Class 9 Exam > Class 9 Notes > Science Class 9 > Mnemonics: Atoms and Molecules
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1. Historical Milestones of Atomic Theory
Mnemonic: "Kanad’s Kat Demo Look Lavoisier Proves"
- Kanad’s Kat → Kanad & Katyayama (Suggested matter divides into Parmanu)
- Demo Look → Democritus & Leucippus (Proposed indivisible atoms)
- Lavoisier→ Lavoisier (late 18th century) (Established laws of chemical combination)
- Proves → Proust (Conducted experiments to support chemical combination laws)
Mnemonic Explanation: This phrase imagines Kanad’s philosophical ideas leaping forward with Lavoisier and Proust proving them through experiments. It covers the key historical figures and their contributions to atomic theory, from early philosophical ideas to experimental validation.
- Maharishi Kanad and Pakudha Katyayama in India suggested that matter can be divided into smaller particles called Parmanu.
- Democritus and Leucippus in Greece proposed that matter can be divided until it reaches indivisible particles called atoms.
- These ideas were based on philosophical considerations and lacked experimental validation until the eighteenth century.
- In the late 18th century, Antoine L. Lavoisier laid the foundation of chemical sciences by establishing two important laws of chemical combination.
- Lavoisier and Joseph L. Proust conducted numerous experiments to establish the laws of chemical combinations.
- These laws of chemical combination became crucial in understanding the combination and behavior of elements and compounds.
2. Law of Conservation of Mass
Mnemonic: “Mass is a Guest—Never Leaves, Never Arrives”
Mnemonic Explanation: In any reaction, the total mass stays the same—no gain, no loss.
Law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction.
3. Law of Constant Proportion
Mnemonic: “Every Water Drop is an H:O = 1:8 Shop”
Mnemonic Explanation: No matter the source of water, H and O are always in a 1:8 mass ratio.
The law states that in a chemical substance, the elements are always present in definite proportions by mass.
4. John Dalton’s Atomic Theory (Key Postulates)
Mnemonic: “SIMPLE”
S – Smallest particle (atom)
I – Indivisible
M – Mass is same for same element
P – Properties same for same element
L – Locked ratios form compounds
E – Elements don’t change during reactions
Mnemonic Explanation:
- All matter consists of atoms participating in chemical reactions.
- Atoms are indivisible and remain unchanged in reactions.
- Atoms of the same element are identical in mass and properties.
- Atoms of different elements have distinct masses and properties.
- Atoms combine in simple whole-number ratios to form compounds.
- The composition of atoms in a compound is constant.
5. Modern Symbols of Elements
Mnemonic for symbol rules: “First Big, Next Small”
Mnemonic Explanation: The first letter is capital (e.g., H), and if there is a second, it’s lowercase (e.g., He, Na, Cl).
Examples (for tricky ones):
Fe – Ferrum (Iron)
Na – Natrium (Sodium)
K – Kalium (Potassium)
6. Difference Between Atom, Molecule, and Ion
Mnemonic: “Atom single netural and molecule group neutral with always Charged Ion”
Mnemonic Explanation:
Atom: Single neutral particle
Molecule: Neutral group of atoms bonded
Ion: Charged atom or group
7.Writing Chemical Formula (Crossover Method)
Mnemonic: “Swap and Drop”
Mnemonic Explanation: Swap the valencies and drop them as subscripts.
Example: 
8. Molecular Mass
Mnemonic: “Add All Atomic Masses”
Mnemonic Explanation: H2O = 2H + 1O = 2×1 + 16 = 18u
9. Formula Unit Mass
Mnemonic: “Ionic Version of Molecular Mass”
Mnemonic Explanation: Just like molecular mass, but for ionic compounds like NaCl, CaCl2.
10. Difference Between Atomic Mass and Molecular Mass
Mnemonic: “Atom = 1, Molecule = Many”
Mnenomic Explanation:
Atomic Mass = mass of a single atom.
Molecular Mass = sum of all atoms in the molecule.
The document Mnemonics: Atoms and Molecules | Science Class 9 is a part of the Class 9 Course Science Class 9.
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FAQs on Mnemonics: Atoms and Molecules - Science Class 9
| 1. What are the historical milestones of atomic theory? |  |
Ans. The historical milestones of atomic theory include the ancient Greek concept of atoms by Democritus, John Dalton's atomic theory in the early 19th century, J.J. Thomson's discovery of the electron in 1897, Ernest Rutherford's nuclear model of the atom in 1911, Niels Bohr's planetary model in 1913, and the development of quantum mechanics in the 20th century, which refined our understanding of atomic structure.
| 2. What is the Law of Conservation of Mass? | |
Ans. The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. This means that the total mass of the reactants in a chemical reaction equals the total mass of the products. This principle was established by Antoine Lavoisier in the late 18th century and is fundamental to chemistry.
| 3. What is the Law of Constant Proportion? | |
Ans. The Law of Constant Proportion, also known as the Law of Definite Proportions, states that a chemical compound always contains its constituent elements in fixed ratio by mass, regardless of the sample size or source. This principle was outlined by Joseph Proust in the early 19th century.
| 4. What are the key postulates of John Dalton’s Atomic Theory? | |
Ans. John Dalton’s Atomic Theory includes several key postulates: (1) All matter is made of atoms, which are indivisible and indestructible. (2) Atoms of a given element are identical in mass and properties. (3) Compounds are formed by the combination of different types of atoms in fixed ratios. (4) A chemical reaction involves the rearrangement of atoms, not their creation or destruction.
| 5. What is the difference between atomic mass and molecular mass? | |
Ans. Atomic mass is the weighted average mass of an element's isotopes measured in atomic mass units (amu), while molecular mass is the sum of the atomic masses of all atoms in a molecule. Atomic mass refers to single atoms, whereas molecular mass applies to molecules composed of two or more atoms.
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