Topic 8
5 subtopics · Cambridge IGCSE Chemistry 0620
Key Idea
Elements are arranged in order of increasing proton number. Periods = horizontal rows (number of electron shells). Groups = vertical columns (number of outer electrons, similar properties).
Explanation
Key features: - 118 elements arranged by proton number. - Periods (horizontal): elements in same period have same number of electron shells. - Groups (vertical): elements in same group have same number of outer electrons and similar chemical properties. - Metals are on the left/centre. Non-metals are on the right. Periodic trends across a period (left to right): - Proton number increases. - Atomic radius decreases. - Metallic character decreases; non-metallic character increases. Periodic trends down a group: - Atomic radius increases (more electron shells). - Reactivity of metals increases (easier to lose outer electrons). - Reactivity of non-metals decreases (harder to gain electrons).
Practice Questions
Which statement correctly describes elements in the same group of the periodic table?
Key Idea
Group I metals (Li, Na, K) are very reactive metals that react vigorously with water and oxygen. Reactivity increases down the group.
Explanation
Properties of Group I metals: - Soft metals, low density (Li, Na, K float on water). - Low melting points (decrease down the group). - 1 outer electron: easily lost, form +1 ions. Reaction with water: Metal + water to metal hydroxide + hydrogen 2Na + 2H2O to 2NaOH + H2 Reactivity trend down Group I: - Li: fizzes steadily. - Na: fizzes vigorously, melts into a ball. - K: catches fire, burns with a lilac flame. Reason: going down the group, outer electron is further from nucleus (more electron shells), less attraction, easier to lose, more reactive.
Practice Questions
Explain why potassium is more reactive than lithium. [2 marks]
Key Idea
Group VII non-metals (F, Cl, Br, I) are reactive non-metals that form -1 ions. Reactivity decreases down the group. More reactive halogens displace less reactive ones.
Explanation
Properties of halogens: - F2: pale yellow gas. Cl2: yellow-green gas. Br2: orange-brown liquid. I2: grey-black solid. - All exist as diatomic molecules (X2). - 7 outer electrons: gain 1 electron to form -1 ions (halide ions). Reactivity trend down Group VII: - Decreases down the group. F is most reactive, I is least reactive. - Reason: going down, atoms are larger, outer shell is further from nucleus, less attraction for incoming electron. Displacement reactions: - A more reactive halogen displaces a less reactive halide ion from solution. - Cl2 + 2KBr to 2KCl + Br2 (Cl displaces Br) - Cl2 + 2KI to 2KCl + I2 (Cl displaces I) - Br2 + 2KI to 2KBr + I2 (Br displaces I) - Br2 + KCl: NO REACTION (Br cannot displace Cl)
Practice Questions
Chlorine water is added to potassium iodide solution. Describe what you would observe and write a balanced equation for the reaction. [3 marks]
Key Idea
Group 0 elements (He, Ne, Ar, Kr, Xe) are unreactive (inert) because they have full outer electron shells.
Explanation
Properties: - All are colourless, odourless gases at room temperature. - Very low reactivity: full outer shells, no tendency to gain or lose electrons. - Monatomic (exist as single atoms, not molecules). - Boiling points increase down the group. Uses: - Helium: balloons, airships (non-flammable, low density). - Neon: advertising signs (glows red-orange). - Argon: filling light bulbs, welding (inert atmosphere).
Practice Questions
Explain why noble gases are unreactive. [2 marks]
Key Idea
Transition elements are found in the d-block of the periodic table. They have characteristic properties: variable oxidation states, coloured compounds, catalytic activity.
Explanation
Properties of transition elements: - High melting points, high densities, hard metals. - Good conductors of heat and electricity. - Variable oxidation states (e.g. Fe: +2 and +3; Cu: +1 and +2; Mn: +2, +4, +7). - Form coloured compounds and ions (e.g. Cu2+ = blue, Fe3+ = yellow/brown, Fe2+ = green, MnO4- = purple). - Act as catalysts (e.g. Fe in Haber process, V2O5 in Contact process, Pt in catalytic converters, MnO2 in H2O2 decomposition).
Practice Questions
State three properties that are characteristic of transition elements, giving one example for each. [3 marks]