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Class IX Science
Notes for Atoms and Molecules
Notes for Atoms and Molecules
Syllabus
Particle nature, Basic units, Atoms and molecules, Law of constant proportions, Atomic and molecular masses.
Facts that Matter
• Law of Chemical Combination
Given by Lavoisier and Joseph L. Proust as follows:
(i) Law of conservation of mass: Mass can neither be created nor destroyed in a chemical reaction.
e.g.,A B-C D
Reactants R Products
Mass of reactants = Mass of products
(ii) Law of constant proportion: In a chemical substance the elements are always present in definite proportions by mass.
E.g., in water, the ratio of the mass of hydrogen to the mass of oxygen is always 1 : 8 respectively.
These laws lacked explanation. Hence, John Dalton gave his theory about the matter. He said that the smallest particle of matter is called ‘atom’.
• Dalton’s Atomic Theory
1. Every matter is made up of very small or tiny particles called atoms.
2. Atoms are not divisible and cannot be created or destroyed in a chemical reaction.
3. All atoms of a given element it are same in size, “mass and chemical properties.
4. Atoms of different elements are different in size, mass and chemical properties.
5. Atoms combine in the ratio of small whole number to form compounds.
6. The relative number and kinds of atoms are constant in a given compound. 9
• Atom
Atoms are the smallest particles of an element which can take reaction.
Size of an atom: Atomic radius is measured in nanometres.
Atomic radii of hydrogen atom = 1 × 10–10m.
Symbols of atoms:
(a) Symbols for some elements as proposed by Dalton:
(b) Symbols of some common elements:
| Name of the element | Latin name | Symbol | |
| Hydrogen | � | H | |
| Helium | � | He | |
| Carobon | � | C | |
| Copper | Cuprum | Cu | |
| Cobalt | Co | ||
| Chlorine | Cl | ||
| Cadmium | Cd | ||
| Boron | B | ||
| Barium | Ba | ||
| Bromine | Br | ||
| Bismuth | Bi | ||
| Sodium | Natrium | Na | |
| Potassium | Kalium | K | |
| Iron | Ferrum | Fe | |
| Gold | Aurum | Au | |
| Silver | Argentum | Ag | |
| Mercury | Hydragyrum | Hg |
• Molecule
It is the smallest particle of an eleme4It- or a dolnpound which can wxist independently.
• Molecules of an element constitutes same type of atoms. 1.w
• Molecules may be monoatomic, di-atomic or polyatomic. IT .
• Molecules of compounds join together in defmite proportionsrand constitutes different type of atoms.
• Atomicity
The number of atoms constituting a Molecule is known as its atomicity.
| Name of the element | Atomicity | Molecules formula | |
| Helium | Monoatomic | He | |
| Neon | Monoatomic | Ne | |
| Argon | Monoatomic | Ar | |
| Sodium | Monoatomic | Na | |
| Iron | Monoatomic | Fe | |
| Aluminium | Monoatomic | Al | |
| Hydrogen | Di-atomic | H2 | |
| Oxygen | Di-atomic | O2 | |
| Chlorine | Di-atomic | Cl2 | |
| Nitrogen | Di-atomic | N2 | |
| Phosphorus | Polyatomic (Tetra) | P4 | |
| Sulphur | Polyatomic (Octa) | S8 |
• Ions
The charged particles (atoms) are called ions, they charge or negative charge on it:
Negatively charged ionis called anion (C1�).
Positively charge ion is called cation (Na ).
• Valency
The combining capacity of an element is known as its valency: Valency is used to fmd out how atom of an element will combine with the atom of another element to form a chemical compound.
(Every atom want, to become stable, to do so it may loose, gain or share electrongs.
(i) If an atom consists of 1, 2 or 3 electrons in its valgncesI ell then its valency is 1, 2 or 3 respectively,
(ii) If an atom consists of 5, 6 or 7 electrons in the outermost shell, then it will gain 3, 2 or 1 electron respectively and its valency will be 3, 2 or 1 respectively.
(ii) If an atom has 4 electrons in the outermost shell than it will she this electron and hence its valency will be 4.
(iv) If an atom has 8 electrons in the outermost shell then its valency is 0.• Chemical Formulae
Rules: (i) The valencies or charges on the ion must balance.
(ii) A metal and non-metal compound should show the name or symbols of the metal first.
e.g., Na Cl– → NaCl
(iii) If a compound consist of polyatomic ions. The ion before writing the number to indicate the ratio.
e.g., [SO4]2– → polyatomic radical
H1 SO42– → H2SO4.
Chemical formula of some simple compounds
(a) Calcium hydroxid
(b) Aluminium oxide
• Molecular Mass
It is the sum of the atomic masses of all the atoms in a molecule of the substance. It is expressed in atomic mass unit (u).
e.g., 2H O2 H2O [H = 1, 0 = 16]
1 × 2 16 = 18 u
• Formula Unit Mass
It is the sum of the atomic masses of all atoms in a formula unit of a compound. The constituent particles are ions.
e.g., Na Cl– → NaCl
1 × 23 1 × 35.5 = 58.5 u
• Mole Concept
Definition of mole: It is defined as one mole of any species (atom, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecular mass in grams.
1 mole = 6.022 × 1023 in number
Molar mass = mass of 1 mole → is is always expressed in r gram, and is also known as gram atomic mass.
1u of hydrogen has → 1 atom of hydrogen
lg of hydrogen has → 1 mole of hydrogen
= 6.022 × 1023 atoms of hydrogens
FAQs on Class 9 chapter 3 atoms and molecules notes
| 1. What are atoms and molecules? |  |
Ans. Atoms are the basic building blocks of matter, consisting of a nucleus (containing protons and neutrons) surrounded by electrons. Molecules, on the other hand, are formed when two or more atoms chemically combine together.
| 2. How do atoms and molecules differ from each other? | |
Ans. Atoms are individual particles that cannot exist independently, while molecules are formed when atoms chemically bond together. Molecules have specific properties and can exist independently.
| 3. Can atoms be broken down into smaller particles? | |
Ans. Yes, atoms can be further broken down into smaller particles called subatomic particles. These include protons, neutrons, and electrons, which are the fundamental constituents of atoms.
| 4. What is the role of atoms and molecules in chemical reactions? | |
Ans. Atoms and molecules play a crucial role in chemical reactions. During a chemical reaction, atoms rearrange their bonds to form new molecules with different properties. This rearrangement is responsible for the formation of new substances.
| 5. Why is the study of atoms and molecules important? | |
Ans. The study of atoms and molecules is important because it helps us understand the fundamental nature of matter and its behavior. It forms the basis of various scientific disciplines such as chemistry and physics, enabling advancements in fields like medicine, materials science, and environmental studies.
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