Atomic Foundations
of Matter
๐งช handwritten-style notes for quick revision ๐งช
๐ Table of Contents
โ๏ธ Law of Conservation of Mass
Given by Antoine Lavoisier (1789) โ the "Father of Modern Chemistry".
He stated: "...in every operation an equal quantity of matter exists both before and after the operation."
๐งซ Quick Activity Recap
- Salt + Water (physical change): mass of solution = mass of salt + mass of water. No change in mass.
- Vinegar + Baking soda in open flask: gas (COโ) escapes โ final reading โ initial reading (looks like mass lost).
- Same reaction in a closed/tied balloon setup: gas is trapped โ final reading = initial reading โ
โก๏ธ So mass is always conserved โ the mismatch in the open flask was only because gas escaped, not because mass was destroyed.
๐งญ Law of Constant Proportions
Also called Law of Definite Proportions or Proust's Law โ given by Joseph Louis Proust.
Example: Water from a river, borewell or ocean โ when purified โ always has Hydrogen : Oxygen = 1 : 8 by mass.
So 9 g of pure water always gives 1 g hydrogen + 8 g oxygen.
โ๏ธ Dalton's Atomic Theory
Both laws above are explained by John Dalton's postulates (1808):
- All matter is made of tiny particles called atoms.
- Atoms are indivisible โ cannot be created or destroyed in a reaction.
- Atoms of the same element are identical in mass & properties.
- Atoms of different elements differ in mass & properties.
- Atoms combine in simple whole-number ratios to form compounds.
- The relative number & kind of atoms is constant in a given compound.
English scientist, taught in Manchester. Presented his atomic theory in 1808 โ a turning point in the study of matter.
๐ How Atoms Combine?
Atoms need 8 electrons in the valence shell (2 for K-shell) to be stable โ i.e. a full octet.
To complete the octet, atoms combine by:
- Sharing of electrons โ forms a Covalent Bond
- Transfer of electrons โ forms an Ionic Bond
Noble gases like Helium already have a stable outer shell โ they exist as single atoms.
๐ค Covalent Bond (Sharing)
Formed when atoms share electron pairs. The shared pair attracts both nuclei and holds the molecule together.
| Molecule | Electrons shared | Bond type | Formula |
|---|---|---|---|
| Hโ | 1 each | Single | HโH |
| Clโ | 1 each | Single | ClโCl |
| Oโ | 2 each | Double | O=O |
| HCl | 1 each | Single | HโCl |
| HโO | 1 eโป from each H to O | 2 single bonds | HโOโH |
โ๏ธ Naming Covalent (Molecular) Compounds
- First element keeps its name; second element ends in -ide.
- Prefixes show atom count: mono-1, di-2, tri-3, tetra-4, penta-5, hexa-6.
- Mono- is skipped for the first element, used for the second.
- Drop the vowel before another vowel: monoxide, pentoxide (not monooxide).
HโS โ hydrogen sulfide (no prefix before H) | HโO = water, NHโ = ammonia (common names)
โก Ionic Bond (Transfer)
Metals (usually <4 valence eโป) lose electrons โ form cations (+). Non-metals (usually >4 valence eโป) gain electrons โ form anions (โ).
Example โ NaCl: Na loses 1 eโป โ Naโบ; Cl gains 1 eโป โ Clโป. Naโบ + Clโป โ NaCl.
Ionic compounds form 3-D crystal lattices (not single molecules) โ each Naโบ is surrounded by 6 Clโป and vice-versa.
๐งพ Naming ionic compounds
Cation name first, then anion name (ending in -ide). Metal + non-metal โ ionic compound.
| Common Cations | Common Anions |
|---|---|
| Naโบ, Kโบ, Agโบ, Liโบ (valency 1) | Clโป, Fโป, Brโป, Iโป, OHโป, NOโโป (valency 1) |
| Caยฒโบ, Mgยฒโบ, Baยฒโบ, Znยฒโบ, Cuยฒโบ, Feยฒโบ (valency 2) | Oยฒโป, Sยฒโป, COโยฒโป, SOโยฒโป (valency 2) |
| Alยณโบ, Feยณโบ (valency 3) | POโยณโป (valency 3) |
Polyatomic ions (e.g. NHโโบ, OHโป, NOโโป, COโยฒโป, SOโยฒโป) are groups of atoms carrying a charge together โ names usually don't end in -ide.
โ๏ธ Writing Chemical Formulae
Covalent compounds โ criss-cross valencies
C (valency 4) + Cl (valency 1) โ criss-cross โ CClโ
Ionic compounds โ criss-cross charges
- Write cation symbol first, then anion symbol.
- Write charges below the symbols (not as superscripts).
- Criss-cross the numbers โ these become subscripts.
- Simplify subscripts to the smallest whole-number ratio.
- Use brackets ( ) when 2+ polyatomic ions of the same kind are needed, e.g. Mg(OH)โ.
Caยฒโบ + COโยฒโป โ CaCOโ | Alยณโบ + OHโป โ Al(OH)โ | Alยณโบ + SOโยฒโป โ Alโ(SOโ)โ
๐ฌ Properties: Ionic vs Covalent
| Property | Ionic Compounds | Covalent Compounds |
|---|---|---|
| Example | NaCl, CuSOโ | Camphor, Naphthalene, Sugar |
| Solubility | Soluble in water | Insoluble in water, soluble in kerosene/petrol |
| Conducts electricity (solid) | โ No (ions fixed in lattice) | โ No |
| Conducts electricity (in water) | โ Yes (ions free to move) | โ Usually no (e.g. sugar solution) |
| Melting/Boiling point | High | Low |
โ๏ธ Molecular Mass & Formula Unit Mass
Molecular Mass (for covalent compounds)
Sum of atomic masses of all atoms in one molecule.
COโ = (12ร1) + (16ร2) = 44 u
Formula Unit Mass (for ionic compounds)
Ionic compounds don't form molecules (they form crystal lattices), so we use "formula unit" = simplest whole-number ratio of ions.
Ca(NOโ)โ = (40ร1) + {(14ร1)+(16ร3)}ร2 = 164 u
๐ At a Glance โ Full Chapter Revision
- Mass is neither created nor destroyed in a chemical reaction โ Law of Conservation of Mass.
- A compound always has the same elements in a fixed ratio by mass โ Law of Definite/Constant Proportions.
- A molecule = neutral, independent, 2+ atom particle showing all properties of the substance.
- Atoms combine to become stable, held by a chemical bond.
- Covalent bond = sharing of electrons.
- Ionic bond = transfer of electrons โ forms cations & anions.
- Covalent formula = number of atoms of each element.
- Ionic formula = simplest whole-number ratio of ions.
- Molecular mass = sum of atomic masses in a molecule.
- Formula unit mass = sum of atomic masses in one formula unit of an ionic compound.