Periodic law and the periodic table
Periodic law: when elements are placed in order of atomic number (number of protons), their properties repeat in a pattern. Mendeleev found the pattern using atomic mass; Moseley later showed that atomic number is the true order.
- A period is a row. From left to right, atoms get smaller (more protons pull the same shell closer), metals change to non-metals, and the tendency to gain electrons rises.
- A group is a column. Elements in a group have the same number of outer electrons, so they behave alike. Going down a group, atoms get bigger and metals get more reactive.
- Group 1 (Li, Na, K) are soft reactive metals. Group 17 (F, Cl, Br) are reactive non-metals. Group 18 (He, Ne, Ar) hardly react.
Structure: atoms, ions and bonds
An atom has a nucleus (protons and neutrons) and electrons in shells. Chemistry is about the outer electrons.
- Ionic bond: a metal gives electrons, a non-metal takes them. Ions of opposite charge attract (Na⁺ and Cl⁻). Result: hard, brittle crystals with high melting points that conduct when melted or dissolved.
- Covalent bond: two non-metal atoms share electrons (H₂, H₂O, CO₂). Result: molecules, often low melting points, usually no electrical conduction.
- Metallic bond: metal atoms share a sea of free electrons. Result: shiny, bendable, conducts well.
Type of bond explains the properties. Check the elements: metal + non-metal gives ionic; non-metal + non-metal gives covalent.
Reaction rate and energy
Kinetics asks "how fast?". A reaction happens when particles collide with enough energy. Rate goes up when:
- the temperature is higher (particles move faster, more strong hits),
- the concentration is higher (more particles close together),
- the solid is in smaller pieces (more surface to hit),
- a catalyst is used (it gives an easier path and is not used up).
Thermodynamics asks "how much heat?". An exothermic reaction gives out heat (burning, rusting, neutralisation). An endothermic reaction takes in heat (dissolving ammonium nitrate in water, photosynthesis). Energy to start a reaction is called activation energy. A reaction can be exothermic and still need a spark to start.
Many reactions are reversible: the products can turn back. At equilibrium the forward and backward rates are equal, and the amounts stop changing.
Solutions and solubility
A solution is a clear mixture of a solute (what dissolves) in a solvent (what dissolves it, often water). Water is a good solvent because its molecules pull ions and polar molecules out of the crystal.
- Concentration = amount of solute in a given amount of solution. Mass percent = (mass of solute ÷ mass of solution) × 100.
- Solubility = the most solute that dissolves in 100 g of water at a given temperature. A solution that cannot take more is saturated.
- For most solids, solubility rises with temperature. For gases it falls (warm soda goes flat).
- Stirring and crushing do not change how much dissolves, only how fast.
Electrolysis
Electrolysis uses electric current to cause a chemical change in a melted or dissolved ionic compound. The liquid that conducts is the electrolyte; the two plates are electrodes.
- Cathode (−): positive ions (cations) gain electrons. Metals deposit here (Cu²⁺ + 2e⁻ → Cu).
- Anode (+): negative ions (anions) lose electrons. Chlorine gas forms from chloride (2Cl⁻ → Cl₂ + 2e⁻).
Memory: cathode attracts cations; anode attracts anions. Uses: purifying copper, plating objects, making chlorine and sodium hydroxide, extracting aluminium.
Compare with a battery. A battery makes current from a reaction; electrolysis uses current to force a reaction.
Tie it together
Follow one example, copper chloride, through all five ideas: Cu (a metal, group in the middle block) and Cl (group 17 non-metal) → bond type ionic → it dissolves in water as Cu²⁺ and Cl⁻ (faster in warm water) → the ions let the solution conduct → on electrolysis copper plates the cathode and chlorine bubbles at the anode.
Try it: kitchen test
Take two glasses with the same amount of water, one cold and one warm (not hot). Add one spoon of sugar to each without stirring. Predict which dissolves first, then check the time. In the 3D, go to the Solutions station and move the energy slider to see the same thing.
Key formulas and definitions
- Atomic number Z = number of protons = number of electrons (neutral atom)
- Mass percent = (mass of solute ÷ mass of solution) × 100
- Rate: higher temperature, concentration, surface area or catalyst → faster
- Exothermic: heat out (ΔH < 0). Endothermic: heat in (ΔH > 0)
- Cathode: Mⁿ⁺ + ne⁻ → M. Anode: 2Cl⁻ → Cl₂ + 2e⁻
Worked examples
1. Which is bigger: a sodium atom or a chlorine atom? Why?
Sodium. Both are in period 3, and atoms get smaller from left to right because more protons pull the same shell in. Na is at the left, Cl is near the right.
2. 10 g of salt is dissolved in 90 g of water. Find the mass percent.
Mass of solution = 10 + 90 = 100 g. Mass percent = 10 ÷ 100 × 100 = 10%.
3. Magnesium ribbon reacts with acid. Name two ways to make it faster.
Warm the acid (faster particles), or use more concentrated acid. Crushing magnesium into powder, or adding a small catalyst if one exists, also speeds it up.
4. Molten sodium chloride is electrolysed. What is formed at each electrode?
Sodium ions go to the cathode (−): Na⁺ + e⁻ → Na (metal). Chloride ions go to the anode (+): 2Cl⁻ → Cl₂ + 2e⁻ (chlorine gas).
Common mistakes
- Saying atoms get bigger across a period. They get smaller.
- Forgetting that cathode is negative and attracts positive ions (cations).
- Thinking a catalyst is used up. It comes back unchanged.
- Thinking dissolved solids always dissolve more in hot water. This is true for most solids, but not for gases.