Molecules & Compounds
Section: 2. Atoms, Elements & Compounds | Syllabus: Cambridge AS Level Physics 9702
What is a Molecule? Molecule A group of two or more atoms chemically bonded together by covalent bonds. Molecules can contain atoms of the same element or different elements. Molecules are the basic units of covalent substances.
They exist as discrete particles - separate, independent groups of atoms. Types of Molecules 1. Diatomic Molecules (Two Atoms) Diatomic Molecule A molecule consisting of only two atoms bonded together.
Elements that exist as diatomic molecules: Remember: "HOFBrINCl" Seven elements exist as diatomic molecules in their natural state: H₂ - Hydrogen O₂ - Oxygen F₂ - Fluorine Br₂ - Bromine I₂ - Iodine N₂ - Nitrogen Cl₂ - Chlorine Diatomic compounds (two different atoms): HCl - Hydrogen chloride CO - Carbon monoxide NO - Nitrogen monoxide 2.
Polyatomic Molecules (More Than Two Atoms) Polyatomic Molecule A molecule consisting of three or more atoms bonded together. Molecule Formula Number of Atoms Type Water H₂O 3 atoms (2H + 1O) Triatomic Carbon dioxide CO₂ 3 atoms (1C + 2O) Triatomic Ammonia NH₃ 4 atoms (1N + 3H) Polyatomic Methane CH₄ 5 atoms (1C + 4H) Polyatomic Glucose C₆H₁₂O₆ 24 atoms Polyatomic Molecular Elements vs Molecular Compounds Molecular Elements Molecular Element A molecule made up of atoms of the same element only .
Examples: H₂ - Hydrogen molecule (2 hydrogen atoms) O₂ - Oxygen molecule (2 oxygen atoms) N₂ - Nitrogen molecule (2 nitrogen atoms) Cl₂ - Chlorine molecule (2 chlorine atoms) P₄ - Phosphorus molecule (4 phosphorus atoms) S₈ - Sulfur molecule (8 sulfur atoms) Molecular Compounds Molecular Compound A molecule made up of atoms of two or more different elements chemically bonded together.
Examples: H₂O - Water (hydrogen + oxygen) CO₂ - Carbon dioxide (carbon + oxygen) NH₃ - Ammonia (nitrogen + hydrogen) CH₄ - Methane (carbon + hydrogen) HCl - Hydrogen chloride (hydrogen + chlorine) C₂H₅OH - Ethanol (carbon + hydrogen + oxygen) Property Molecular Element Molecular Compound Number of elements Only ONE element TWO or more elements Can be separated?
Cannot be chemically broken down (it's an element) Can be broken down into simpler substances Examples H₂, O₂, N₂, Cl₂, P₄, S₈ H₂O, CO₂, NH₃, CH₄, HCl Molecular Formulae Molecular Formula A chemical formula showing the actual number of atoms of each element in one molecule of a substance.
Reading Molecular Formulae H₂O means: - 2 atoms of hydrogen (H₂) - 1 atom of oxygen (O) - Total: 3 atoms in the molecule NH₃ means: - 1 atom of nitrogen (N) - 3 atoms of hydrogen (H₃) - Total: 4 atoms in the molecule C₆H₁₂O₆ means: - 6 atoms of carbon (C₆) - 12 atoms of hydrogen (H₁₂) - 6 atoms of oxygen (O₆) - Total: 24 atoms in the molecule Important The subscript number tells you how many atoms of that element are in ONE molecule.
If there's no subscript, it means there's just 1 atom of that element. Simple Molecular Structures Simple molecules are small, discrete units with a fixed number of atoms. Each molecule is separate from other molecules.
Atoms within a molecule are held together by strong covalent bonds Different molecules are held together by weak intermolecular forces Most simple molecular substances are gases or liquids at room temperature They have low melting and boiling points (because weak forces between molecules) Common Simple Molecules in Detail Water (H₂O) Molecular formula: H₂O 3 atoms: 2 hydrogen + 1 oxygen Oxygen forms 2 covalent bonds (one with each H) Bent molecular shape Polar molecule - important solvent Carbon Dioxide (CO₂) Molecular formula: CO₂ 3 atoms: 1 carbon + 2 oxygen Carbon forms 2 double bonds (O=C=O) Linear molecular shape Gas at room temperature Ammonia (NH₃) Molecular formula: NH₃ 4 atoms: 1 nitrogen + 3 hydrogen Nitrogen forms 3 covalent bonds Pyramidal molecular shape Alkaline gas, very soluble in water Methane (CH₄) Molecular formula: CH₄ 5 atoms: 1 carbon + 4 hydrogen Carbon forms 4 covalent bonds Tetrahedral molecular shape Main component of natural gas Hydrogen Chloride (HCl) Molecular formula: HCl 2 atoms: 1 hydrogen + 1 chlorine 1 single covalent bond Linear molecular shape (diatomic) Gas that dissolves in water to form hydrochloric acid Representing Molecules There are different ways to represent molecules: 1.
Molecular Formula Shows the number of each type of atom: H₂O, CO₂, NH₃ 2. Structural Formula Shows how atoms are connected: Water: H-O-H Carbon dioxide: O=C=O Ammonia: H-N-H with one H below 3. Displayed Formula Shows all atoms and all bonds explicitly as lines 4.
Dot and Cross Diagram Shows all outer shell electrons and how they're shared Representation Type What It Shows Example (Water) Molecular Formula Number of each atom H₂O Structural Formula How atoms are bonded H-O-H Dot & Cross All electrons in outer shells Shows shared pairs and lone pairs Molecules vs Other Structures Feature Simple Molecules Ionic Compounds Giant Covalent Structure Small, discrete molecules Giant ionic lattice Giant covalent structure Bonding within Strong covalent bonds Strong ionic bonds Strong cova…
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