Atoms, Elements and the Periodic Table

Ever wondered why some elements explode in water whilst others barely react at all? It all comes down to their atomic structure and position on the periodic table.

Elements contain just one type of atom, each with the same number of protons. Compounds form when different atoms bond chemically together. Mixtures are simpler - they're just substances mixed together without chemical bonding, so you can separate them using physical methods like filtration or distillation.

Isotopes are atoms of the same element with different numbers of neutrons. This gives them different masses but the same chemical properties. The relative atomic mass you see on the periodic table is actually an average of all the isotopes.

Group 1 metals (alkali metals) are incredibly reactive because they only have one electron in their outer shell. As you go down the group, they get more reactive because the outer electron is further from the nucleus and easier to lose. Group 7 elements (halogens) are the opposite - they get less reactive down the group because it's harder for larger atoms to gain an electron.

Quick Tip: Remember that atoms have equal numbers of protons and electrons, giving them no overall charge. Protons = +1, neutrons = 0, electrons = -1.

Chemical Bonding and Structure

The way atoms stick together determines everything from whether a substance conducts electricity to how hard it is. There are three main types of bonding you need to know.

Ionic bonding happens when metals transfer electrons to non-metals, creating charged ions that attract each other. These substances have high melting points and conduct electricity when molten or dissolved because the ions can move freely.

Covalent bonding occurs when non-metal atoms share electrons. Simple covalent structures like water have low melting points because weak forces hold the molecules together. Giant covalent structures like diamond are incredibly hard with high melting points due to millions of strong bonds.

Metallic bonding creates a "sea" of delocalised electrons around positive metal ions. This explains why metals conduct electricity and can be hammered into shape. Alloys are harder than pure metals because adding different-sized atoms disrupts the regular layers, preventing them from sliding over each other.

Carbon forms some fascinating structures: diamond (super hard), graphite (conducts electricity and is slippery), and fullerenes (hollow spheres and tubes with unique properties).

Remember: The type of bonding determines the properties - ionic conducts when molten, covalent doesn't conduct, metallic always conducts.

Quantitative Chemistry

Maths in chemistry isn't just about numbers - it's about understanding exactly what happens in reactions. Once you grasp these concepts, you can predict how much product you'll get from any reaction.

Formula mass (Mr) is simply adding up all the atomic masses in a compound. One mole of any substance contains 6.02×10²³ particles and has a mass equal to its Mr in grams. This massive number is called Avogadro's constant.

Balancing equations ensures the same number of each type of atom appears on both sides. Use the formula: mass = Mr × moles to convert between mass and moles in calculations.

Limiting reactants are like ingredients in a recipe - if you run out of one, the reaction stops. The reactant that runs out first limits how much product you can make. Calculate which reactant you need more of using the balanced equation ratios.

Concentration tells you how much solute is dissolved in a solution, calculated as concentration = mass/volume. Remember to convert cm³ to dm³ by dividing by 1000.

Exam Tip: Always show your working in calculations - you can get marks for the method even if your final answer is wrong.

Chemical Reactions and Electrolysis

Understanding which metals are more reactive than others helps predict what happens when they meet. The reactivity series runs from potassium (most reactive) down to gold (least reactive).

Displacement reactions occur when a more reactive element kicks out a less reactive one from its compound. Metals above hydrogen in the reactivity series react with acids to produce hydrogen gas.

Metal extraction depends on reactivity. Very unreactive metals like gold exist naturally. Metals less reactive than carbon can be extracted by heating with carbon (reduction). More reactive metals need electrolysis - passing electricity through molten compounds.

During electrolysis, positive ions move to the negative electrode (cathode) and gain electrons. Negative ions move to the positive electrode (anode) and lose electrons. In aqueous solutions, sometimes water molecules interfere and you get hydrogen or oxygen instead of the metal.

Acids and alkalis are opposites on the pH scale. Strong acids completely ionise in water, whilst weak acids only partially ionise. Concentration is different from strength - you can have dilute strong acids or concentrated weak acids.

Key Point: Electrolysis requires molten ionic compounds or solutions - solid ionic compounds can't conduct electricity because the ions are fixed in position.

Energy Changes in Reactions

Every chemical reaction involves energy changes, which explains why some reactions feel hot whilst others feel cold. Understanding bond energies helps you predict whether reactions release or absorb energy.

Exothermic reactions transfer energy to the surroundings, making them feel warm. Common examples include combustion, oxidation, and neutralisation. These reactions are useful for hand warmers and self-heating cans.

Endothermic reactions absorb energy from the surroundings, making them feel cold. Thermal decomposition and some acid reactions are endothermic. Sports injury packs use endothermic reactions for instant cooling.

Bond breaking always requires energy (endothermic), whilst bond formation always releases energy (exothermic). The overall energy change depends on which process needs more energy.

To calculate energy changes: work out the energy needed to break all bonds in reactants, then subtract the energy released when forming all bonds in products. A negative result means exothermic, positive means endothermic.

The activation energy is the minimum energy needed to start any reaction - think of it as the energy hill that reactants must climb before they can react.

Memory Aid: Exo = Exit (energy leaves the reaction), Endo = Enter (energy enters the reaction from surroundings).