Kerala Syllabus Class 9 Chemistry Chapter 1 Structure of Atom Notes Solutions

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Kerala SCERT Class 9 Chemistry Chapter 1 Solutions Structure of Atom

Kerala Syllabus Std 9 Chemistry Chapter 1 Structure of Atom Notes Solutions Questions and Answers

Class 9 Chemistry Chapter 1 Let Us Assess Answers Structure of Atom

Question 1.
Some observations related to experiments on cathode rays are given. Write the inference based on each observation.
a. A paddle wheel placed in the path of cathode rays rotates.
b. A shadow is formed if an object is placed in the path of cathode rays.
c. When an electric field is applied perpendicular to the path of cathode rays, the rays deflect towards the positive plate.
Answer:
a. A paddle wheel placed in the path of cathode rays rotates shows that the particles of cathode rays have mass

b. A shadow is formed if an object is placed in the path of cathode rays, indicating that the cathode
rays travel in a straight line.

c. When an electric field is applied perpendicular to the path of cathode rays, the rays deflect towards the positive plate because the cathode rays are composed of negatively charged particles (electrons)

Question 2.
The atomic number of an atom is 16 and mass number is 32.
Answer:
a. How many electrons, protons and neutrons are present in this atom?
b. Write the electron configuration of this atom.
c. Draw the orbit electron configuration of this atom.
Answer:
a) Atomic number of atom = 16
Mass number of atom = 32
Number of protons = Atomic number = 16
No: of electrons = 16
No: of neutrons = Mass number – Atomic number = 32 – 16 = 16

b. Electronic configuration of atom = 2, 8, 6

c. Orbit electronic configuration of the atom

Question 3.
Electrons are present in the K, L and M shells of an atom.
a. Which of these shells has the highest energy?
b. If M shell contains only 3 electrons, write the atomic number of this atom.
c. What is the number of electrons in this atom?
d. If the nucleus of this atom contains 16 neutrons, what is its mass number?
Answer:
a. M shell has maximum energy in K, L, and M shells

b. M shell contains 3 elecrons, indicates that inner shells are filled. i.e., K and L shells contain maximum of 2 and 8 electrons respectively.
Total number of electrons = 2 + 8 + 3 = 13
Atomic number = no: of protons = no: of electrons in a neutral atom = 13

c. No: of electrons in the atom = sum of electrons in each shell = 2 + 8 + 3= 13

d. No: of neutrons = 16
Mass number = No: of protons + No: of neutrons = 13 + 16 = 29

Question 4.
The orbit electron configuartion of an atom is given below.

a. What is the mass number of this atom?
b. Write its electron configuration.
Answer:
a. From the figure
No: of protons = 13
No: of neutrons = 14
Mass number of atom = No: of protons + No: of neutrons = 13 + 14 = 27

b. From the figure
No: of electrons = 13
Electron configuration = 2, 8, 3

Question 5.
The symbols of some elements are given.
\({ }_12^{24} \mathrm{Mg}\), \({ }_6^{12} \mathrm{C}\), \({ }_7^{15} \mathrm{N}\), \({ }_6^{14} \mathrm{C}\), \({ }_11^{24} \mathrm{Na}\)
a. Select a pair of isotopes from the given elements. Write the reason for selecting it.
b. Select a pair of isobars from the given elements.
Answer:
a. \({ }_6^{12} \mathrm{C}\) and \({ }_6^{14} \mathrm{C}\)C are the pair of isotopes. Isotopes have same atomic number and different mass number. They are atoms of same element with different mass number.

b. \({ }_12^{24} \mathrm{C}\)Mg and \({ }_11^{24} \mathrm{C}\)Na are the pair of isobars. Isobars are the atoms of different elements with same mass number and different atomic number.

Question 6.
Match the items in column A & B suitably.

A	B
Plum pudding model	James Chadwick
Planetary model of atom	Goldstein
Canal rays	J.J. Thomson
Neutron	Rutherford

Answer:

A	B
Plum pudding model	J.J. Thomson
Planetary model of atom	Rutherford
Canal rays	Goldstein
Neutron	James Chadwick

Question 7.
The atomic number and mass number of an element are 15 and 31 respectively.
a. What is the number of valence electrons in this atom?
b. How many neutrons are present in this atom?
c. Draw the orbit electron configuration of this atom.
Answer:
a. Atomic number of an atom = No: of protons = No: of electrons = 15
Electron configuration = 2,8,5
Valence electrons No: of electrons in outermost shell = 5

b. Mass number = 31
Mass number = atomic number + No: of nuetrons
No: of neutrons = Mass number – atomic number = 31 – 15=16
c. Orbit electron configuration of the atom

Question 8.
Isotope of an element is used to determine the age of fossils.
a. Which is this isotope?
b. Which are the other two main isotopes of this element?
c. Write the number of neutrons in each isotope.
Answer:
a. \({ }_6^{14} \mathrm{C}\) is the radioactive isotope of carbon used to determine the age of fossils.

b. \({ }_6^{12} \mathrm{C}\) and \({ }_6^{13} \mathrm{C}\) are the other two isotopes of carbon.

c. No: of neutrons in \({ }_6^{14} \mathrm{C}\) isotope = 8
No: of neutrons in \({ }_6^{13} \mathrm{C}\) isotope = 7
No: of neutrons in \({ }_6^{12} \mathrm{C}\) isotope = 6

Extended Activities

Question 1.
Prepare a presentation on scientists connected to the history of atom and their contributions and present it in the classroom.
Answer:
Here are some points for your reference:

• John Dalton – In 1809, found out all substances are made up of very minute particles called atoms.
• Heinrich Geissler – In 1854, developed discharge tubes and vaccum pumps.
• William Crookes – In 1875, passed high voltage electricity through gaes at low pressure in test tube.
• Eugen Goldstein – In 1886, found out canal rays.
• J.J Thomson – In 1906 found out cathode rays. Put forward the plumpudding model of atom
• Henry Becquerel – in 1896 discovered radioactivity.
• Ernest Rutherford – In 1911 conducted gold foil experiment. Found out the nucleus. Put forward the planetary model of atom.
• Neils Bohr – In 1913 put forward the Bohr model of atom.
• James Chadwick – In 1932 discovered neutron.

Question 2.
Prepare a timeline chart on the main events that led to the discovery of different subatomic particles.
Answer:
In 1886 Eugen Goldstein discovered canal rays. From the study of canal rays discovered protons.
Rutherford named the particle proton. In 1906 J.J.Thomson discovered electrons through discharge tube experiments. In 1932 James Chadwick discovered neutrons.

Question 3.
You have learned about isotopes. Find more examples for radio isotopes. Prepare an article on the uses of each radio isotope and publish it in the science magazine. Use word processor for this work.
Answer:

Radioactive isotopes	Uses
Iodine-131	For the study of thyroid glands and treatment
Uranium-235	Used as nuclear fuel
Cobalt-60	For the treatment of cancer
Sodium-24	To find out leakage in industrial pipe lines
Iron-59	To detect anaemia
Carbon-14	To detect the age of fossils
Dueterium	To produce heavy water
Phosphorus-31	As a tracer to find out material transportation in plants

Question 4.
If you get a chance to conduct an interview with Rutherford, what questions would you ask him? Prepare a questionnaire.
Answer:
Here are some questions for your reference:

• Can you remember the teachers who influenced you the most?
• What was your basic subject of interest?
• Which topic did you selected for your research?
• How long did you spend in research?
• Which do you think is your important discovery?

Structure of Atom Class 9 Notes Questions and Answers Kerala Syllabus

Question 1.
Analyse table

How do molecules of different substances differ?
Answer:

• Differ in the component elements
• Differ in the ratio of the component elements

Question 2.
What are the important particles in an atom?
Answer:

• Electron
• Proton
• Neutron
  They are called sub-atomic particles.

Question 3.
How was it proved that electrons have mass?
Answer:
When a paddle wheel was placed in the path of cathode rays it rotates. This shows that electrons, the particles in cathode rays have mass.

Question 4.
Cathode rays, cast shadows of opaque objects placed in their path. What can be inferred from this?
Answer:
Cathode rays cast shadows of opaque objects placed in their path, indicating that the cathode rays travel in a straight line.

Question 5.
Some properties of the sub-atomic particles like electron, proton and neutron are given in the table. Complete the following table.

Answer:

Question 6.
Some statements are given. Which of them are related to J.J. Thomson?
a) Proposed the idea of the orbit
b) Conducted discharge tube experiments
c) Discovered neutron
d) Discovered electron
e) Proposed the plum pudding model
Answer:
b) Conducted discharge tube experiments
d) Discovered electron

Question 7.
Prepare a questionnaire about the scientists who conducted research on atomic structure and their contributions. Conduct a quiz programme in your classroom based on this.
Answer:
Here are some questions for your reference

• Who developed discharge tubes and vaccum tubes?
  – Heinrich Geissler
• Who discovered electrons?
  – J.J. Thomson
• Name the scientist who discovered the charge to mass ratio of electrons.
  – J.J. Thomson
• Who discovered canal rays?
  – Eugen Goldstein
• What is the charge of the particles of cathode rays?
  – Negative
• Who found the charge and mass of electron?
  – Robert Millikan
• Scientist who discovered radioactivity?
  – Henry Becquerel
• The Particles that do not deviate in electric and magnetic fields is
  – Neutron

Question 8.
What are the particles in the nucleus of an atom?
Answer:
Protons and neutrons are present in nucleus of an atom.

Question 9.
What is the mass number of an atom having 2 protons and 2 neutrons?
Answer:
No: of neutrons = 2
No: of protons = 2
Mass number of the atom = No: of protons + No: of neutrons = 2 + 2 = 4

Question 10.
Find the number of protons, electrons and neutrons in chlorine and calcium atoms

Answer:
1. Chlorine \({ }_17^{35} \mathrm{C}\)Cl
Atomic number of Cl, Z = 17
Mass number of Cl, A = 35
No: of protons = 17
No: of electrons = 17
Mass number = No: protons + No: of neutrons
∴ No: of neutrons = Mass number – No: of protons A – Z = 35 – 17 18

2. Calcium \({ }_20^{40} \mathrm{C}\)Ca
Atomic number of Ca. Z = 20
Mass number of Ca, A =40.
No: of protons = 20
No: of electrons = 20
Mass number = No: protons + No: of neutrons
∴ No: of neutrons Mass number – No: of protons = A – Z = 40 – 20 = 20

Question 11.
Complete the table given below.

Answer:
Atomic number, Z = No: of protons = No: of electrons
Mass Number, A = No: of protons (Z) + No: of neutrons
No: of neutrons = Mass number – Atomic number = A – Z

1. Hydrogen \({ }_1^{1} \mathrm{H}\)
Atomic number of H, Z = 1
Mass number of H, A =1
‘No: of protons = 1
No: of electrons = 1
Mass number = No: protons + No: of neutrons
∴ No: of neutrons = Mass number – No: of protons = A – Z = 1-1=0

2. Lithium \({ }_3^{7} \mathrm{Li}\)
Atomic number of Li, Z = 3
Mass number of Li. A = 7
No: of protons = 3
No: of electrons = 3
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 7 – 3 = 4

3. Oxygen \({ }_8^{16} \mathrm{O}\)
Atomic number of O, Z = 8
Mass number of O, A = 16
No: of protons = 8
No: of electrons = 8
Mass number = No: protons + No; of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 16 – 8 = 8

4. Sodium Na \({ }_11^{23} \mathrm{Na}\)
Atomic number of Na, Z = 11
Mass number of Na, A = 23
No: of protors = 11
No: of electrons = 11
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 23 – 11 = 12

5. Neon \({ }_10^{20} \mathrm{Ne}\)
Atomic number of Ne, Z = 10
Mass number of Ne, A = 20
No: of protons = 10
No: of electrons = 10
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 20 – 10 = 10

6. Titanium \({ }_22^{48} \mathrm{Ne}\)
Atomic number of Ti, Z = 22
Mass number of Ti, A =48
No: of protons = 22
No: of electrons = 22
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 48 – 22 = 26

7. Uranium \({ }_92^{235} \mathrm{U}\)
Atomic number of U, Z = 235
Mass number of U, A = 92
No: of protons = 92
No: of electrons = 92
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 235 – 92 = 143

8. Thorium \({ }_90^{232} \mathrm{Th}\)
Atomic number of Th, Z = 90
Mass number of Th, A = 232
No: of protons = 90
No: of electrons = 90
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 232 – 90 = 142

9. Zinc \({ }_30^{65} \mathrm{Th}\)
Atomic number of Zn, Z = 30
Mass number of Zn, A = 65
No: of protons = 30
No: of electrons = 30
Mass number = No: protons + No: of neutrons
∴, No: of neutrons = Mass number – No: of protons = A – Z = 65 – 30 = 35

Question 12.
According to the Bohr atom model, where is the electron situated in an atom?
Answer:
Electrons are situated in orbits of an atom.

Question 13.
What are the symbols given to the energy levels 1,2,3, and 4?
Answer:

n	Energy level
1	K
2	M
3	L
4	N

The arrangement of electrons in an atom is done in accordance with certain laws.

Orbit number (n)	Name	Maximum number of electrons that can be accommodated (2n2)
1	K	2 × l2 = 2
2	L	2 × 22 = 8
3	M	2 × 32 = 18
4	N	2 × 42 = 32
5	O	2 × 52 = 50

1. The maximum number of electrons that can be accomodated in an orbit is 2n2, where n is the Table
2. Normally, filling up of electrons in higher energy orbits will take place only after the lower energy orbits are filled.
3. The maximum number of electrons that can be accomodated in the outermost orbit of an atom is 8.

Question 14.
Let us write the electron configuration of some elements. Complete the following table

Answer:

Orbit Electron Configuration-Diagrammatic Representation
The orbit electron configuration of Hydrogen \({ }_{1}^{1} \mathrm{H}\)
No: of electrons in hydrogen = 1

The orbit electron configuration of Boron \({ }_{5}^{10} \mathrm{H}\)
No: of electrons in Boron = 5

Question 15.
Diagrammatically represent the orbit electron configuration of \({ }_{13}^{27} \mathrm{Al}\)
Answer:
The atomic number of aluminium, Z = 13
Mass number of aluminium, A = 27
Neutrons of aluminium = A – Z = 27 – 13 = 14
The orbit electronic’ configuration of aluminium is

Question 16.
The orbit electron configuration of an atom is given.

Analyse the figure and find the following.
(i) Atomic number
(ii) Number of protons
(iii) Number of neutrons
(iv) Mass number
Answer:
(i) Atomic number is the number of protons in an atom.
Atomic number of given atom, Z = 18
(ii) Number of protons (from the figure) = 18
(iii) Number of neutrons (from the figure) = 22
(iv) Mass number of the atom, A = sum of protons and neutron = 18 +22 = 40
(v) Electron configuration = 2,8,8

Question 17.
Write the electron configuration of elements from atomic number 1 to 18 and represent their shell electron configuration.
Answer:

Question 18.
Number of which sub-atomic particle determines the element? (proton/neutron)
Answer:
Proton

Question 19.
See figure given below.

Complete table regarding these atoms.

Answer:

Question 20.
What is the atomic number of these atoms?
Answer:
Atomic number in these atoms is 1.

Question 21.
Which is the element having atomic number 1?
Answer:
Hydrogen is the element with atomic number 1.
Then, all these three are hydrogen atoms.

Question 22.
In the number of which particle do these atoms differ?
Answer:
Neutrons

Question 23.
Are the mass number of these atoms same?
Answer:
Mass number of these atoms are not the same.

Question 24.
Which of them has no neutron in the nucleus?
Answer:
Protium has no nuetron in it.

Question 25.
These are the isotopes of hydrogen. Can you define an isotope?
Answer:
Isotopes are the atoms of the same element with same atomic number and different mass number. Isotopes exhibits the same chemical properties. But they show slight variations in the physical properties. Heavy water is the oxide of deuterium (an isotope of hydrogen) is used in nuclear reactors.

Question 26.
Let us see whether hydrogen alone has isotopes. See the figure given below.

Answer:

\({ }_6^{12} \mathrm{C}\), \({ }_6^{13} \mathrm{C}\), \({ }_6^{14} \mathrm{C}\)C are the three natural isotopes of carbon. Among which the most stable and most abundant is the \({ }_6^{12} \mathrm{C}\) isotope. The amount of ¹³C among the isotopes of carbon is approximately 1.1 %. This isotope is used to study the metabolic processes in plants and animals. ¹⁴C is a radioactive isotope. This is used to determine the age of fossils.

ISOTOPES	USES
Iodine-131	To study the functioning of thyroid gland and its treatment
Uranium-235	Fuel in nuclear reactors
Cobalt-60	Cancer treatment
Sodium-24	To detect the leakage in industrial pipelines
Iron-59	To diagnose Anaemia

Question 27.
Orbit electron configuration of argon (Ar), potassium (K) and calcium (Ca) is given below. Analyse the figure and complete table.


Answer:
From the figure:

Question 28.
What is the pecularity of the mass numbers of these elements?
Answer:
Mass number of the elements Ar, K and Ca are the same.i.e., A = 40

Question 29.
Are the atomic numbers the same?
Answer:
Atomic numbers are not the same for these elements.
Isobars are atoms having the same mass number and different atomic numbers. They are
atoms of different elements in which the number of nucleons (proton  neutron) is equal.
Atoihs having the same number of neutrons are called isotones. Examples: \({ }_6^{14} \mathrm{C}\) and \({ }_7^{15} \mathrm{C}\)