Ukraine 8 клас Chemistry
Chapters: 4
1. Quantitative laws of chemistry
Formulas of binary compounds · Relative masses and mass fraction · Molar mass and amount of substance
- Atoms and Molecules – In a chemical reaction mass is neither created nor destroyed (conservation of mass), and a compound always has its elements in the same ratio by mass (constant proportions). Dalton explained both: matter is made of tiny atoms that join in small whole numbers. Atoms join to form molecules; charged atoms or groups are ions. Formulae are written by crossing valencies. Molecular mass (or formula unit mass for ionic compounds) is the sum of the atomic masses in the formula, in u.
2. Gases of the environment
Air and oxygen · Ozone · Gas laws in chemistry · Oxides and water; hydrogen · Carbon oxides, methane and climate
- Oxygen – Oxygen (O, atomic number 8) is a colourless, odourless gas that makes up about 21% of air by volume. It exists as O₂ molecules. In the lab it is made by breaking down hydrogen peroxide with a manganese dioxide catalyst (2H₂O₂ → 2H₂O + O₂) or by heating potassium manganate(VII); in industry it comes from fractional distillation of liquid air. It relights a glowing splint. Oxygen supports burning and respiration and reacts with most elements to form oxides: metal oxides are basic, non-metal oxides are mostly acidic. Its other form, ozone (O₃), shields Earth from UV rays.
- Avogadro's Law: Equal Volumes, Equal Numbers of Molecules – Avogadro's law: at the same temperature and pressure, equal volumes of all gases hold the same number of molecules. So volume is proportional to the number of moles: V ∝ n, or V₁/n₁ = V₂/n₂. At STP (0 °C, 101.3 kPa) one mole of any gas fills 22.4 L (molar volume, Vm). Useful links: n = V ÷ Vm, N = n × 6.02 × 10²³, m = n × M. Equal volumes have different masses, so relative density D = M₁ ÷ M₂ (D by hydrogen = M ÷ 2; by air = M ÷ 29). In gas reactions, volumes are in the same ratio as the equation coefficients (2H₂ + O₂ → 2H₂O: 2 : 1 : 2).
- Hydrogen: The Lightest Element – Hydrogen (H) is element 1: one proton and one electron. It exists as H₂ molecules, a colourless, odourless gas that is the least dense of all gases and hardly dissolves in water. In the lab it is made from a reactive metal and a dilute acid (Zn + 2HCl → ZnCl₂ + H₂) and collected over water. It burns with a squeaky pop to form only water. It reacts with non-metals to give hydrides such as HCl, NH₃ and H₂S, and it reduces some metal oxides. It has three isotopes: protium, deuterium and tritium. Uses: ammonia and fertilisers, margarine, rocket fuel and clean fuel cells.
- The Greenhouse Effect – The Sun heats Earth with visible light. About 30% is reflected straight back to space (albedo ≈ 0.30). The rest warms the ground, which gives off infrared radiation. Greenhouse gases – water vapour, carbon dioxide, methane, nitrous oxide – let visible light through but absorb infrared and send part of it back down. Earth settles at the temperature where energy in equals energy out. Without greenhouse gases Earth would average about −18 °C; with them it is about +15 °C. Adding more CO₂ and CH₄ (from burning fuels, farming, landfills) strengthens the effect and warms the planet.
3. Structure of the atom
Atomic structure · Electron shells and properties of elements · Periodic law and periodic table
- Structure of the Atom – An atom has a tiny, heavy, positive nucleus made of protons and neutrons. Electrons move around it in fixed shells K, L, M, N. The number of protons (Z) tells the element; protons + neutrons give the mass number (A). Outer electrons decide valency. Isotopes share Z; isobars share A.
- Quantum Mechanical Model: Quantum Numbers, Orbitals and Configuration – Moving electrons also behave like waves: λ = h/mv (de Broglie). Because of this we cannot know exact position and momentum together: Δx·Δp ≥ h/4π (Heisenberg). So we drop sharp orbits and use orbitals: regions where the electron is likely to be, from the Schrödinger equation (ψ² gives probability). Four quantum numbers describe each electron: n (shell, size, energy), l (subshell, shape: 0 to n−1), mₗ (orientation: −l to +l) and mₛ (spin: +½ or −½). s is spherical, p is dumbbell, d is mostly four-lobed. Electrons fill by the Aufbau (n + l) rule, Pauli principle (max 2, opposite spins) and Hund's rule (single first). Half-filled and fully filled subshells are extra stable, so Cr is 3d⁵4s¹ and Cu is 3d¹⁰4s¹.
- Periodic Classification of Elements – Scientists sorted elements so that similar ones sit together. Döbereiner made triads, Newlands found that every 8th element repeats (octaves), Mendeleev arranged elements by atomic mass and left gaps for unknown ones. Moseley showed that atomic number is the real key. Modern periodic law: the properties of elements are a periodic function of their atomic numbers. The modern table has 7 periods and 18 groups. Elements with Z > 100 get temporary IUPAC names built from digit roots (nil, un, bi, tri, quad, pent, hex, sept, oct, enn) plus -ium.
4. Structure of substances
Covalent bonding · Ionic bonding and crystals
- The Solid State – In a solid the particles sit close together and only vibrate in place, so a solid keeps its shape. In crystalline solids the particles repeat in a regular pattern; in amorphous solids (like glass) they do not. The smallest repeating box is the unit cell. Simple cubic holds 1 particle, body-centred cubic 2 and face-centred cubic 4. By the bonds holding them, crystals are ionic, covalent network, molecular or metallic.