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A-Level Trends in the elements across a period and down a group
What the A-Level syllabus expects for Trends in the elements across a period and down a group, and how to practise it.
What the syllabus expects
- Spot how electron configurations shift as one moves along a period and descends a group.
- Give a qualitative description and explanation of how electronegativity, first ionisation energy, and both the atomic and the ionic radius vary:
- moving across a period, set against shielding and rising nuclear charge
- moving down a group, set against added electron shells, shielding and nuclear charge
- Account for how melting point and electrical conductivity change across a period by pointing to whether the element is metallic, giant molecular or simple molecular.
- Explain how volatility changes down Group 17, tracing it to instantaneous dipole-induced dipole attraction.
- Give and account for how the top oxidation number shifts along the period's oxides, namely Na2O, MgO, Al2O3, SiO2, P4O10 and SO3, and along its chlorides NaCl, MgCl2, AlCl3, SiCl4 and PCl5.
- Using electronegativity, explain how the bonding character changes across those oxides and chlorides.
Scope: AlCl3 is the exception - Set out what happens when each oxide, Na2O, MgO, Al2O3, SiO2, P4O10 and SO3, meets water.
- Describe and account for whether these oxides and the hydroxides NaOH, Mg(OH)2 and Al(OH)3 act acidic or basic, noting amphoteric cases that react with both acids and sodium hydroxide.
Scope: the amphoteric test uses sodium hydroxide only - Explain what each chloride, NaCl, MgCl2, AlCl3, SiCl4 and PCl5, does on contact with water.
- From how these oxides and chlorides behave chemically and physically, propose their underlying structure and bonding.
- Using E-standard values, describe and work out the comparative reactivity of:
- Group 2 metals in the role of reducing agents
- Group 17 elements in the role of oxidising agents
- Describe and account for the thermal-stability trend of:
- Group 2 carbonates, tying it to the cation's charge density and how readily the large anion is polarised
- Group 17 hydrides, tying it to bond energies
- Lean on periodic trends to forecast the typical properties of an element that sits in a given group.
- From supplied physical and chemical clues, work out an unknown element's character, where it probably falls in the table, and which element it is.
How it's examined
Questions on this topic most often ask you to explain, describe, outline, state. About 6% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
Describe and account for the way thermal stability changes across the hydrogen halides HCl, HBr and HI, including an equation for the decomposition reaction.
Show the worked answer
Thermal stability decreases from HCl to HBr to HI (HCl > HBr > HI). Going down the group the halogen atom gets larger, so the H-X bond length increases and bond overlap becomes poorer; the H-X bond enthalpy falls (H-Cl ~431 > H-Br ~366 > H-I ~299 kJ/mol). A weaker bond is broken more easily on heating, so HI is the least thermally stable and decomposes most readily. Decomposition equation (e.g. for HI): 2HI(g) -> H2(g) + I2(g).
Example 2 (3 marks)
With reference to section 24 of the data booklet, account for why acidified aqueous potassium iodide develops a brown colour after being left open to the air for some time.
Show the worked answer
The brown colour is due to iodine, I2, being produced. Oxygen from the air oxidises the iodide ions in acidic solution. Relevant standard electrode potentials: O2 + 4H+ + 4e- -> 2H2O, E = +1.23 V; I2 + 2e- -> 2I-, E = +0.54 V. Since E(O2/H2O) > E(I2/I-), Ecell = 1.23 - 0.54 = +0.69 V (positive), so O2 is able to oxidise I- to I2. Overall: O2 + 4H+ + 4I- -> 2I2 + 2H2O.
Example 3 (5 marks)
When a hot glass rod is thrust into a gas jar containing gaseous HI, a violet vapour appears. Carrying out the same experiment with HBr produces no visible change. Explain these observations.
Show the worked answer
The hot rod supplies enough energy to decompose the hydrogen halide: 2HX -> H2 + X2. The violet vapour with HI is iodine vapour (I2), showing that HI has been thermally decomposed to give I2. With HBr, no visible change (no brown/orange Br2 vapour formed appreciably) means HBr is not readily decomposed under the same conditions. The reason lies in bond strength / thermal stability, which follows the trend down Group 17. The H-X bond strength decreases from HCl to HBr to HI because the halogen atom gets larger, the bonding electrons are further from the nucleus, so the bond is longer and weaker. HI has the weakest H-X bond, so it is the least thermally stable and is readily decomposed by the hot rod, liberating violet I2 vapour. HBr has a stronger H-Br bond (more thermally stable), so it is not decomposed by the hot rod and no coloured vapour appears.
More worked questions on this topic
- Group 2 oxalates, MC2O4, follow a thermal-stability trend similar to that of Group 2 carbonates (3 marks)
- Nitrogen sits directly above phosphorus in the Periodic Table. Outline how an oxide of phosphor (3 marks)
- The oxides across Period 3 show a bonding trend that parallels that of the Period 3 chlorides. (3 marks)
- Account for why the Pauling electronegativity of sodium (0.93) differs from that of chlorine (3 (2 marks)
- Account for the decrease in atomic radius on moving across period 3 from sodium through to chlo (2 marks)
- Al3+ has an ionic radius of 0.050 nm, whereas Ti2+ has one of 0.086 nm. Explain why these two i (2 marks)
- With reference to section 24 of the data booklet, deduce how the reducing power of the group 17 (2 marks)
- The thermal-stability trend of the Group 2 ethanoates parallels that seen for the Group 2 carbo (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · How atoms bond and how that governs a substance's behaviour · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · The mole and reacting-quantity calculations · Enthalpy, entropy and the feasibility of reactions · all of A-Level H2 Chemistry