This part draws together patterns that run through the whole course. It revisits periodic trends and explains them through atomic structure, examines the special properties of transition metals, and consolidates the quantitative skills of stoichiometry that underpin every part of chemistry, from formulae and equations to yield and volumes.
Periodic trends across a period
Moving across a period from left to right, the number of protons increases while electrons are added to the same shell, so the nuclear charge grows but shielding stays roughly constant. As a result the atoms get smaller, because the stronger nuclear pull draws the electron shell inward. Metallic character decreases as elements change from metals on the left to non-metals on the right. The oxides change too, trending from basic (sodium and magnesium oxides) through amphoteric (aluminium oxide) to acidic (sulfur and phosphorus oxides). These changes are not random but follow directly from the steadily increasing nuclear charge acting on a fixed number of electron shells, which is the central idea of periodicity.
Periodic trends down a group
Moving down a group, each successive element has one more occupied electron shell, so atomic size increases and the outer electrons lie further from the nucleus and are better shielded. This affects reactivity in opposite ways for metals and non-metals. Down Group I, the outer electron is more easily lost because it is further from the nucleus, so the metals become more reactive, reacting more vigorously with water. Down Group VII, an incoming electron is captured less strongly for the same reason, so the halogens become less reactive, with chlorine more reactive than bromine and iodine. Recognising that the same change in atomic structure produces opposite reactivity trends in metals and non-metals is an important insight.
Characteristic properties of transition metals
The transition metals form a block between Groups II and III and show distinctive properties that set them apart from the main-group metals. They are typically hard, dense and high-melting, and are much less reactive than Group I metals. They show variable oxidation states, so iron forms both iron(II) and iron(III) compounds and copper forms copper(I) and copper(II). Their compounds and solutions are often coloured, for example copper(II) salts are blue and iron(III) salts are yellow-brown. Many transition metals and their compounds act as catalysts, such as iron in the Haber process and manganese(IV) oxide in the decomposition of hydrogen peroxide. These features make transition metals especially important in industry and biology.
Moles, formulae and equations
Quantitative chemistry rests on the mole, the amount of substance containing the Avogadro constant, 6.02 x 10^23 particles. The number of moles equals mass divided by molar mass, n = m / M, which links the mass you weigh to the number of particles reacting. Empirical formulae are found by converting the mass or percentage of each element to moles and finding the simplest whole-number ratio, while the molecular formula is a whole-number multiple of the empirical formula. A balanced equation gives the mole ratio in which substances react, which is the key to every reacting-mass calculation. These tools are the foundation for working out reacting masses, yields and gas volumes consistently.
Reacting masses, yield and concentration
From a balanced equation you can calculate how much product a given mass of reactant should give. Convert the known mass to moles, use the mole ratio to find moles of the required substance, then convert back to mass with n = m / M. The theoretical yield is the maximum predicted; the percentage yield is the actual yield divided by the theoretical yield times 100, and is always less than 100% because of losses and incomplete reactions. For solutions, concentration in mol/dm3 equals moles divided by volume in dm3, so n = c x V, which is the basis of titration calculations. Mastering these conversions lets you move confidently between masses, moles, volumes and concentrations.
Gas volumes and Avogadro's law
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules, which is Avogadro's law. This means the mole ratio of reacting gases equals the ratio of their volumes, so volumes of gases can be used directly in calculations without converting to mass. At room temperature and pressure the molar volume of any gas is about 24 dm3/mol, so the number of moles of a gas equals its volume divided by 24 dm3/mol. Worked example: the volume of carbon dioxide produced by completely burning 0.10 mol of carbon, C + O2 -> CO2, is 0.10 mol x 24 = 2.4 dm3. This makes gas calculations quick once the molar volume is known.
Key terms
Periodicity
The regular repetition of element properties across periods and down groups.
Nuclear charge
The positive charge of the nucleus that increases across a period and pulls electrons inward.
Shielding
The reduction in the nucleus's pull on outer electrons by inner electron shells.
Atomic size
The radius of an atom; decreases across a period and increases down a group.
Amphoteric oxide
An oxide such as aluminium oxide that reacts with both acids and alkalis.
Transition metal
A d-block metal showing variable oxidation states, coloured compounds and catalytic activity.
Variable oxidation state
The ability of a transition metal to form ions of more than one charge.
Catalyst
A substance, often a transition metal, that speeds a reaction without being consumed.
Mole
The amount of substance containing 6.02 x 10^23 particles, the Avogadro constant.
Empirical formula
The simplest whole-number ratio of atoms of each element in a compound.
Percentage yield
The actual yield divided by the theoretical yield, expressed as a percentage.
Molar volume
The volume occupied by one mole of any gas, about 24 dm3/mol at room conditions.
Avogadro's law
Equal volumes of gases at the same temperature and pressure contain equal numbers of molecules.
Exam technique
Explain periodic trends through nuclear charge, shielding and number of shells, not just the direction of change.
Remember Group I reactivity increases down the group while Group VII reactivity decreases, for the same structural reason.
List the transition metal properties precisely: variable oxidation states, coloured compounds and catalytic activity.
Use n = m / M for solids, n = c x V for solutions, and n = V / 24 for gases at room conditions.
In reacting-mass questions, always convert to moles, apply the mole ratio, then convert back.
State that percentage yield is below 100% because of losses, side reactions and incomplete reactions.
Quick check
Which set of properties is most characteristic of transition metals compared with Group I metals?
Low melting points, single oxidation state and white compounds
High reactivity with water and colourless solutions
Variable oxidation states, coloured compounds and catalytic activity
Very low density and reaction with cold water to give hydrogen
Show answer
Answer: C. Transition metals are distinguished by variable oxidation states, coloured compounds and ions, and their use as catalysts. Group I metals, by contrast, are very reactive, low-density, low-melting and form colourless compounds with a single oxidation state.