The Periodic Table and Inorganic Chemistry
The periodic table and inorganic chemistry covers physical and chemical trends across Period 3 and down Groups 2 and 7, including atomic radius, melting point, reactions of oxides with water, and reactions of Group 2 and Group 7 elements and their compounds. It draws together ideas about structure, bonding and redox from earlier A Level Chemistry topics.
Before you start
Make sure you're comfortable with these topics first:
Method
- Explain a period or group trend by first stating what happens to nuclear charge, number of electron shells and shielding, then link this to the resulting change in atomic radius or ionisation energy.
- When explaining a Period 3 melting point trend, first identify the type of structure of each element (metallic for Na, Mg, Al; giant covalent for Si; simple molecular for P4, S8, Cl2, Ar) before comparing bond/force strength.
- To predict how a Period 3 oxide reacts with water, remember that oxides of metals on the left (Na2O, MgO) are basic, oxides of non-metals on the right (P4O10, SO3) are acidic, and Al2O3 is amphoteric (reacts with both acids and alkalis).
- For Group 2 reactivity trends, explain that reactivity with water increases down the group because atomic radius and shielding increase, so the outer electrons are more easily lost (lower ionisation energy).
- For Group 7 trends, explain that boiling point increases down the group because the number of electrons per molecule increases, strengthening London (van der Waals) forces, while oxidising power decreases down the group because atomic radius and shielding increase, weakening the attraction for an incoming electron.
- For a halogen displacement or disproportionation question, compare oxidising power (a halogen higher in the group oxidises the halide ion of one lower down) or track oxidation states before and after the reaction to confirm which atoms are oxidised and reduced.
Worked example
Explain why the first ionisation energy of magnesium (738 kJ/mol) is higher than that of sodium (496 kJ/mol), and why the first ionisation energy of silicon (786 kJ/mol) is higher still.
- Compare sodium and magnesium: both outer electrons are removed from the same (3s) sub-shell, but magnesium has one more proton than sodium, giving a greater nuclear charge.
- Shielding is approximately the same for both elements (same number of inner shells), so the increased nuclear charge in magnesium pulls the outer 3s electrons in more strongly.
- This makes the outer electron in magnesium harder to remove than in sodium, giving magnesium the higher first ionisation energy.
- Moving from magnesium to silicon, nuclear charge continues to increase across the period while shielding stays approximately constant, so atomic radius continues to decrease.
- Silicon's outer electron is therefore held more tightly than magnesium's, requiring more energy to remove it.
- Final answer: first ionisation energy increases Na < Mg < Si mainly because nuclear charge increases across the period while shielding stays roughly constant, so atomic radius decreases and outer electrons are held more tightly
Practice questions
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Q1State how atomic radius changes across Period 3 from sodium to argon.Show answer
Answer: It decreases
Q2State the type of structure of silicon.Show answer
Answer: Giant covalent (macromolecular) structure
Q3Write an equation for the reaction of magnesium oxide, MgO, with water, and state the approximate pH of the resulting solution.Show answer
Answer: MgO + H2O -> Mg(OH)2; pH approximately 9-10 (weakly alkaline, as Mg(OH)2 is only sparingly soluble)
Q4Explain why reactivity with water increases down Group 2 from magnesium to barium.Show answer
Answer: Atomic radius and shielding increase down the group, so the outer electrons are further from the nucleus and less strongly held, making them easier to lose, so the element reacts more readily
Q5Explain why the boiling point of chlorine is lower than the boiling point of bromine.Show answer
Answer: Bromine molecules have more electrons than chlorine molecules, so the London (van der Waals) forces between bromine molecules are stronger and need more energy to overcome, giving bromine the higher boiling point
Q6Chlorine water is added to a colourless solution of potassium iodide. Write an ionic equation for the reaction that occurs, and state the colour change observed.Show answer
Answer: Cl2 + 2I- -> 2Cl- + I2; the colourless solution turns brown/orange (as iodine, I2, is formed)
Exam-style questions
Written in the style of a A Level Science exam paper, with a full mark scheme.
Aluminium oxide, Al2O3, reacts with both dilute hydrochloric acid and dilute sodium hydroxide solution, but does not react with water. Write an equation for its reaction with dilute hydrochloric acid, and state and explain the term used to describe an oxide that reacts with both acids and alkalis.
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A student adds excess barium chloride solution to 40.0 cm3 of 0.0500 mol/dm3 sodium sulfate solution. Calculate the mass, in g, of barium sulfate precipitate formed. Mr of BaSO4 = 233.4.
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Describe how you would carry out a chemical test on a solution to confirm the presence of sulfate ions, SO4^2-, including the reagents used, the order of addition, and the expected positive result. Explain why dilute hydrochloric acid is added before the barium chloride solution.
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