CBSE Class 9 Science Structure Of Atom Notes Set A

Download CBSE Class 9 Science Structure Of Atom Notes Set A in PDF format. All Revision notes for Class 9 Science have been designed as per the latest syllabus and updated chapters given in your textbook for Science in Standard 9. Our teachers have designed these concept notes for the benefit of Grade 9 students. You should use these chapter wise notes for revision on daily basis. These study notes can also be used for learning each chapter and its important and difficult topics or revision just before your exams to help you get better scores in upcoming examinations, You can also use Printable notes for Class 9 Science for faster revision of difficult topics and get higher rank. After reading these notes also refer to MCQ questions for Class 9 Science given our website

CBSE Class 9 Structure of Atom. Learning the important concepts is very important for every student to get better marks in examinations. The concepts should be clear which will help in faster learning. The attached concepts made as per NCERT and CBSE pattern will help the student to understand the chapter and score better marks in the examinations.

Chapter-4

"Structure Of Atom" class_9_science_concept_4

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⇒ First recorded evidence that atoms existed.

⇒ Using his theory, Dalton rationalized the various laws of chemical combination

Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.

⇒ Limitations

o The indivisibility of an atom was proved wrong , for, an atom can be further subdivided into protons, neutrons and electrons.

o The atoms of same element are similar in all respects , but isotopes of same element have different mass.

Dalton's theory was based on the premise that the atoms of different elements could be distinguished by differences in their weights.

2. J J Thomson Experiments:

STRUCTURE OF ATOM_1

⇒ Discovered electrons in 1897.

⇒ Showed us that the atom can be split into even smaller parts.

His discovery was the first step towards a detailed model of the atom .

⇒ An atom is a uniform sphere of positive charges (due to presence of protons) as well as negative charges (due to presence of electrons).

⇒ Atom as a whole is electrically neutral because the negative and positive charges are equal in magnitude.

⇒ An electron is a negatively charged component of an atom which exists outside the nucleus. Each electron carries one unit of negative charge and has a very small mass as compared with that of a neutron or proton.

STRUCTURE OF ATOM_2

JJ Thomson used cathode ray tubes to demonstrate that the cathode ray responds to both magnetic and electric fields.

Since the ray was attracted to a positive electric plate placed over the cathode ray tube (beam deflected toward the positive plate) he determined that the ray must be composed of negatively charged particles.

He called these negative particles "electrons."

Limitation: Model failed to explain how protons and electrons were arranged in atom so close to each other.

Eugene Goldstein:

STRUCTURE OF ATOM_3

⇒ E. Goldstein in 1886 discovered the presence of new radiations in a gas discharge and called them canal rays. These rays were positively charged radiations which ultimately led to the discovery of another sub-atomic particle.

⇒ Used a Cathode Ray Tube to study "canal rays" which had electrical and magnetic properties opposite of an electron

⇒ Canal Rays: The positively charged radiation produced in the discharge tube at low pressure and high voltage are called canal rays.

Protons:

The canal rays have positively charged sub-atomic, particles known as protons (p).

Q.1 What was the model of an atom proposed by Thomson? Q.2 What is the nature of charge on electrons?

Q.3 What are canal rays ? 

Q.4 State the nature of the constituents of canal rays.

3. Rutherford’s Scattering Experiments:

STRUCTURE OF ATOM_4

Experiment: Rutherford took a thin gold foil and made alpha particles , [ He2+ ] positively charged Helium fall on it.

 STRUCTURE OF ATOM_5

⇒ Limitation: In Rutherford’s atomic model , Nucleus & electrons are held together by electrostatic force of attraction which would lead to the fusion between them. This does not happen in the atom.

Atomic radius ~ 100 pm = 1 x 10-10 m

Nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m

STRUCTURE OF ATOM_6

⇒ In 1932, James Chadwick proved that the atomic nucleus contained a neutral particle which had been proposed more than a decade earlier by Ernest Rutherford officially discovered the neutron in 1932,

⇒ Chadwick received the Nobel Prize in 1935.

A neutron is a subatomic particle contained in the atomic nucleus. It has no net electric charge, unlike the proton's positive electric charge.

Q.1 Who discovered the nucleus of the atom?

Q.2 What is the charge on alpha particles ?

Q.3 Which observation of Rutherford’s scattering experiment established the presence large empty space in atom?

Q.4 What is the nature of charge on nucleus of atom? Q.5 Who discovered neutron ?

4. Sub Atomic Particles:

STRUCTURE OF ATOM_7

Protons & Neutrons collectively are known as NUCLEONS.

Q.1 Why is the relative mass of an electron is taken as negligible ?

Q.2 Give the actual masses of electron & proton in kg?

Q.3 What are nucleons?

5.Atomic Number & Mass Number:

Atomic number of an element is defined as the number of unit positive charges on the nucleus (nuclear charge) of the atom of that element or as the number of protons present in the nucleus.

Atomic number, Z = Number of unit positive charge on the nucleus

= Total number of unit positive charges carried by all protons present in the nucleus.

= Number of protons in the nucleus (p)

= Number of electrons revolving in the orbits (e)

Eg :- Hydrogen – Atomic number = 1 (1 proton)

        Helium - Atomic number = 2 (2 protons)

Mass number[ A] : It is defined as the sum of the number of protons & neutrons present in the nucleus of an atom.

Mass Number = Mass of protons + Mass of neutrons

Eg :- Carbon – Mass number = 12 (6 protons + 6 neutrons) Mass = 12u

         Aluminium – Mass number = 27 (13 protons + 14 neutrons) Mass = 27u

STRUCTURE OF ATOM_8
Q.1 What happens when an electron jumps from lower to higher energy level?

Q.2 Which energy shell is nearest to the nucleus of an atom?

Q.3 Which energy shell has higher energy L or N ?

7.Electronic configuration & Valency:

Bohr and Bury Scheme - Important Rules

STRUCTURE OF ATOM_9

The outermost shell of an atom cannot accommodate more than 8 electrons, even if it has a capacity to accommodate more electrons. This is a very important rule and is also called the OCTET RULE. The presence of 8 electrons in the outermost shell makes the atom very stable.

Q.1 An atoms has atomic number 13. What would be its configuration.

Q.2 What is octet rule?

Q.3 How many electrons M shell can accommodate?

Q.4 If an atom has complete K and L shell, what would be its atomic number?

8. Isotopes & Isobars:
STRUCTURE OF ATOM_10

Q.1 Why atoms of isotopes are chemically same?

Q.2 Give the representation of three isotopes of carbon which are C-12 , C-13 and C-14.

QUESTION BANK [ *HOTS ]

1 Mark Questions:

1. Write the names of three elementary particles which constitute an atom.

2. Name the scientist & his experiment to prove that nucleus of an atom is positively charged.

3. Which is heavier , neutron or proton ?

4. *How many times a proton is heavier than an electron?

5. What was the model of an atom proposed by Thomson ?

6. How many electrons at the maximum can be present in the first shell ?

7. What type of charge is present on the nucleus of an atom?

8. Give the number of protons in 35Cl17

9. *What are iso bars ?

10. Name the particles which determine the mass of an at

2 Marks Questions:

1. Define the following terms: a) Atomic number b) Mass number

2. Write the charges on sub atomic particles.

3. Identify the isotopes out of A , B , C & D ? 33A17 40B20 37C17 38D19

4.* Give one Achievement and one limitation of J.J Thomson’s model of atom?

5. What are valence electrons? Give example.

6. *Which kind of elements have tendency to lose electron ? Give example.

7. How many electrons are present in the valence shell of nitrogen & argon?

8. State the maximum capacity of various shells to accommodate electrons.

9. Give the symbol , relative charge & mass of the three sub atomic particles.

10. From the symbol 3S16 state :

i) Atomic number of sulphur, ii) Mass number of sulphur iii) Electronic configuration of sulphu

Marks Questions:

1. Why do Helium has Zero valency?

2. An atom contains 3 protons , 3 electrons and 4 neutrons .What is its atomic number , mass number & valency?

3. *How are the isotopes of hydrogen represented ?

4. Write the complete symbol for the atom with the given atomic number [Z] & mass number[A].

i) Z= 17 , A = 15 ; ii) Z=4 , A = 9; iii) Z= 92 ; A=233

5. *What would be the electronic configuration of Na, Al3+ , O2, Cl -.

5 Marks Questions:

1.* a) Give the observations as well as inferences of Rutherford’s Scattering experiment for determining the structure of an atom.

b) On the basis of above experiment write the main features of atomic mod

2. Write the main postulates of Bohr’s Model of Atom.

You are expected to know…………

⇒ The scientists who discovered subatomic particles.

⇒ Rutherford established the existence of nucleus.

⇒ Difference between Atomic number and Mass number

⇒ Electronic configuration & its relation with Valency.

⇒ Difference between Isotope and Isobar.

STRUCTURE OF ATOM_11

 

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