Atoms and Molecules Where invisible particles create visible wonders

Philosophical Roots of the Atom

Parmanu / Atom

The ultimate, indivisible unit of matter proposed by early Indian and Greek philosophers.

Key Characteristics:

  • c. 500 BC – Maharishi Kanad: endless division ends at Parmanu.
  • Democritus & Leucippus called similar particles “atomos”, meaning indivisible.
  • Concepts were philosophical; no experimental proof existed until the 18th century.

Example:

These early notions inspired later scientists like Dalton to develop a testable atomic theory.

Law of Conservation of Mass

Conservation of Mass

During a chemical reaction, total mass remains constant; matter is neither created nor destroyed, so mass before and after reaction is equal in a closed system.

Key Characteristics:

  • Mass of reactants equals mass of products.
  • Balance readings stay unchanged before & after reaction.
  • Requires a sealed container to prevent loss or gain of matter.

Example:

Mixing barium chloride and sodium sulphate solutions in a corked flask shows identical total mass on the balance before and after mixing.

Multiple Choice Question

Question

In a sealed container, 5 g of calcium reacts with 2 g of oxygen. Using the mass data and the law of conservation of mass, what should be the combined mass of products?

1
6 g
2
5 g
3
7 g
4
3 g

Hint:

Total mass stays constant in a closed system: add the masses of all reactants.

Law of Constant Proportions

Fixed Mass Ratio Rule

In every pure compound, constituent elements combine in the same definite proportion by mass, independent of source or preparation.

Key Characteristics:

  • Holds true for every sample of a given compound.
  • Elemental mass ratio remains constant (definite proportions).
  • Provides the foundation for consistent chemical formulae.

Example:

Water: H : O = 1 : 8  |  Ammonia: N : H = 14 : 3 — ratios never vary.

Dalton’s Atomic Theory

Building on the laws of conservation of mass and constant proportions, Dalton explained chemical combination with six clear postulates.

1

Matter is Atomic

All matter consists of minute, discrete particles called atoms.

2

Indivisible in Reactions

Atoms are neither created nor destroyed during chemical reactions; they only rearrange.

3

Atoms of an Element are Identical

Every atom of the same element has identical mass and chemical properties.

4

Different Elements, Different Atoms

Atoms of distinct elements differ in mass and other properties.

5

Whole-Number Ratios

Atoms combine in simple, small, whole-number ratios to form compounds.

6

Fixed Composition

In any compound, the relative number and kinds of atoms remain constant.

Pro Tip:

Indivisible atoms keep total mass conserved (law of conservation of mass), while fixed whole-number ratios secure constant proportions in every compound.

How Small is an Atom?

AFM image of silicon surface with scale bar comparing atom, molecule, grain of sand

Atomic-force microscope image of silicon; scale bar highlights the nanometre range.

The Nanometre Scale

A nanometre (nm) is one-billionth of a metre: \(1\text{ nm}=1\times10^{-9}\text{ m}\).

About a million atoms stacked would form a sheet as thin as this page.

Key Points:

  • Hydrogen atom ≈ \(1\times10^{-10}\text{ m}\) (0.1 nm)
  • Water molecule ≈ \(1\times10^{-9}\text{ m}\) (1 nm)
  • Grain of sand ≈ \(1\times10^{-4}\text{ m}\) (0.1 mm)

Modern Symbols of Elements

Chemical Symbol

IUPAC-approved one- or two-letter abbreviation for an element. First letter is uppercase; second, if present, lowercase.

Key Characteristics:

  • Single-letter symbols for certain elements, e.g., H, O, C.
  • Two-letter symbols echo English names, e.g., Al, Co.
  • Latin-derived symbols: Na (natrium), Fe (ferrum), K (kalium).

Example:

Symbol set: H, Al, Co, Na, Fe, K demonstrates naming rules.

Match the Element to Its Symbol

Drag Hydrogen, Carbon, Sodium, Chlorine and Iron onto their matching symbols — H, C, Na, Cl and Fe.

Draggable Items

Hydrogen
Carbon
Sodium
Chlorine
Iron

Drop Zones

H

C

Na

Cl

Fe

Atomic Mass Unit (u)

\[1 \text{ \,u } = \frac{1}{12}\, m_{^{12}\mathrm{C}}\]

Variable Definitions

u unified atomic mass unit
m^{12}C mass of one carbon-12 atom
^{12}C carbon-12 isotope (6 p, 6 n)

Applications

Why Carbon-12?

Stable, abundant isotope; gives most relative atomic masses near whole numbers.

Sample Relative Masses

H ≈ 1 u | O ≈ 16 u | Na ≈ 23 u

Use in Calculations

Forms the base for molar masses and stoichiometric predictions in chemistry.

Source: NCERT Science 9, Chapter 3

Molecules and Atomicity

Atomicity

A molecule is the smallest particle able to exist alone. The number of atoms it holds is called its atomicity.

Key Characteristics:

  • Monoatomic – single atom molecules; He, Ar
  • Diatomic – two atoms bonded; O₂, N₂, Cl₂
  • Polyatomic – three or more atoms; P₄, S₈

Example:

He (atomicity 1), O₂ (atomicity 2) and P₄ (atomicity 4) illustrate increasing atomicity.

Ions and Valency

Ion

An ion is an atom or group of atoms that carries a net charge after losing or gaining electrons.

Key Characteristics:

  • Cations (+) form by electron loss, e.g., Na\(^{+}\), Ca\(^{2+}\).
  • Anions (−) form by electron gain, e.g., Cl\(^{-}\), O\(^{2-}\).
  • Polyatomic ions act as single units, e.g., \( \text{SO}_4^{2-} \), \( \text{NH}_4^{+} \).
  • Valency is the combining capacity; for simple ions it equals the magnitude of charge.
  • Knowing valency helps predict chemical formulae.

Example:

Na has valency 1 and O has valency 2, so they combine as \( \text{Na}_2\text{O} \).

Rules for Writing Chemical Formulae

Use valency to build neutral formulae for binary and polyatomic compounds.

1

List ions & valencies

For a binary compound, write the cation first and anion next with their valencies, e.g. \( \text{Mg}^{2+} \) and \( \text{Cl}^{-} \).

2

Apply cross-over rule

Cross the valency numbers to subscripts, reduce to the smallest ratio: \( \text{Mg}^{2+} \) with \( \text{Cl}^{-} \) gives \( \text{MgCl}_2 \).

3

Use brackets for polyatomic ions

If more than one polyatomic ion is needed, enclose it in brackets before adding the subscript: \( \text{Ca}^{2+} \) and \( \text{OH}^{-} \) → \( \text{Ca(OH)}_2 \).

Pro Tip:

Always verify that total positive and negative charges cancel; the final chemical formula must be electrically neutral.

Build the Formula!

Drag ions, balance charges and build Na₂O, Al₂O₃ and CaCO₃ to master chemical formula writing.

Draggable Items

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Drop Zones

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Tip:

Ensure total positive charge equals total negative charge for a neutral compound.

Key Takeaways

Atoms, Molecules & Chemical Combination

Law of Conservation of Mass

Total mass of reactants equals total mass of products in every reaction.

Law of Constant Proportions

A compound always contains the same elements in a fixed mass ratio.

Dalton’s Atomic Theory

Indivisible atoms combine in simple whole-number ratios, explaining both laws.

Atomic Mass Unit (u)

Atom masses are measured relative to 1/12 of a carbon-12 atom.

Molecules & Ions

Atoms join as neutral molecules or charged ions to build all matter.

Writing Chemical Formulae

Balance element valencies or ion charges to obtain correct formulas.