Atoms, Molecules & Ions – Comprehensive Study Notes

Lesson Outcomes

• Identify fundamental subatomic particles within an atom: protons, neutrons, electrons

• Comprehend the development of atomic theory and its link to the laws governing matter

• Correctly write and interpret chemical formulas for compounds

Matter: Core Review

• Definition: anything with mass and occupying space; manifests as solid, liquid, gas

  • Solid

  • Definite shape & volume

  • High density, incompressible, rigid, low diffusivity

  • Liquid

  • Definite volume, indefinite shape (takes container’s form)

  • High density, virtually incompressible, moderate diffusivity

  • Gas

  • Indefinite shape & volume (fills container)

  • Widely-spaced rapid particles, low density, highly compressible, high diffusivity

Atomic Theory: Central Tenets

• Matter consists of minute particles called atoms

• For any single element, its atoms are identical in mass & properties

• Atoms of different elements possess different masses & properties

• Chemical compounds form when atoms combine in simple whole-number ratios

• Chemical reactions involve combination, separation, rearrangement of atoms; atoms are neither created nor destroyed (conservation principle)

Fundamental Laws of Matter (Chemical Combination)

• Law of Conservation of Mass: total mass remains constant during physical/chemical change
  $m<em>{\text{reactants}} = m</em>{\text{products}}$
  Example: burning wood → water vapor + CO₂ + carbon residue; mass sum unchanged

• Law of Constant Composition (Definite Proportions): a given compound always contains the same elements in the same mass ratio
  Example: $\text{CO}_2$ always shows a C:O mass ratio of $12:32$ (or $3:8$ in simplest form)

• Law of Multiple Proportions: when two elements form more than one compound, masses of one element that combine with a fixed mass of the other are in small whole-number ratios Data set (N–O compounds, mass of N fixed at $14~\text{g}$):

  • $\text{NO}:14:16$

  • $\text{NO}_2:14:32$ ⇒ O mass doubled (ratio $1:2$)

  • $\text{N}_2\text{O}:28:16$ (same O as first but double N)

  • $\text{N}<em>2\text{O}</em>2:28:32$ ⇒ ratio $1:1$

  • $\text{N}<em>2\text{O}</em>5:28:80$ ⇒ ratio $2:5$
    Carbon/Oxygen pair for comparison:

  • $\text{CO}:12:16$

  • $\text{CO}_2:12:32$ ⇒ ratio $1:2$

Atom & Its Structure

• Subatomic particles

  • Proton (p⁺): +1 charge, ~$1.007\,\text{amu}$, inside nucleus

  • Neutron (n⁰): 0 charge, ~$1.008\,\text{amu}$, inside nucleus

  • Electron (e⁻): −1 charge, $0.00055\,\text{amu}$ (≈1⁄1836 of proton), in electron cloud/orbitals

• Size scale: nucleus ≈ $10^{-5}$ times the atom’s diameter (100 000× smaller). Analogy: if the atom is a stadium, nucleus < a grain of sand.

Historical Development of Atomic Models

• Solid Sphere Model (1803, John Dalton)

  • Atoms = indivisible solid spheres; identical for each element

  • Recognized elemental uniqueness but ignored subatomic structure

• Plum Pudding Model (1904, J.J. Thomson)

  • Discovery of electrons (“corpuscles”)

  • Atom = diffuse positive ‘pudding’ with embedded negative electrons

  • Explained overall neutrality but no nucleus; couldn’t predict later observations

• Nuclear Model (1911, Ernest Rutherford)

  • Gold-foil experiment: most α-particles passed; a few large deflections ⇒ atom mostly empty space with dense positive nucleus

  • Left question of electron stability in orbit

• Planetary/Bohr Model (1913, Niels Bohr)

  • Electrons travel in fixed quantised orbits (energy levels) around nucleus

  • Explained hydrogen emission lines; predicted stable orbits

  • Fails for multi-electron/heavier atoms; electrons should radiate energy classically

• Quantum/Cloud Model (1926, Erwin Schrödinger)

  • Electrons are wave-particles; described by wave function $\psi$

  • Probability clouds (orbitals) define regions of likely electron presence, not fixed paths

  • Remains best representation; accommodates Heisenberg uncertainty principle

Molecules: Formation & Properties

• Molecule: group of ≥2 atoms bonded chemically; foundational unit for many substances (water, air components, biomolecules)

• Bonding usually via covalent sharing of electrons → increased stability (octet rule attainment)

• Properties depend on atom types & spatial arrangement

• Illustrative reaction (water formation):
  $2\,\text{H} + 1\,\text{O} \longrightarrow \text{H}_2\text{O}$

• Common molecular examples:

  • $\text{O}_2$ (oxygen gas): 1 O atom + 1 O atom

  • $\text{CO}_2$: 1 C + 2 O

  • $\text{CH}_4$ (methane, mentioned in task)

Ions: Charged Species

• Ion: atom / molecule with net electric charge due to electron transfer

  • Cation: positive, electron loss ⇒ fewer e⁻ than p⁺

  • Example: $\text{Na} \rightarrow \text{Na}^+ + e^-$

  • Anion: negative, electron gain ⇒ more e⁻ than p⁺

  • Example: $\text{Cl} + e^- \rightarrow \text{Cl}^-$

• Motivation: achieve noble-gas-like stable electron configuration (full valence shell)

• Significance: salt formation, electrical conduction (electrolytes), nerve impulses, acid-base chemistry, etc.

Synthesis / Wrap-Up Highlights

• Atomic theory underpins modern chemistry: matter = atoms, impossible to create/destroy in ordinary reactions, combine in definite ratios

• Progressive models refined understanding from indivisible spheres → probabilistic quantum clouds

• Atoms (protons, neutrons, electrons) build molecules (covalent lattices) & ions (charged species) which dictate reactivity & functionality in physical, biological, and industrial contexts

Practical Application: Performance Task (Class Activity)

• Group assignment: construct 3-D model of an assigned molecule (e.g., $\text{H}<em>2\text{O}$, $\text{CO}</em>2$, $\text{CH}_4$)

• Materials: marshmallows = atoms (color-coded), toothpicks = bonds

• Steps:

  1. Examine structural diagram / formula for correct atom counts & geometry

  2. Assemble with marshmallows (different colors/sizes for distinct elements)

  3. Present to class: name molecule, elemental composition, bond arrangement, geometric shape (linear, bent, tetrahedral), and real-world significance