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Political Science NCERT Solutions Class 9 Science Chapter 3 – Atoms and Molecules
Class 9 Science
Last Updated: 22 July 2026
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Important Points
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Frequently Asked Questions (FAQ)
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Tough Words (Glossary)
Divisibility
Meaning: Ability to be divided into smaller parts
अर्थ: विभाज्यता
"Ancient thinkers debated the divisibility of matter."
Philosopher
Meaning: A thinker who studies fundamental questions
अर्थ: दार्शनिक
"Greek philosophers proposed early atomic ideas."
Postulate
Meaning: To suggest or assume as a basis for reasoning
अर्थ: प्रतिपादन करना / मान लेना
"Kanad postulated the existence of Parmanu."
Doctrine
Meaning: A set of beliefs taught or held
अर्थ: सिद्धांत / मत
"Katyayama elaborated the doctrine of tiny particles."
Parmanu
Meaning: Smallest indivisible particle (ancient Indian term)
अर्थ: परमाणु (प्राचीन भारतीय संकल्पना)
"Kanad named the smallest particle Parmanu."
Atom
Meaning: Smallest unit of an element retaining its properties
अर्थ: परमाणु
"An atom is the building block of matter."
Indivisible
Meaning: Not able to be divided
अर्थ: अविभाज्य
"Democritus called atoms indivisible."
Validate
Meaning: To confirm with evidence
अर्थ: सत्यापित करना
"The ideas could not be validated until experiments improved."
Element
Meaning: Pure substance made of one kind of atom
अर्थ: तत्व
"Oxygen is an element."
Compound
Meaning: Substance formed by chemical combination of elements
अर्थ: यौगिक
"Water is a compound of hydrogen and oxygen."
Law of Conservation of Mass
Meaning: Mass is neither created nor destroyed in a chemical reaction
अर्थ: द्रव्यमान संरक्षण का नियम
"Burning follows the law of conservation of mass."
Law of Constant Proportions
Meaning: Elements in a compound are present in fixed mass ratios
अर्थ: नियत अनुपात का नियम
"Water always has hydrogen and oxygen in 1:8 by mass."
Definite
Meaning: Clearly fixed and not changing
अर्थ: निश्चित
"Compounds have definite composition."
Proportion
Meaning: A comparative relation in size or amount
अर्थ: अनुपात
"The proportion 14:3 is fixed in ammonia."
Atomic Theory
Meaning: Theory describing atoms as building blocks of matter
अर्थ: परमाणु सिद्धांत
"Dalton’s atomic theory explained the laws of combination."
Postulates
Meaning: Statements assumed to be true as a basis of theory
अर्थ: उपपत्तियाँ / प्रतिज्ञापन
"Learn the postulates of Dalton’s theory."
Weaver
Meaning: A person who weaves cloth
अर्थ: बुनकर
"Dalton was born in a weaver’s family."
Turning Point
Meaning: A decisive change in a situation
अर्थ: टर्निंग पॉइंट / मोड़
"The atomic theory was a turning point in chemistry."
Symbol (Chemical)
Meaning: One- or two-letter code for an element
अर्थ: रासायनिक चिह्न
"Fe is the symbol for iron."
IUPAC
Meaning: Body that standardises chemical names and symbols
अर्थ: आईयूपीएसी (रासायनिक नामकरण संस्था)
"IUPAC approves element names."
Uppercase
Meaning: Capital letter form
अर्थ: बड़े अक्षर
"The first letter of a symbol is uppercase."
Lowercase
Meaning: Small letter form
अर्थ: छोटे अक्षर
"The second letter, if present, is lowercase."
Atomic Mass
Meaning: Mass of an atom relative to carbon-12
अर्थ: परमाणु द्रव्यमान
"Chlorine has an atomic mass of about 35.5 u."
Atomic Mass Unit (u)
Meaning: 1/12 of the mass of a carbon-12 atom
अर्थ: परमाणु द्रव्यमान इकाई (u)
"Oxygen’s mass is expressed in atomic mass units."
Isotope
Meaning: Atoms of the same element with different masses
अर्थ: समस्थानिक
"Carbon-12 is the standard isotope for atomic masses."
Relative
Meaning: Considered in relation to something else
अर्थ: सापेक्ष
"Atomic masses are relative to carbon-12."
Aggregate
Meaning: To collect into a group or mass
अर्थ: एकत्र होना / समूह बनना
"Ions aggregate to form solids."
Molecule
Meaning: Two or more atoms chemically bonded
अर्थ: अणु
"An oxygen molecule contains two atoms."
Diatomic
Meaning: A molecule consisting of two atoms
अर्थ: द्विपरमाणुक
"O2 is a diatomic molecule."
Ozone
Meaning: A form of oxygen with three atoms (O3)
अर्थ: ओज़ोन
"Ozone protects us from UV radiation."
Atomicity
Meaning: Number of atoms in a molecule
अर्थ: परमाणुता
"The atomicity of ozone is three."
Ion
Meaning: A charged atom or group of atoms
अर्थ: आयन
"Salt contains positive and negative ions."
Cation
Meaning: A positively charged ion
अर्थ: धनायन
"Na+ is a common cation."
Anion
Meaning: A negatively charged ion
अर्थ: ऋणायन
"Cl− is a common anion."
Polyatomic Ion
Meaning: Charged group of several atoms
अर्थ: बहुपरमाणुक आयन
"Sulfate (SO4)2− is a polyatomic ion."
Valency
Meaning: Combining capacity of an element
अर्थ: संयोजकता
"Oxygen has a valency of two."
Chemical Formula
Meaning: Symbolic representation of a compound
अर्थ: रासायनिक सूत्र
"NaCl is the chemical formula of common salt."
Crossover Method
Meaning: Method of balancing valencies to write formulae
अर्थ: क्रॉसओवर विधि
"Use the crossover method to write MgCl2."
Binary Compound
Meaning: Compound of two different elements
अर्थ: द्विघटकी यौगिक
"HCl is a binary compound."
Formula Unit
Meaning: Simplest ratio of ions in an ionic compound
अर्थ: सूत्रक इकाई
"NaCl is a formula unit, not a molecule."
Molecular Mass
Meaning: Sum of atomic masses in a molecule
अर्थ: आणविक द्रव्यमान
"The molecular mass of CO2 is 44 u."
Formula Unit Mass
Meaning: Sum of atomic masses in an ionic formula unit
अर्थ: सूत्रक इकाई द्रव्यमान
"NaCl has a formula unit mass of 58.5 u."
Independently
Meaning: Without help; on its own
अर्थ: स्वतंत्र रूप से
"Most atoms do not exist independently."
Macroscopic
Meaning: Large enough to be seen with the naked eye
अर्थ: सामान्य आँखों से दिखने योग्य
"We study macroscopic properties of matter in labs."
Microscopic
Meaning: Too small to be seen with the naked eye
अर्थ: सूक्ष्म
"Atoms are microscopic entities."
Magnitude (Order of)
Meaning: Approximate size or scale of a quantity
अर्थ: आदेश / परिमाण का स्तर
"Millions of atoms fit across a paper’s thickness—an order-of-magnitude idea."
Latin Name
Meaning: Name derived from Latin language
अर्थ: लैटिन नाम
"Fe comes from the Latin name ‘ferrum’."
Allotrope
Meaning: Different structural forms of the same element
अर्थ: समरूपी रूप / अलॉट्रॉप
"Oxygen and ozone are allotropes."
Notation
Meaning: A system of symbols used to represent things
अर्थ: लेखन-पद्धति / संकेतन
"Chemical notation uses symbols like H, O, and Na."
Mass Ratio
Meaning: Proportion of masses of elements in a compound
अर्थ: द्रव्यमान अनुपात
"The mass ratio of H:O in water is 1:8."