Atoms and Molecules Class 9 Notes CBSE Science Chapter 3 (Free PDF Download)

Law of Conservation of Mass:

• In a chemical action, the law of conservation of mass stipulates that mass can not be created or destroyed.

• According to this law, the overall mass of the products remains equal to the total mass of the reactants after any physical or chemical change.

Law of Constant roportion:

• This law was expressed by another French chemist, Joseph Proust, as follows: "A chemical compound always comprises the same elements mixed in the same proportion by mass."

Law of Multiple Proportions:

• As established by John Dalton, when two elements combine to form two or more compounds, the mass of the element that combines with the fixed mass of the other bears a simple whole-number ratio $\left( \text{1803} \right)$.

Dalton’s Atomic Theory:

• According to Dalton's atomic theory, all matter, whether an element, a compound, or a mixture, is made up of microscopic particles called atoms.

• This theory's postulates are as follows::

1. All matter is made up of atoms, which are very small tiny particles that engage in chemical reactions.

2. In a chemical reaction, atoms are indivisible particles that cannot be formed or destroyed.

3. A given element's atoms have the same mass and chemical characteristics.

4. The masses and chemical characteristics of atoms of various elements differ.

5. Compounds are formed when atoms join in a ratio of tiny whole numbers.

6. In a given compound, the number and types of atoms remain constant.

Atom:

• An atom is an element's defining structure that can't be broken chemically.

• The electron, proton, and neutron are the three particles that make up an atom.

• An atom's nucleus holds the entire mass of the atom.

• An atom's electrons are grouped in shells/orbitals.

• The atomic symbol is made up of three parts: 

The symbol $\text{X}$ - standard element symbol; 

The atomic number $\text{A}$- represents the number of protons; 

The mass number $\text{Z}$ - represents the total amount of protons and neutrons in an element.

• The radius of an atom is measured in nanometres.

Atomic Mass:

• The atomic mass was proposed by Dalton as an atomic hypothesis.

• The average mass of an atom, or a set of atoms, is the sum of the masses of the electrons, neutrons, and protons.

• The atomic mass is the mass of an atomic particle.

• This is often stated in terms of a unified atomic mass unit, as per the international agreement (AMU).

• The average mass of one atom of an element, as compared to $\dfrac{1}{12}$ th the mass of one carbon- $12$ atom, is called atomic mass.

Valency:

• The electrons in the atom's outermost orbit are referred to as valence electrons.

• The valency of an atom is determined by its ability to lose, gain, or share valence electrons in order to complete its octet.

Molecule:

• The total masses of the elements present in a molecule is the molecule's molecular mass.

• The atomic mass of an element is multiplied by the number of atoms in the molecule, and the masses of all the elements in the molecule are added to get the molecule's mass.

• The number of atoms in a single molecule of an element is known as its atomicity.

• For example, each of the molecules of hydrogen, nitrogen, oxygen, chlorine, iodine, and bromine has two atoms, and hence they all have two atomicities each.

Compound:

• When two or more elements join chemically in a defined mass ratio, the result is known as a compound.

• Compounds are substances made up of two or more different types of atoms in a specific ratio.

Ions:

• An ion is an atom or molecule with a net positive or negative charge due to the gain or loss of one or more of its valence electrons.

• A negatively charged particle is an anion, and a positively charged particle is a cation.

• Ionic compounds are chemical compounds in which ions are held together by ionic bonds, which are a type of specialised bond.

• The positive and negative charges in an ionic substance are always in equal amounts.

Molecular Mass:

• The total masses of the elements present in a molecule is known as the molecule's molecular mass.

• The atomic mass of an element is multiplied by the number of atoms in the molecule, and then the masses of all the elements in the molecule are added.

Mole and Avogadro Number:

• A mole is the number of entities existing in a substance, such as atoms, molecules, and ions.

• A mole is \[\text{6}\text{.022 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{23}}}\] molecules of any substance.

• One of the most practical ways of describing the number of reactants and products in a reaction is to use the mole idea.

• Avogadro's number has a value of about \[\text{6}\text{.022 }\!\!\times\!\!\text{ 1}{{\text{0}}^{\text{23}}}\].

• Avogadro's number is a formula that calculates the number of particles in one mole (or mol) of a substance.

• It's possible that these particles are electrons, molecules, or atoms.

• No. of Moles can be calculated as;

$\text{1 Mole = }\dfrac{\text{Mass of a substance}}{\text{Gram atomic mass}}$

$\text{n = }\dfrac{\text{Molecular Weight}}{\text{Empirical Formula Weight}}$

$\text{n = }\dfrac{\text{Given No}\text{. of particles}}{\text{Avogadro Number}}$

$\text{n = }\dfrac{\text{N}}{{{\text{N}}_{A}}}$

Some Important formulae:

• \[\text{No}\text{. of Molecules = }\dfrac{\text{Given Mass}}{\text{Molar Mass}}\text{  }\!\!\times\!\!\text{  Avogadro Number}\]

Hence,

$\text{N = }\dfrac{\text{m}}{\text{M}}\text{  }\!\!\times\!\!\text{  }{{\text{N}}_{\text{A}}}$

• $\text{1 Mole = }\dfrac{\text{Mass of a substance}}{\text{Gram atomic mass}}$

• $\text{Mass of a substance = }\dfrac{\text{Molar Mass of a substance}}{\text{No}\text{. of Moles}}$

• $\text{Percentage composition of an Element = }\dfrac{\text{Total weight of element in a molecule}}{\text{Gram Molecular Weight}}\times 100$

• $\text{RMM = }\dfrac{\text{Mass of one molecule of the substance}}{\left( \dfrac{\text{1}}{\text{12}} \right)\text{ Mass of the atom of Carbon }\left( {{\text{C}}^{\text{12}}} \right)}$

• $\text{Gram Molecular Volume = }\dfrac{\text{Gram Molecular Weight}}{\text{Weight / Volume of gas at STP}}$

CBSE Class 9 Science Notes Chapter 3 Atoms And Molecules

What is an Atom in Chapter 3 Science Class 9 Notes?

An atom is the smallest constituent of an element, which cannot be broken down further by any chemical reaction. The atomic number ‘A’ of an element is the number of protons present in it. The letter ‘Z’ represents the mass number of an element that is equal to the total number of neutrons and protons of the element. There are specific symbols to represent the mass number and the atomic number of an element. 

What Does Daltonic Theory Define in Class 9 Chemistry Chapter 3 Notes?

Class 9th science chapter 3 notes define Daltonic theory in details – 

• Very small particles called atoms constitute all matter 

• There is no creation or destruction of atoms 

• In a particular element, all atoms are identical in both physical and chemical properties

• Atoms of different elements vary in both physical and chemical properties 

• In chemical combination, atoms of various elements undergo combination in order to form compounds.

What are Atomic Mass and Atomic Mass Unit in Class 9 Science Notes Chapter 3?

The total mass of a single atom amounts to Atomic Mass. It is an approximate equivalent to the number of neutrons and protons present. The measurement of atomic mass is represented through the Atomic Mass Unit (AMU). It is also known as Daltons, D. 

What is a Molecular Mass and Molar Mass in Class 9 Ch 3 Science Notes?

The aggregate of all atomic masses contained in a molecule is held to be molecular mass. On the other hand, molar mass pertains to the weightage of one sample mole.

The chapter 3 science class 9 notes pdf explains the concept of substance which has mass and occupies space. Here a molecular weight is the sum of the total mass in grams and atoms. They form a molecule per mole in an element.

One can check Vedantu, which is a reliable education portal offering multiple benefits. They provide notes of chapter 3 science class 9 and exercises. Moreover, students can check their live classes and training sessions available for a budget-friendly price. To revise from top-notch atoms and molecules class 9 notes, download the official app today!

The students can revise the chapter on Atoms and Molecules from this article. They are also provided with a free-to-download pdf which will help the students to retain this till the exam and study this anytime.

The CBSE Chapter 3 revision notes help the students do a comprehensive revision and grasp all the important topics of this chapter - Atoms and Molecules.

Key Takeaways of NCERT Revision Notes Class 9 Science Chapter 3 - Atoms and Molecules Free PDF

The Atoms and Molecules class 9 questions with solutions allow the students to take several benefits. They are:

• Vedantu's revision notes are designed to provide you with the most essential information in a concise format. They focus on key concepts and formulas, ensuring you save time and focus on what truly matters.

• These notes are prepared by expert educators who understand the challenges students face. The content is simplified, making complex topics easier to understand and free to download offline.

• Visual aids, diagrams, and charts are often included in these notes to aid in better understanding.Vedantu's revision notes often include practice questions and examples. This helps you apply what you've learned and solidify your understanding

• By using these notes, you save time on extensive note-making. This extra time can be invested in problem-solving, revision, and mock tests.

• Vedantu often tailors revision notes to suit different types of learners, ensuring you have a resource that aligns with your unique study style.

Conclusion

Vedantu provides you with a comprehensive understanding of the chapter Atoms and Molecules, along with answers to common questions. Here you will learn about the building blocks of matter, the structure of atoms, and how they combine to form molecules. Our revision notes cover all the key concepts and definitions that you need to excel in your exams. You can easily access these valuable resources in downloadable PDF format which is FREE. Vedantu's Class 9 Science Chapter 3 notes are a reliable companion for academic success, and we are here to help you every step of the way.