Atoms and molecules class 9 revision notes | Atoms and molecules class 9 extra questions

The most important chapter in class 9th is Atoms and molecules. The importance of the chapter can be inferred from the fact that Questions on the concept of this chapter come in exams like JEE, NEET, IT etc. 

If you will strengthen these concepts from class 9, You will not have that much problem in classes like 11th and 12th. And I have given you the solution of this problem in this article and providing you best

Atoms and molecules class 9 notes

Main topics we are going to cover:

• Atoms and molecules class 9 notes
• Atoms and molecules class 9 notes pdf overview
• Atoms and molecules class 9 notes pdf download link
• class 9 science chapter 3 extra questions
• The 5 Best Atoms and Molecules class 9 Notes pdf YouTube Videos.
• 5 Quick Tips Regarding Atoms and Molecules class 9 Notes.

Atoms and molecules class 9 notes

So, Today we are going to provide you atoms and molecules class 9 notes in written and pdf format.

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Atoms and molecules class 9 notes pdf overview start from here:-

● Idea of divisibility:

■ Maharishi kanada- He postulated that if we keep dividing the matter we will get smaller and smaller particles. Ultimately we will achieve the smallest particles (parmanu) which may not divided further.

■ Pakudha Kaccayana- He postulated that there are various forms of matter because the particles of matter exist together in combinations.

■ Democritus and Leucippus- They suggested that when we keep on dividing the matter there comes a time when no more division of particles is possible. They called these particles atoms (meaning indivisible)

● Laws of Chemical Combination:

1. Law of conservation of mass- Mass nether be created nor destroyed in a chemical change.

2. Law of Constant Proportion- Elements are always present in definite proportions by mass in a chemical substance.

For example: Hydrogen and oxygen are present in water in a ratio of 1:8. Thus, if 9g of water is decomposed, 1g of hydrogen and 8g of oxygen are always obtained.

■ Dalton Atomic Theory:

John Dalton Proposed a theory in which he said, all matter whether it is an element, a compound or a mixture consist of tiny invisible particles called 'atoms'.

● Postulated of Dalton Atomic Theory:

1. The matter is made up of tiny Particles called atoms that cannot be divided.
2. Atoms are never formed or destroyed during a chemical reaction.
3. Atoms of an element have same nature. They have some size, mass and character.
4. Atoms of different elements exhibit variant nature. They do not have same characteristics.
5. Atoms from compounds by combining in a ratio of whole numbers.
6. A compound contains constant number and kinds of atoms.

■ What is an Atom?

Atoms are the building blocks of matter. Just Like bricks are the building blocks of a building.

● What is the size of an atom?

Atoms are extremely small. More than millions of atoms when stacked would make a layer as thick as sheet of Paper.

Atomic radius is measured in nanometres

1/10⁹ m = 1 nm

1m = 10⁹ nm

● Symbols of Atoms:

Here are some examples of the symbols that are used to represent different atoms.

The Symbols for representing an atom are generated from the first two letters of the elements name. The First letter is always in uppercase (capital) While the Second letter is written in lowercase (small)

For example - Hydrogen H, Aluminium Al not AL, Cobalt Co not CO

Now-a-days, IUPAC [International Union of Pure and Applied chemistry] approves

names of elements

■ The Atomic Mass:

Dalton's Atomic theory suggested that each element has a distinguish atomic mass. Since the size of the atom is very small, it is difficult to calculate it's mass.

Therefore scientists started evaluating the mass of an atom by comparing it with mass of a standard atom.

● Earlier 1/16 of mass of an oxygen atom was used as standard for calculating mass of a standard atom.

"One atomic mass unit is the mass of 1/12 the mass of a carbon - 12 atom."

● How do Atoms exist?

Atoms cannot survive independently. So, atoms join together and form molecules or ions.

■ What is a Molecule?

• A molecule is a collecting of various atoms that combine chemically with each other.
• Atoms are bound together by certain force of attraction.
• Atoms of same elements or different elements can bind together by certain force of attraction.
• Atoms of same elements or different elements can bind together to form molecules.

● Molecules of Elements:

The molecules of an element formed by combinations of similar types of atoms.

Like - Helium (He) is made up one atom while oxygen is made up of two atoms.

● Atomicity -  The number of atoms in a molecule of an element is called it's Atomicity. Like - Helium is monoatomic because he is an atom of a molecule, oxygen is diatomic because of two atoms.

● Molecules of Compounds:

Molecules of Compounds constitute atoms of different elements that combine together in a fixed proportion. For example - Water made up of 2 hydrogen and 1 oxygen.

■ What is an ION?

• Compounds contains metals and non metals. These elements include charged species which are known as ions.
• Ion is a particle that has positive or negative charge.

Anion - negatively charged

Cation - Positively charged

• When a group of atoms carries a charge in a compound it is called polyatomic ion.

How Ion's are made? ➸ will study in ch4.

■ Chemical Formula:

The chemical formula of a compound is a symbolic representation of its composition.

To write a chemical formula, we must know two things - 

1.  Symbols of elements
2.  Valency

●  Valency: It is known as the combining capacity of an Element. In simple words

how atoms of one element will mix with atoms of another element.

For example - Hydrogen ion is represented as H+ which means it's valency is 1.

Similarly oxygen ion is represented as O^2- 

Which means it's valency is 2.

■ Rules of writing a chemical Formula:

• Valencies of on the ions must balance.
• If metal and non metal both are present in a compound, So name of metal is always written first in the chemical formula.

For example - Sodium chloride is written as Nacl. So, sodium is a metal and chlorine is a non metal 

• In case of polyatomic ions, the ion is written in brackets before writing the number of ions associated to it. There is no need of backers, If there is a Single ion.

■ Formula of Simple Compounds:

• Step - 1 write the symbols of corresponding elements of the compounds as explained above.
• Step - 2 write the valencies of the elements of compound.
• Step - 3 Crossover the valencies of elements 

For example:

We know Valency of sodium (Na) is +1 and oxygen (0) is -2, Cross multiply the both elements Valency.

■ Molecular Mass

The sum of the atomic masses of all the atoms in a molecule of the substance.

Molecular mass is expressed in a atomic mass unit (amu)

IUPAC changes amu to u (unified mass u)

 For example: molecular mass of HNO3 can be calculated as: 

Atomic mass of H = 1u

Atomic mass of N = 14u

Atomic mass of O = 16u

Now, molecular mass of HNO3 = 1+14+16×3=63u

■ Formula Unit Mass:

The sum of atomic masses of all atoms in a formula unit of a compound is called formula unit mass.

We calculated it in the same way as molecular mass.

Example: Formula unit mass of Na2O

= 2× atomic mass of Na+1×atomic mass of o

= 2×23u+1×61u=62u

[MOST IMPORTANT]

■ Mole Concept 

●  Mole - Mole is a quantity that has a mass equal to the atomic or molecular mass of species 

The word ''mole'' was introduced in 1896 by Wilhelm Ostward.

1 mole of any substance = 6.022×10^23 particles 

This is called Avogadro number or Avogadro constant which is represented as N0 

Mole is basically a quantity with a fixed value of 6.022×10^23

The mass of 1 mole of a substance is the same as that it's atomic mass or molecular mass in grams. 

● Gram atomic mass - The Atomic mass of a substance when expressed in grams is called gram atomic mass. 

●  Gram molecular mass - The molecular mass of a substance when expressed in grams is known as gram molecular mass.

For example: atomic mass of sulphur is 32u then Gram atomic mass of sulphur is 32g Simply, write the atomic mass in gram.

■ Important Formulas of atoms and molecules class 9 

(i) Number of moles (n) = given mass/molar mass = M/N

(ii) Number of moles (n) = given number of particles/Avogadro's number = N/No 

(iii) m/M = N/No => m= M×N/No 

All the questions that will appear in the exam will come from the three formulas.

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class 9 science chapter 3 extra questions

 If you have done reading your notes and watched a fully explained video of the chapter atoms and molecules class 9. Then, you have to practice some extra and important questions of the chapter

I am  providing you some class 9 science chapter 3 extra questions that will help you alot in your practice time! Basically I collect some important questions from past 8 to 10 years class 9 CBSE exams and combine then in one pdf format.

Because our term 2 is in subjective form so I will provide you very short, short and long answer type questions in the pdf. This pdf contain only extra questions for class 9 science and ncert exemplar class 9 science pdf if you want me to upload some content for maths please comment down.

 

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Ncert exemplar class 9 science pdf 

Ncert Exemplar class 9 science pdf is the best source to practice some questions because term 2 are in subjective form and exemplar provide us best questions in subjective forms.

(Exemplar questions start from here)



Multiple Choice Questions

1. Which of the following correctly represents 360 g of water?

(i) 2 moles of H20

(ii) 20 moles of water

(iii) 6.022 × 1023 molecules of water

(iv) 1.2044×1025 molecules of water

(a) (i) (b) (i) and (iv)

(c) (ii) and (iii) (d) (ii) and (iv)

2. Which of the following statements is not true about an atom?

(a) Atoms are not able to exist independently

(b) Atoms are the basic units from which molecules and ions are formed

(c) Atoms are always neutral in nature

(d) Atoms aggregate in large numbers to form the matter that we can see,

feel or touch

3. The chemical symbol for nitrogen gas is

(a) Ni (b) N2 (c) N+ (d) N

4. The chemical symbol for sodium is

(a) So (b) Sd (c) NA (d) Na

5. Which of the following would weigh the highest?

(a) 0.2 mole of sucrose (C12 H22 O11)

(b) 2 moles of CO2

(c) 2 moles of CaCO3

(d) 10 moles of H2O

6. Which of the following has maximum number of atoms?

(a) 18g of H2O

(b) 18g of O2

(c) 18g of CO2

(d) 18g of CH4

7. Which of the following contains maximum number of molecules?

(a) 1g CO2

(b) 1g N2

(c) 1g H2

(d) 1g CH4

8. Mass of one atom of oxygen is

(a) 16/6.023×10^23 g

(b) 32/6.023×10^23 g

(c) 1/6.023×10^23 g

(d) 8u

9. 3.42 g of sucrose are dissolved in 18g of water in a beaker. The number of

oxygen atoms in the solution are

(a) 6.68 × 1023

(b) 6.09 × 1022

(c) 6.022 × 1023

(d) 6.022 × 1021

10. A change in the physical state can be brought about

(a) only when energy is given to the system

(b) only when energy is taken out from the system

(c) when energy is either given to, or taken out from the system

(d) without any energy change

Short Answer Questions

11. Which of the following represents a correct chemical formula? Name it.

(a) CaCl (b) BiPO4 (c) NaSO4 (d) NaS

12. Write the molecular formulae for the following compounds

(a) Copper (II) bromide

(b) Aluminium (III) nitrate

(c) Calcium (II) phosphate

(d) Iron (III) sulphide

(e) Mercury (II) chloride

(f) Magnesium (II) acetate

13. Write the molecular formulae of all the compounds that can be formed by

the combination of following ions

14. Write the cations and anions present (if any) in the following compounds

(a) CH3COONa

(b) NaCl

(c) H2

(d) NH4NO3

15. Give the formulae of the compounds formed from the following sets of

elements

(a) Calcium and fluorine

(b) Hydrogen and sulphur

(c) Nitrogen and hydrogen

(d) Carbon and chlorine

(e) Sodium and oxygen

(f) Carbon and oxygen

16. Which of the following symbols of elements are incorrect? Give their correct symbols

(a) Cobalt CO

(b) Carbon c

(c) Aluminium AL

(d) Helium He

(e) Sodium So

17. Give the chemical formulae for the following compounds and compute the

ratio by mass of the combining elements in each one of them. (You may

use appendix-III).

(a) Ammonia

(b) Carbon monoxide

(c) Hydrogen chloride

(d) Aluminium fluoride

(e) Magnesium sulphide

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6 Quick Tips Regarding Atoms and Molecules class 9.

• Read your NCERT first 
• Watch a one shot video on YouTube
• Download and revise the notes 5,6 times
• Don't forgot to make mindmaps and flowchart of the chapter 
• Do PYQ (previous year questions) of the chapter
• Do exemplar questions of the chapter

Now you are ready to score well in your exams from this particular chapter.

FAQ (frequently asked questions)

Question

What is an Atom?

Answer:

Atoms are the building blocks of matter. Just Like bricks are the building blocks of a building.

Question

What is the size of an atom?

Answer:

Atoms are extremely small. More than millions of atoms when stacked would make a layer as thick as sheet of Paper.

Question

What is a Molecule?

Answer:

A molecule is a collecting of various atoms that combine chemically with each other.

Atoms are bound together by certain force of attraction.

Atoms of same elements or different elements can bind together by certain force of attraction.

Atoms of same elements or different elements can bind together to form molecules.

Question

What is an ION?

Answer: 

Compounds contains metals and non metals. These elements include charged species which are known as ions.

Ion is a particle that has positive or negative charge.

Question

Who introduced the word mole?

Answer:

The word ''mole'' was introduced in 1896 by Wilhelm Ostward.

Question

What are the formulas in atoms and molecules class 9?

Answer:

(i) Number of moles (n) = given mass/molar mass = M/N

(ii) Number of moles (n) = given number of particles/Avogadro's number = N/No 

(iii) m/M = N/No => m= M×N/No 

Question

Is Ncert exemplar Class 9 good?

Answer:

The answer is Yes! Ncert Exemplar class 9 is good for term 2 preparation for class 9 students because it provide us best and quality Questions that will help you in your exams.

Question

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Answer:

Yes, you can download Ncert exemplar from cbse site and edugyane blog in free of cost!

Question

Who published Ncert exemplar?

Answer:

The national council of educational research and training (NCERT) published the Ncert Exemplar from classes 6 to 12 for cbse Students.

Also read

• Class 9 gravitation notes 
• Class 9 term 2 syllabus of science
• Class 9 term 2 syllabus of maths
• Perfect time table for students 

Conclusion

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