Atoms and Molecules From the smallest particles emerge the wonders of matter.

Early Ideas of Divisibility

Parmanu / Atom

Around 500 BC, Indian and Greek thinkers proposed that continuous cutting of matter ends with one indivisible particle—Kanad’s “Parmanu” and Democritus’ “Atom”.

Key Characteristics:

  • Maharishi Kanad (India): named the smallest particle “Parmanu”.
  • Democritus (Greece): called the indivisible particle “Atom”.
  • Ideas were philosophical; no experiments supported them then.

Example:

Keep halving a grain of rice mentally until only an undividable speck remains—this speck is the imagined atom.

Laws of Chemical Combination

Two Fundamental Laws

Law of Conservation of Mass states that during a chemical reaction, total mass of reactants equals total mass of products. Law of Constant Proportions says a pure compound always has the same elements in a fixed mass ratio, wherever obtained.

Formulated by Antoine Lavoisier (1789) and Joseph Proust (1799), these laws laid the groundwork for modern stoichiometry.

Law of Conservation of Mass

Diagram of conical flask with hanging ignition tube (Fig 3.1 style)

Closed flask set-up showing ignition tube immersion (Fig 3.1)

Closed-Flask Precipitation Experiment

A corked conical flask holds sodium sulphate solution; an ignition tube containing barium chloride solution is suspended inside and weighed.

The flask is tilted to mix both liquids, forming a white barium sulphate precipitate. Re-weighing shows the same total mass.

Key Points:

  • Initial and final balance readings are identical.
  • Cork keeps the system closed; no matter escapes.
  • Mass is conserved in every chemical reaction.

Law of Constant Proportions

Fixed Mass Ratio

A pure compound always contains the same elements combined in an unchanging mass ratio, whatever its source or preparation.

Examples → Water (H₂O): H : O = 1 : 8  •  Ammonia (NH₃): N : H = 14 : 3

Dalton’s Atomic Theory

Dalton listed six postulates that explain the laws of conservation of mass and constant proportions.

1

Matter is Atomic

All matter is made of extremely small particles called atoms that take part in chemical reactions.

2

Indivisible & Indestructible

Atoms cannot be created, divided or destroyed during any chemical change.

3

Identical in an Element

Atoms of the same element have identical mass and chemical properties.

4

Different Across Elements

Atoms of different elements differ in mass and chemical behaviour.

5

Simple Ratios Form Compounds

Atoms combine in simple whole-number ratios to produce compounds.

6

Fixed Composition

In a given compound, the kind and relative number of atoms remain constant.

Pro Tip:

Postulates 1–2 justify conservation of mass, while 5–6 explain constant proportions in compounds.

How Big is an Atom?

Scaled bar showing atom, water molecule, grain of sand, apple

Relative sizes from atomic radius to an apple (not to scale)

The Nanometre World

Atomic radius of hydrogen is about 0.1 nm or \(1 \times 10^{-10}\) m.

One nanometre is one-billionth of a metre; stack 10 million atoms to match this page’s thickness.

Relative Size Chart:

  • Hydrogen atom ≈ 0.1 nm
  • Water molecule ≈ 0.3 nm
  • Grain of sand ≈ 0.1 mm (1 × 105 nm)
  • Apple ≈ 10 cm (1 × 108 nm)

Modern Symbols of Elements

Chemical Symbol

A unique one- or two-letter code approved by IUPAC for every element. The first letter is always capital, the second (if any) lowercase. Several symbols stem from Latin names.

Key Characteristics:

  • One-letter or two-letter code, e.g., H, O, Cl.
  • First letter uppercase; second lowercase, e.g., Na, Al.
  • Latin-derived when English name repeats, e.g., Fe (ferrum), K (kalium).
  • Standardised globally by IUPAC for clear scientific communication.

Example:

Hydrogen = H, Aluminium = Al, Iron = Fe (from “ferrum”).

Atomic Mass Unit (u)

\[1\,\text{u} \;=\; \frac{1}{12}\,\text{mass of one }^{12}\text{C atom}\]

Variable Definitions

u atomic mass unit
\(^{12}\)C carbon-12 atom used as reference

Applications

Relative Atomic Mass

Atomic masses are compared to the C-12 scale and expressed in u.

Molar Mass Link

1 mol of any element has mass (in g) equal to its atomic mass in u.

Universal Standard

Provides a consistent unit for chemistry and physics calculations.

Molecules

Molecule

Smallest particle of an element or compound that can exist alone and retains all its chemical properties.

Key Characteristics:

  • Atomicity = number of atoms present in a molecule.
  • Monoatomic (atomicity 1): single-atom molecules e.g., He, Ar.
  • Diatomic (atomicity 2): two-atom molecules e.g., O₂.
  • Polyatomic (atomicity >2): many-atom molecules e.g., S₈.

Example:

O₂ illustrates diatomic molecules, while S₈ shows polyatomic nature, helping you distinguish types by atomicity.

Ions and Polyatomic Ions

Ion

An ion is an atom or group of atoms carrying a net charge due to electron loss or gain. Positive ions are cations, e.g., Na⁺ and NH₄⁺. Negative ions are anions, such as Cl⁻, or the polyatomic ion SO₄²⁻. Ions act as discrete formula units in ionic compounds.

Writing Chemical Formulae

Use the valency crossover rule to derive correct formulae for binary and polyatomic compounds.

1

List Symbols

Write chemical symbols of the combining elements or polyatomic ions. Metal or cation is written first.

2

Assign Valencies

Above each symbol note its valency / charge—the combining capacity that must be satisfied.

3

Cross & Drop Sign

Cross over the numerical valencies and write them as subscripts to the other element. Drop the charge sign.

4

Simplify

Divide all subscripts by their highest common factor. Remove ‘1’. The formula is charge-balanced and simplest.

Pro Tip:

Always verify total positive charge equals total negative charge; if not, revisit valencies and subscripts.

Build the Formula!

{% if intro %}

Balance charges with valency. Drag Mg²⁺, Na⁺, NH₄⁺ onto Cl⁻, SO₄²⁻ and PO₄³⁻ to build \( \mathrm{MgCl_2}, \mathrm{Na_2SO_4}, (\mathrm{NH_4})_3\mathrm{PO_4} \).

{% endif %}

Draggable Items

{% for item in draggable_items %}
{{ item.label }}
{% endfor %}

Drop Zones

{% for zone in drop_zones %}

{{ zone.label }}

{% endfor %}
{% if tip %}

Tip:

Total positive charge must equal total negative charge in the final compound.

{% endif %}

Molecular & Formula Unit Mass

\[ M = \sum_{i} n_{i} \times A_{i} \]

Variable Definitions

\(M\) Molecular / formula unit mass (u)
\(n_{i}\) Number of atoms/ions of element i
\(A_{i}\) Relative atomic mass of element i (u)

Applications

Water (H₂O)

\(M = 2 \times 1 + 16 = 18\,\text{u}\)

Calcium Chloride (CaCl₂)

\(M = 40 + 2 \times 35.5 = 111\,\text{u}\)

Source: NCERT Science – Atoms & Molecules

Quick Check

Question

Which postulate of Dalton’s atomic theory explains the Law of Conservation of Mass?

1
Atoms combine in small whole-number ratios.
2
Atoms are indivisible and can neither be created nor destroyed.
3
Atoms of the same element are identical.
4
Atoms of different elements have different masses.

Key Takeaways

Atoms → Molecules → Matter

Law of Conservation

Total mass stays constant in every chemical reaction.

Law of Definite Proportions

Elements combine in fixed mass ratios to form a compound.

Atomic Concept

An atom is the smallest reactive unit and remains unchanged during reactions.

Building Matter

Atoms join as molecules or ions, giving rise to all visible matter.

Formula Writing

Use valency or ion charge to cross-combine symbols into neutral chemical formulas.