RBSE Class 9 Science Notes Chapter 3 Atoms and Molecules

These comprehensive RBSE Class 9 Science Notes Chapter 3 Atoms and Molecules will give a brief overview of all the concepts.

RBSE Class 9 Science Chapter 3 Notes Atoms and Molecules

→ According to Indian philosopher Maharshi Kanad, if we go on dividing matter into smaller parts, we shall get smaller particles and lastly a stage would be reached, beyond which further division of particles will not be possible. He called these particles “Parmanu”, known as “Atoms” nowadays.

→ Laws of Chemical Combination : A compound is formed by chemical combination of different elements, which is basically governed by two following laws :

(i) Law of conservation of mass
(ii) Law of constant proportions

→ Law of Conservation of Mass : Antoine Lavoisier gave this law in 1774. According to which,
“Whenever a physical or chemical change takes place, the total mass of the reactants is equal to the total mass of the products of the reaction.”

→ Law of Constant Proportion : It is also called law of definite proportion. This law. was given by chemist Joseph Proust.

This law states that every pure compound always contains the same elements combined in the same proportion by mass.

→ Atoms : All the matter is made up of atoms. An atom is the smallest particle of an element which cannot be further divided and that retains all the characteristic properties of that element.

→ Size of an atom is normally indicated by its radius. Hydrogen is the smallest atom and has the radius of the order of 0.37 × 10^-10m or 37 pm (1 pm = 10^-12 m).

→ Dalton’s Atomic Theory : The following are the postulates of Dalton’s atomic theory :

• All forms of matter are made up of very small discrete particles called atoms.
• An atom cannot be further divided into smaller particles.
• Atoms can neither be created nor destroyed in the chemical process.
• Atoms of a particular element are identical in all respects:
• The atoms of one element are different from those of any other element.
• Atoms of different elements may combine in simple whole number ratio to form compounds.

→ Drawbacks of Dalton’s Atomic Theory :

• The assumption that atom is indivisible is no longer true.
• Atoms of same element may not be identical in all respect (e.g. isotopes).
• The atoms of different elements may be same in some respect (isobar).
• Atom is no longer indestructible.
• It fails to explain how and why different elements combine with each other to form compounds.

→ Berzelius Atomic Symbols : J. J. Berzelius proposed in 1813, chemical symbols of the elements should be based on their Latin or Greek names.

Table : Symbols of some elements

→ Atomic Mass : Atomic mass of an element is the mass of an atom expressed in atomic mass unit. It is compared with mass of an atom of carbon (C-12 isotope) taken as 12 amu.

→ Atomic Mass Unit : Atomic mass unit (u) is equal to \(\frac{1}{12}\)th the mass of a carbon-12 atom.

1 atomic mass unit = \(\frac{\text { Mass of C-12 atom }}{12}\)

→ Relative Atomic Mass : Relative atomic mass is the average mass of the atoms of an element as compared to \(\frac{1}{12}\)th the mass of one carbon-12 atom.

→ Gram Atomic Mass : When atomic mass is expressed in gram is called gram atomic mass.

→ Molecules : A molecule is the smallest particle of an element or compound that has independent existence.
The molecules of elements contain one kind of atoms only.
The molecules of a compound contain two or more different types of atoms chemically bonded together.

→ Atoms and Molecules Molecular mass/weight : The molecular mass of a substance (an element or a compound) is the mass of one molecule of that substance as compared with 1/12th mass of an atom of carbon (C-12).

→ Ions : An ion is a charged particle that can be positive or negative in charge. These are two type of ions : (i) cations (ii) anions.

(i) Cations : A positively charged ion is known as cation. For example : Sodium ion-Na⁺, Magnesium ion-Mg²⁺.
(ii) Anions : A negatively charged ion is known as anion. For example : Cl^– (chloride ion), O^-2 (oxide ion), etc.

→ Chemical Formula of a Compound : The chemical formula is a symbolic representation of its composition. Chemical formula of a compound describes the composition of a molecule of the compound in terms of the symbols and the number of atoms of each element.

Mole Concept :

→ Mole : Mole is the quantity in number having a mass equal to its atomic mass in gram.

1 mole of atoms = 6.022 × 10²³ atoms
1 mole of molecules = 6.022 × 10²³ molecules
Mole concept can be summarised as :

→ Molar Mass : Mass of 1 mole molecule of any substance is called its molar mass.