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CBSE Class 9 · Science

Atomic Foundations of Matter

Official NCERT chapter from Science (Grade 9, 2026 edition) (book code iesc1). ExamMaster notes are original teaching at CBSE Class 9 depth.

This lesson follows the official chapter “Atomic Foundations of Matter” in Science (Grade 9, 2026 edition). The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 9
  • Medium level
  • 8 concepts

1Law of Constant Proportions Law of Definite Proportions

Constant (definite) proportions: a given compound, made cleanly, has the same elements in the same mass ratio every time you make that compound — as the school law. Water is not “any H-and-O mix you like”. The heading is one law said two common ways, not two rival laws.

A kitchen splash of “some hydrogen, some oxygen” is not the compound.

Figure. Water from a well and from rain keep the same mass ratio: hydrogen 1 to oxygen 8. The law of definite proportions is that the ratio does not move with the source.

How it works

  1. Name the compoundThis water, this taught oxide.
  2. Say the mass ratio stays, for that compoundThe law.
  3. Refuse a different mix as the same compoundDefinite.

2How Atoms Combine?

Atoms combine by a taught job: transfer (ionic story) or share (covalent story). Combine is that job, not “they like each other”. One pair (Na with Cl as a school ionic; two O as a school share, if taught) is enough.

A handshake cartoon is not the job.

Figure. Atoms combine in fixed counts: two hydrogen units and one oxygen unit make one water unit. The arrow is the combination, not a drawn bond.

How it works

  1. Name the two atoms as taughtThe pair.
  2. Say transfer or share as the lesson didThe combine.
  3. Keep the job on those twoNo extra romance.

3Writing Chemical Formulae

A chemical formula is a writing of how many of each atom the school compound is treated as: H2O, CO2, NaCl. Writing means valency-cross or the taught count, then a check that the combining numbers settle. H2O is not HO because it looked shorter.

A pretty subscript you guessed is not a formula.

Figure. Criss-cross the valencies: magnesium 2 and chlorine 1 swap to write MgCl2. The crossing is a bookkeeping device, not a picture of the ions.

How it works

  1. Write the symbols and the valencies as taughtThe given.
  2. Cross or fill so the capacities settleThe write.
  3. Read H2O as two H and one OThe formula.

Water’s school write

Hydrogen valency 1, oxygen valency 2 (as taught). Write the formula.

  • CapacitiesH:1, O:2
  • Settletwo H with one O
  • FormulaH2O

Pro tip. The write is a settle of capacities, not a shorter doodle.

4Properties of the Ionic and the Covalent Bond

Ionic versus covalent properties at this class are school contrasts: a taught ionic solid may dissolve and the solution may conduct, as shown; a taught covalent may not, or may melt lower — only the contrasts the lesson ran. Property is a look, not a brand.

Saying “ionic is stronger” as a vibe is not a property.

Figure. Ionic and covalent substances differ by test: melt and conduction, not by a drawn bond. Ionic: high melt, conducts when aqueous. Covalent: low melt, no current.

How it works

  1. Name the bond-type the lesson claimedIonic or covalent.
  2. Name one look (dissolve, conduct, melt) the lesson tied to itThe property.
  3. Keep the contrast humble — this pair, this testNot every compound on Earth.

5Molecular Mass of Covalent Compounds

Molecular mass of a covalent school formula is the sum of the atomic masses you were given: H = 1, O = 16 ⇒ H2O is 18 u. It is a sum, not a new kind of weigh. Use the formula’s counts.

Using 1+16 = 17 for water drops an H.

Figure. Molecular mass of water: 1 + 1 + 16 = 18 u. Each number is an atomic mass unit count, not a new kind of gram.

How it works

  1. Write the formulaH2O, CO2…
  2. Multiply each atomic mass by its countThe parts.
  3. AddMolecular mass.

6Formula Unit Mass of Ionic Compounds

Formula-unit mass is the same kind of sum for an ionic school formula: NaCl as 23 + 35.5 if those were the given masses, or a whole-number stand-in if the lesson used one. The name changes because the solid is a lattice-story, not a single molecule-ride — the arithmetic is still a sum.

Inventing a different arithmetic because the word is “formula unit” is a miss.

Figure. Formula-unit mass of NaCl: 23 + 35.5 = 58.5 u. The unit is the ionic pair, not a covalent molecule.

How it works

  1. Write the ionic formulaNaCl, as taught.
  2. Sum the given atomic masses with the countsThe unit mass.
  3. Keep the word “formula unit” as the lattice-name, not a new calculatorSame sum-job.

7A definition is a test you can run

Compound / formula / proportion is a test: a named ratio or a settled write. If you only say “chemistry”, you have a heading.

A lab-coat photo is not the test.

Figure. Constant composition is a test you can run: two samples are the same compound only when both give the 1:8 hydrogen-to-oxygen mass ratio.

How it works

  1. Name the compound or the two elementsThe object.
  2. Give the ratio, the write, or the sumThe test.
  3. Then the word has contentThe definition ran.

8Name the given before the unknown

The given are the valencies or the atomic masses. The unknown is the formula or the sum. Copy O = 16 before you add.

Using 18 as oxygen because water is 18 is a silent given (that 18 is already the sum).

Figure. Name the given atomic masses before the unknown formula-unit or molecular mass. Hydrogen 1 and oxygen 16 are given; 18 u is what you find.

How it works

  1. Copy symbols, valencies, massesThe given.
  2. Name formula or massThe unknown.
  3. Then cross or addGiven first.
H2O with H = 1, O = 16 has molecular mass
  1. 18 u
  2. 17 u
  3. 16 u

Two H and one O: 2+16.

Notes

  • Mapped to the official NCERT chapter “Atomic Foundations of Matter”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • H2O: 2×1 + 16 = 18 u (given masses)
  • formula from settling valencies

Recap

Hold these pegs from the official chapter “Atomic Foundations of Matter”. The wording is ExamMaster’s teaching, not a textbook recap.

Law of Constant Proportions Law of Definite Proportions
Constant (definite) proportions: a given compound, made cleanly, has the same elements in the same mass ratio every time you make that compound — as the school law.
How Atoms Combine?
Atoms combine by a taught job: transfer (ionic story) or share (covalent story).
Writing Chemical Formulae
A chemical formula is a writing of how many of each atom the school compound is treated as: H2O, CO2, NaCl.
Properties of the Ionic and the Covalent Bond
Ionic versus covalent properties at this class are school contrasts: a taught ionic solid may dissolve and the solution may conduct, as shown; a taught covalent may not, or may melt lower — only the contrasts the lesson…
Molecular Mass of Covalent Compounds
Molecular mass of a covalent school formula is the sum of the atomic masses you were given: H = 1, O = 16 ⇒ H2O is 18 u.
Formula Unit Mass of Ionic Compounds
Formula-unit mass is the same kind of sum for an ionic school formula: NaCl as 23 + 35.5 if those were the given masses, or a whole-number stand-in if the lesson used one.

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