physical Science Chapter 4 Important Questions – 2 Marks

Question 1.
What is an electron ? Who discovered it ?
Answer:
Electron is a negatively charged particle present outside the nucleus of an atom. It was discovered by J.J.Thomson in 1897.

Question 2.
What will be the number of neutrons if an atom has mass number = 23 and the number of electrons = 11?
Answer:
No.of electrons = 11; Mass number = 23
Atomic number = number of electrons = 11
Number of Neutrons = Mass number – Atomic number = 23 – 11 = 12
∴ Number of neutrons = 12

Question 3.
There are 15 protons and 16 neutrons in the nucleus of an element. Calculate its atomic number and its atomic mass.
Answer:
Atomic number = Number of protons = 15
Atomic mass = Number of protons + Number of neutrons =15 + 16 = 31

Question 4.
An element ‘Z’ forms the following compound, when it reacts with hydrogen, chlo¬rine, oxygen and phosphorus.
ZH₃, ZCl₃, Z₂O₃ and ZP
i) What is the valency of element Z ?
Answer:
The valency of Z is 3.

ii) Element ‘Z’ is metal or non-metal ?
Answer:
Z is a metal, because it is electropositive and reacts with non-metals.

Question 5.
Name the isotopes used in the treatment of goitre and cancer.
Answer:
Goitre – Isotope of iodine
Cancer – Isotope of cobalt

Question 6.
Write a major challenge before the scientists at the end of the 19^th century towards atomic structure.
Answer:

1. A major challenge before the scientists at the end of the 19^th century was to reveal the structure of the atom as well as to explain its important properties.
2. The elucidation of the structure of atoms is based on a series of experiments.

Question 7.
Explain Thomson’s model of an atom.
Answer:
Thomson’s model of an atom:

1. According to Thomson’s model, an atom consists of a positively charged sphere with electrons embedded in it.
2. The negative and positive charges are equal in magnitude, making the atom electrically neutral.

Question 8.
Why did Rutherford not expect to see large deflections in his experiment with the gold foil ?
Answer:
Rutherford did not expect to see large deflections in his experiment with the gold foil because the α – particles were much heavier than the protons.

Question 9.
What did Rutherford conclude from his gold foil experiment ?
Answer:
Rutherford concluded that the atom has a small, dense, positively charged nucleus at its center, with electrons orbiting around it.

Question 10.
Who discovered the neutron ? What is the mass of a neutron ?
Answer:

1. The neutron was discovered by J. Chadwick in 1932.
2. The mass of a neutron is nearly equal to that of a proton.

Question 11.
Why Bohr’s model of the atom is more successful ?
(OR)
How Neils Bohr’s model of the atom has explained the stability of the atom ?
Answer:
Neils Bohr’s model of the atom was more successful. He proposed that electrons are distributed in different shells with discrete energy around the nucleus. If the atomic shells are complete, then the atom will be stable and less reactive.

Question 12.
Explain the properties of sub-atomic particles.
Answer:
the three sub-atomic particles of an atom are: i) electrons, ii) protons and iii) neutrons. Electrons are negatively charged; protons are positively charged and neutrons have no charges. The mass of an electron is about 1/2000 times the mass of an hydrogen atom. The mass of a proton and a neutron is taken as one unit each.

Question 13.
How is valency related to the number of electrons in the outermost shell of an atom?
Answer:

1. The valency of an atom is directly related to the number of electrons present in its outermost shell.
2. An atom’s valency is determined by the number of electrons it can gain, lose or share to achieve a stable octet in its outermost shell.

Question 14.
How is valency determined for atoms with close to a full outermost shell ?
Answer:
Valency is determined by subtracting the number of electrons in the outermost shell from eight, which gives the number of electrons that the atom needs to. gain, lose, or share to achieve a fully-filled outermost shell.

Question 15.
What is the notation for au atom?

Answer:
The atomic number, mass number and symbol of the elements are written as : where A is the mass number and X is the symbol of the element and Z is the atomic number.

Question 16.
Observe the table given below.

Shell	Shell Number	Maximum number of electrons in a shell
K	1	2(1)2 = 2
L	2	2(2)2 = 8
M	3	2(3)2 = 18
N	4	2(4)2 = 32

Answer the following questions.
i) Which Shell has highest number of electrons ?
ii) Write the general form of the formula to find maximum number of electrons in each shell.
Answer:
i) Shell ’N’
ii) 2n²(n = 1, 2, 3)

Question 17.
Explain the following with one example of each :
i) Atomic number
ii) Mass number
Answer:
i) Atomic number : It ts the number of protons of an atom and is denoted by U. For example atomic no. of C = 6.
ii) Mass.number : It is defined as the sum of total number of protons and neutrons present in the nucleus of an atom. For example, nitrogen has 7 protons and 7 neutrons, therefore, its mass number is 14.

Question 18.
The number of electrons in the outermost ‘L’ shell of an atom is 5.
a) Write its electronic configuration.
b) What is its valency and why ?
Answer:
a) 2, 5.
b) 3, because it needs three more electrons to complete its octet.

Question 19.
An iron X^2- contains 10 electrons and 8 neutrons. What are the atomic number and mass number of the element M ?
Answer:
Atomic number = Number of protons
So, atomic number of the element = 8
Mass no. of the element = 8 + 8 = 16

Question 20.
Which of the Na⁺ and He has completely filled K and L shells ? Give reason to support your answer.
Answer:
Na⁺ has completely filled K and L shells. Na atom gets converted to Na” by losing one electron from its outermost shell. He atoms has only K shell.

Structure of the Atom Class 9 Important Questions – 3 Marks

Question 1.
An atom of an element has three electrons in its 3^rd orbit, which is the outermost shell?
i) The electronic configuration
ii) Atomic number
iii) Number of protons
iv) Valency
Answer:
i) Electronic configuration – 2, 8, 3
ii) Atomic number – 13
iii) Number of protons – 13
iv) Valency – 3

Question 2.
How many electrons, protons and neutrons will be there in an element \({ }_9^{19} \mathrm{X}\)? What will be the valency of the element ?
Answer:
Number of protons = Atomic number = 9
Number of (protons + neutrons) = Mass number = 19
Number of electrons = 9 ; Number of protons = 9
Number of neutrons = Mass number – Atomic number = 19 – 9 = 10
Electronic configuration of X = 2, 7
Valency of X = 1

Question 3.
Why chemical properties of all the isotopes of an element Eire same ?
Answer:
This is because isotopes have same atomic number, so the number of valence electrons present in them are same and it is the valence electrons which take part in chemical reactions. So, the isotopes of an element have same chemical properties.

Question 4.
An atom of an element has 2 electrons in the M – shell i.e., third shell. What will be the atomic number of this element ? Name this element. Find the valency of this element. Also, find the number of neutrons in the atom of this element.
Answer:

1. Atomic number is 12.
2. Element is Magnesium.
3. Valency is 2 (+2).
4. Number of neutrons = Atomic mass – Number of protons = 24 – 12 = 12

Question 5.
Describe Thomson’s model of an atom.
Answer:

• Thomson proposed that an atom consists of a positively charged sphere and the electrons are embedded in it.
• The negative and positive charges are equal in magnitude, making the atom as a whole electrically neutral.
• Thomson compared the structure of an atom to that of a Christmas pudding, where electrons are like currants in a spherical Christmas pudding and the positive charge is spread all over like the red edible part of the watermelon.

Question 6.
Draw the diagram of the scattering of α-particles by a gold foil.
Answer:

Question 7.
How is the atomic structure according to Rutherford ?
Answer:
On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features :

1. There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an alom resides in the nucleus.
2. The electrons revolve around the nucleus in circular paths.
3. The size of the nucleus is very small as compared to the size of the atom.

Question 8.
What are the draw backs of Rutherford’s model of atom ?
Answer:

1. The revolution of the electron in a circular orbit is not expected to be stable.
2. Any particle in a circular orbit would undergo acceleration.
3. During acceleration, charged particles would radiate energy.
4. Thus, the revolving electron would lose energy and finally fall into the nucleus.
5. If this were so, the atom should be highly unstable and hence matter would not exist in the form that we know.
6. We know that atoms are quite stable.

Question 9.
Write the applications of isotopes.
(OR)
What are the applications of isotopes in our daily life ?
Answer:
Applications of isotopes :

1. An isotope of uranium is used as a fuel in nuclear reactors.
2. An isotope of cobalt is used in the treatment of cancer.
3. An isotope of iodine is used in the treatment of goitre.

Question 10.
How are Electrons Distributed in Different Orbits (Shells) ?
(OR)
Write Bohr and Bury’s theory about the distribution of electrons into different orbits of an atom.
Answer:
The following rules are followed for writing the number of electrons in different energy levels or shells

1. The maximum number of electrons present in a shell is given by the formula 2n²,
   where ‘n’ is the orbit number or energy level index, 1, 2, 3
2. The maximum number of electrons that can be accommodated in the outermost orbit is 8.
3. Electrons are not accommodated in a given shell, unless the inner shells are filled. That is, the shells are filled in a step-wise manner.

Question 11.
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 ?
Answer:

1. If the α -particle scattering experiment is carried out using a foil of a metal other than gold, the observation may vary depending on the properties of the metal used.
2. The number of alpha particles that scatter at different angles and their intensities depends on the atomic structure of the metal foil.
3. The scattering angles and intensities may be different for different metals due to their different atomic sizes and shapes.

Question 12.
How will you find the valency of chlorine, sulphur and magnesium ?
Answer:

1. The valency of chlorine (Cl) is 1 because it has seven electrons in its outermost shell and needs one more electron to complete its octet. Chlorine can gain one electron to form a chloride ion (Cl^–).
2. The valency of sulphur (S) is 2 because it has two electrons in its outermost shell and needs to lose those two electrons to achieve a stable octet.
3. The valency of magnesium (Mg) is 2 because it has two electrons in its outermost shell and needs to lose those two electrons to achieve a stable octet.

Question 13.
Draw the figures showing arrangement of electrons for the given elements.
1. Helium, Oxygen, Argon.
Answer:

2. How many neutrons are present in the nucleus of Sodium ?
Answer:
Sodium = \({ }_{11}^{23} \mathrm{Na}\) ; Neutrons = 23 – 11 = 12

Question 14.
Compare Rutherford and Thomson’s models of the atom on the following basis:
1) Where is the positive charge placed ?
2) How are the electrons placed ?
3) Are they stationary inside the atom or moving ?
Answer:
1) According to Thomson, the positive charge is uniformly distributed throughout the atom. Whereas according to Rutherford, the positively charged protons are inside the nucleus.

2) According to Thomson, electrons are embedded in the positively charged atom, but according to Rutherford, electrons are revolving around the nucleus in well – defined orbits.

3) According to Thomson, electrons are stable inside the atom but according to Ruth-erford, electrons are moving inside the atom.

Question 15.
i) Name the scientist who discovered neutrons.
ii) State the charge and mass of a neutron.
iii) Where is neutron located in an atom ?
Answer:
i) J.Chadwick
ii) Neutron has no charge and its mass is equal to that of a proton.
iii) It is located in the nucleus of an atom.

Question 16.
A, B and C are three metals with atomic number 4, 11 and 13 respectively. Arrange, the metals in the increasing order of their valency.
Answer:
‘A’ atomic number 4 has electronic configuration = 2, 2.
‘B’ atomic number 11 has electronic configuration = 2, 8, 1
‘C’ atomic number 13 has electronic configuration = 2, 8, 3
Increasing order of their valency B < A < C.

Important Questions on Structure of the Atom Class 9 – 5 Marks

Question 1.
a) Calculate the number of electrons, protons and neutrons in an atom of an element with atomic number 20 and mass number 40. Write electronic configuration and draW the structure of the atom.
b) An atom has complete K and L shells. Is this an atom of a metal, non – metal or noble gas ? Justify.
Answer:

b) Since, the outermost shell of the atom is complete, the atom is of a noble gas.

Question 2.
Which of the following electronic configurations are not possible ? Give reasons.
a) i) X : 2, 8, 4
ii) Y : 3, 8, 2
iii) Z : 2, 8, 9

b) Write the electronic configurations of the following elements and predict their valencies.
Flourine : 9, Aluminium: 13, Argon: 18
Answer:
a) Electronic configurations Y : 3, 8, 2 and Z : 2, 8, 9 are not possible because according to Bohr-Bury Scheme, the first shell can have maximum two and the outermost shell can have a maximum of eight electrons only.

Question 3.
a) Why are anode rays called canal rays ?
b) Mention two postulates of J.J.Thomson’s model.
c) Compare the properties of protons and electrons.
Answer:
a) The anode rays produced at the anode of the discharge tube are called canal rays, because they pass through the holes of the cathode.

b) i) Atom consists of positively charged sphere and electrons are embedded in it.
ii) The negative and positive charges are equal in magnitude.

c)

Protons	Electrons
1) Positively charged.	1) Negatively charged.
2) Mass of 1 proton is equal to mass of 4 atoms.	2) Mass of electron is 1/1840 times that of a proton.

Question 4.
Write Neils Bohr’s postulates about the model of an atom.
Answer:

Neils Bohr put forward the following postulates about the model of an atom :
1) Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.

2) While revolving in discrete orbits the electrons do not radiate energy. These orbits or shells are called energy levels. Energy levels in an atom are shown in Fig. These orbits or shells are represented by the letters K, L, M, N,.,. or the numbers, n = 1, 2, 3, 4, …..

3) Hence the maximum number of electrons in different shells are as follows : First orbit or K-shell will be 2 × 12 = 2, second orbit or L-shell will be 2 × 22 = 8, third orbit or M-shell will be 2 × 32 = 18, fourth orbit or N-shell will be 2 × 42 = 32 and so on.

4) Calculate the maximum number of electrons present in a shell is given by the Neils Bohr :
a) The maximum number of electrons present in a shell is given by the formula 2n2,.
where ‘n’ is the orbit number or energy level index, 1, 2, 3,….
b) Hence the maximum number of electrons in different shells are as follows :
i) First orbit or K-shell will be 2 × 12 = 2,
ii) Second orbit or L-shell will be 2 × 22 = 8,
iii) Third orbit or M-shell will be 2 × 32 = 18
iv) Fourth orbit or N-shell will be 2 × 42 = 32 and so on.

Question 5.
Explain Rutherford’s experiment and his model of an atom.
Answer:
1) Rutherford’s experiment involved firing fast-moving alpha particles at a thin gold foil.
The following observations were made :

• Most of the fast moving α-particles passed straight through the gold foil.
• Some of the α-particles were deflected by the foil by small angles.
• Surprisingly one out of every 12000 particles appeared to rebound.

2) Following a similar reasoning, Rutherford concluded from the α-particle scattering experiment that :

• Most of the space inside the atom is empty because most of the α-particles passed through the gold foil without getting deflected.
• Very few particles were deflected from their path, indicating that the positive charge of the atom occupies very little space.
• 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.
  From the data, he also calculated that the radius of the nucleus is about 105 times less than the radius of the atom.

3) On the basis of his experiment, Rutherford put forward the nuclear model of an atom, which had the following features:

• There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.
• The electrons revolve around the nucleus in circular paths.
• The size of the nucleus is very small compared to the size of the atom.

Question 6.
You know that the mass number of chlorine is 35.5. How do you explain this fractional mass?
(OR)
How do we calculate the mass of an atom ? Explain by taking chlorine as an example.
Answer:

1. Chlorine occurs in nature in two isotopic forms, with masses 35 u and 37 u in the ratio of 3 : 1.
2. The average atomic mass of chlorine atom, on the basis of above data, will be
   [(35 × \(\frac{75}{100}\) + 37 × \(\frac{25}{100}\)) = (\(\frac{105}{4}\) + \(\frac{37}{4}\)) = \(\frac{142}{4}\) = 35.5u]
3. The mass of an atom of any natural element is taken as the average mass of all the naturally occurring atoms of that element. If an element has no isotopes, then the mass of its atom would be the same as the sum of protons and neutrons in it.
4. But if an element occurs in isotopic forms, then we have to know the percentage of each isotopic form and then the average mass is calculated.
5. This does not mean that any one atom of chlorine has a fractional mass of 35.5 u.
6. It means that if you take a certain amount of chlorine, it will contain both isotopes of chlorine and the average mass is 35.5 u.

Question 7.
Write the differences between atomic number and mass number.
Answer:

Atomic Number	Mass Number
1) Atomic number (Z) is the number of protons present in the nucleus of an atom.	1) Mass number (A) is the total number of protons and neutrons present in the nucleus of an atom.
2) Atomic number determines the identity of the element. All atoms of an element have the same atomic number.	2) Mass number determines the isotope of the element. Atoms of the same element can have different mass numbers if they have different numbers of neutrons.
3) Atomic number is denoted by the symbol ‘Z’	3) Mass number is denoted by the symbol ‘A’.

Question 8.
Write the differences between isotopes and isobars.
Answer:

Isotopes	Isobars
Isotopes are atoms of the same element that have different numbers of neutrons in their nuclei.	Isobars are atoms of different elements that have the same mass number, which is the total number of protons and neutrons in the nucleus.
Isotopes have the same number of protons and electrons as other atoms of the same element, but a different number of neutrons.	Isobars have different numbers of protons and electrons as other atoms of the same element, but the same number of neutrons.
Isotopes have similar chemical properties, but different physical properties such as mass and density.	Isobars have different chemical and physical properties.
Examples : i) carbon, \({ }_6^{12} \mathrm{C}\) and \({ }_6^{14} \mathrm{C}\), ii) chlorine, \({ }_{17}^{35} \mathrm{Cl}\) and \({ }_{17}^{37} \mathrm{Cl}\) etc.	Examples: i) \({ }_{18}^{40} \mathrm{Ar}\), \({ }_{19}^{40} \mathrm{K}\), \({ }_{20}^{10} \mathrm{Ca}\) ii) \({ }_{11}^{24} \mathrm{Na}\), \({ }_{12}^{24} \mathrm{Mg}\)

Question 9.
a) Draw neat diagrams indicating the nucleus and arragement of electrons In different shell for the following elements.
i) Helium
ii) Carbon
iii) Argon
Answer:

b) Which of the above element is unstable ? Why ?
Answer:
Carbon is unstable. The nudes of carbon-14 atoms are unstable because they have too many neutrons relative to protons, so they gradually decay.

Question 10.
Define valency by taking examples of nitrogen and boron.
Answer:
Valency : The number of electrons present in outer most orbit of an atom is called its valency.
Valency of Nitrogen :
a) Atomic number of nitrogen is 7.
b) The distribution of electrons is 2, 5.
c) The outer most orbit has 5 electrons.
d) Hence its valency should be 5. But it is easier to nitrogen to gain 3 electrons than to loose 5 electrons for becoming octet.
e) Hence the valency of nitrogen is ‘3’.

Valency of Boron :
a) Atomic number of boron is 5.
b) The distribution of electrons is 2, 3.
c) The outer most orbit has 3 electrons.
d) Hence the valency of boron is 3.

Extra Questions on Structure of the Atom Class 9 – 4 Marks

Question 1.
a) What are isobars ?
b) Atomic number of an element Y is 17.
i) Write its electronic configuration.
ii) What is the number of valence electrons in Y ?
iii) How many electrons are needed to complete the octet of Y ?
iv) Is it a metal or non – metal ?
c) The valency of Na is 1 and not 7. Give reason.
Answer:
a) Atoms of different elements with different atomic number which have same mass number are called isobars.

b) i) 2, 8, 7
ii) 7
iii) 1
iv) Non – metal.

c) Electronic configuration of Na is 2, 8, 1. It can lose one electron to attain the electronic configuration of neon. Therefore, the valency of Na is 1.
The valency of Na may be 7 only when it gains 7 electrons in its valence shell, but the valency of Na is not 7.

Question 2.
Rutherford conducted the gold foil experiment to know how the electrons are arranged inside the atom.
i) What are the observations of gold foil experiment ?
Answer:

Observations of α – Scattering experiment :

1. Most of the fast moving α – Particles passed straight through the gold foil.
2. Some of the α – Particles were deflected by the foil by small angles.
3. One out of every 12000 particles appeared to rebound.

ii) What conclusions does Rutherford gave regarding gold foil experiment ?
Answer:
Conclusions of Rutherford α – Scattering experiment :
a) Most of the space inside the atom is empty.
b) Positive charge of the atom occupies very little space.
c) A very small fraction of α – particles were deflected by 180° indicating.
d) All the positive charge and mass of the atom concentrated in a very small volume
within the atom.

Question 3.
Fill In the missing information in the following table.

Answer:

Question 4.
Composition of nuclei of two atomic species X and Y are given as under.

	X	Y
Protons	6	6
Neutrons	6	8

Give the mass number of X and Y. What is the relation between the two species ?
Answer:
Mass number of X = 6 + 6 = 12
Mass number of Y = 6 + 8 = 14
So these two species have same atomic number (same number of protons) and different mass numbers. Therefore these two species are isotopes.

Question 5.
What information do you know from \({ }_{17}^{35} \mathrm{X}\) ?
Answer:
Given that \({ }_{17}^{35} \mathrm{X}\)
a) The atomic number of element is 17.
b) Hence the element is chlorine, symbol is ‘Cl’.
c) Number of protons = 17.
d) Number of electrons = 17.
e) Mass number = 35.
f) Number of neutrons = 35 – 17 = 18
g) Distribution of electrons in shells

K	L	M	N
2	8	7	–

h) Valency is ‘1’.
i) It gains one electron to become octet.

Question 6.
Fill the blanks in the table using the given information. (Isotopes are not included).

Answer:

Question 7.
The electronic configuration of an element X is 2,8, 2.
a) Find the number of electrons present in the atom of element X.
b) Write the atomic number.
c) This element ‘X’ is a metal or a non-metal.
d) Find out the valency of the element X.
Answer:
a) 12
b) Atomic number = 12
c) ‘X’ is a metal.
d) valency of ‘X’ is + 2.