First Atom Thinkers

Earliest Atomic Idea

Smallest, indivisible particle imagined by Indian sage Maharishi Kanad (Parmanu) and Greek philosopher Democritus (atomos).

Key Characteristics:

  • Maharishi Kanad (≈500 BC) named the uncuttable unit of matter Parmanu.
  • Greek thinkers Democritus & Leucippus called the same concept atomos, meaning “indivisible”.
  • Ideas were philosophical; no experiments, so science embraced atoms only in the 18th century.

Example:

Kanad said a grain of rice can be halved repeatedly until only one Parmanu remains.

Mass Never Disappears

Law of Conservation of Mass

Law of Conservation of Mass: in a closed reaction, reactant mass equals product mass. Step into the virtual lab: weigh a sealed flask, tilt to mix two solutions, watch them react. Re-weigh; if the digital balance is unchanged within ±0.01 g, mass has been conserved. Proving this satisfies the learning outcome—demonstrate that mass never disappears.

Constant Proportions

Law of Constant Proportions

Joseph Proust stated: In any compound, elements combine in a fixed mass ratio, whatever the sample’s source or preparation.

Water always splits into \(1:8\) (H:O) by mass. Ammonia consistently shows \(14:3\) (N:H). Quiz: 28 g N pairs with 6 g H.

Dalton’s Atomic Theory

Dalton (1808) turned chemical laws into a testable model. His six postulates explain tiny atoms, their conservation, and constant composition.

1

Atoms Exist

All matter is made of extremely small, discrete particles called atoms.

2

Element Identity

Atoms of the same element share identical mass & properties; atoms of different elements differ.

3

Indivisible in Reactions

Atoms cannot be created, divided or destroyed in chemical changes, upholding the Law of Conservation of Mass.

4

Simple Whole Ratios

Atoms combine in simple whole-number ratios to form compounds, foreshadowing the Law of Multiple Proportions.

5

Fixed Composition

A given compound always contains the same kinds and numbers of atoms, explaining the Law of Constant Composition.

6

Rearrangement Only

Chemical reactions merely rearrange atoms; total number and kinds of atoms remain constant.

Symbols & Naming

Element symbol diagram

Write Symbols the IUPAC Way

Each element is represented by one or two letters to ensure universal understanding.

Rule: first letter capital, second lowercase. Al is aluminium; AL would be wrong.

Some symbols keep Latin roots: Fe, Na, K.

Check: what is wrong with ‘PB’ for lead?

Key Points:

  • One or two letters only
  • First letter capital, second lowercase
  • Latin names explain Fe, Na, K

Atomic Mass Unit

\[1\,\text{u} = \frac{1}{12}\,m({}^{12}\text{C}) \quad ; \quad A_r = \frac{m_{\text{atom}}}{1\,\text{u}}\]

Variable Definitions

\(1\,\text{u}\) Atomic mass unit
\(m({}^{12}\text{C})\) Mass of one carbon-12 atom
\(A_r\) Relative atomic mass
\(m_{\text{atom}}\) Mass of the given atom

Applications

Quick Comparison

Expresses atomic masses without huge kilogram numbers.

Example

Oxygen atom mass ≈ 16 u, so \(A_r(\text{O}) \approx 16\).

How Many Atoms?

Drag each molecule into the bin that matches its atomicity. O₂ is diatomic; O₃ is triatomic and counts as polyatomic.

Draggable Items

Ar
O₂
P₄
S₈

Drop Zones

Monatomic (1)

Diatomic (2)

Tetraatomic (4)

Polyatomic (>4)

Tip:

Bins glow when the correct molecule is dropped.

Ions & Valency

Ion

An atom or group carrying net charge after losing or gaining electrons.

Key Characteristics:

  • Cations: \( \text{Na}^+, \text{Ca}^{2+} \) — positive, formed by electron loss.
  • Anions: \( \text{Cl}^- , \text{SO}_4^{2-} \) — negative, formed by electron gain.
  • Valency equals electrons lost, gained or shared; shows the combining “arms”.

Example:

\( \text{Na}^+ + \text{Cl}^- \rightarrow \text{NaCl} \). Each ion uses one valency arm.

Multiple Choice Question

Question

Aluminium (metal) combines with oxide ion (non-metal). Using the criss-cross charge-balancing rule, which formula is correct?

1
AlO
2
Al₂O₃
3
Al₃O₂
4
AlO₃

Hint:

Criss-cross the magnitudes of Al³⁺ and O²⁻ to obtain subscripts.

Key Takeaways

Atoms to Equations in a Flash

Two Universal Laws

Mass is conserved, and elements always combine in fixed proportions—bedrock for every balanced equation.

Dalton’s Six Postulates

Matter is made of indivisible atoms that combine in simple ratios, conserve identity, and form compounds with whole-number counts.

Symbols & Masses

Chemical symbols abbreviate element names; atomic masses quantify one atom, letting us count atoms by weighing.

Molecules, Ions & Valency

Atoms join as neutral molecules or charged ions; valency tells how many bonds each atom can form.