Periodic Trends Worked ExamplesIGCSE Chemistry 0620

(a) Reading the bars: $181$, $98$ and $63\,^{\circ}\text{C}$. The melting point decreases down the group.

(b) The drops are $83\,^{\circ}\text{C}$ then $35\,^{\circ}\text{C}$ — they are getting smaller, so the next drop will be smaller still. A sensible prediction is about $40\,^{\circ}\text{C}$ (any value between about $30$ and $50\,^{\circ}\text{C}$ follows the trend; the actual value is $39\,^{\circ}\text{C}$).

(c) Down Group I, (i) the density generally increases and (ii) the reactivity increases.

(d) Reactivity increases down the group, so the element at the top, lithium, is the least reactive.

(a) The halogens are diatomic non-metals: $\text{Cl}_2$, $\text{Br}_2$ and $\text{I}_2$.

Chlorine is a pale yellow-green gas; bromine is a red-brown liquid; iodine is a grey-black solid. So the state changes from gas to liquid to solid down the group.

(b) Down Group VII the density increases and the reactivity decreases.

(c) Reactivity decreases down the group, so chlorine is the most reactive of the three.

A increases (from $11$ to $17$). B increases (from $1$ to $7$). D stays the same — every Period 3 atom has three occupied shells.

Across a period the elements change from metals (sodium, magnesium, aluminium) to non-metals (silicon to argon), so metallic character decreases. Answer: C.

(a) A substance is a gas if its boiling point is below $25\,^{\circ}\text{C}$, a liquid if its melting point is below $25\,^{\circ}\text{C}$ but its boiling point is above, and a solid if its melting point is above $25\,^{\circ}\text{C}$.

Fluorine and chlorine: both points below the line → gases. Bromine: melting point $-7\,^{\circ}\text{C}$ (below $25$) and boiling point $59\,^{\circ}\text{C}$ (above $25$) → liquid. Iodine: melting point $114\,^{\circ}\text{C}$ → solid.

(b) The melting point rises by roughly $100$–$120\,^{\circ}\text{C}$ per period ($+119$, $+94$, $+121$). Adding about this much to iodine's $114\,^{\circ}\text{C}$ gives an estimate of about $230\,^{\circ}\text{C}$ (any value from about $200$ to $260\,^{\circ}\text{C}$ follows the trend).

(c) Its melting point is well above $25\,^{\circ}\text{C}$, so astatine is a solid. Like every halogen it is diatomic: $\text{At}_2$.

(d) Down Group VII density increases and reactivity decreases: astatine is denser and less reactive than iodine.

(a) Melting point decreases down Group I, so the highest is lithium. The grey-black solid halogen is iodine. Lithium forms $\text{Li}^+$ and iodine forms $\text{I}^-$: the product is $\text{LiI}$.

(b) Reactivity increases down Group I → potassium; reactivity decreases down Group VII → chlorine. The product is potassium chloride.

(c) Chlorine is diatomic, so two potassium atoms react with each molecule.

(d) Reaction (b): it uses the most reactive metal and the most reactive halogen, whereas (a) uses the least reactive metal and the least reactive halogen of those given.

(a) Outer electrons: Mg $2$, Si $4$, S $6$, Ar $8$. Type: Mg and Al are metals; Si, S, Cl and Ar are non-metals.

Ions: metals lose their outer electrons — Mg $2+$, Al $3+$. Non-metals gain electrons to fill the outer shell — S $2-$, Cl $1-$. Argon: none.

(b) The number of outer-shell electrons increases by one from each element to the next ($1$ to $8$).

(c) The elements change from metallic to non-metallic character across the period. Argon already has a full outer shell, so it does not lose or gain electrons.

(a) Sodium is more reactive than lithium. It melts into a ball and moves rapidly around the surface, fizzing more vigorously; it disappears faster. (Lithium floats and fizzes steadily without melting.)

(b) The gas is hydrogen: it "pops" with a lighted splint.

(c) The indicator turns purple (or blue), showing that the solution is alkaline — a hydroxide has formed.

(d)

(e) Reactivity increases down Group I. Rubidium is below potassium, so its reaction would be even more violent — dangerous, possibly explosive.

(a) A more reactive halogen displaces a less reactive halogen from a solution of its halide ions — a colour change shows a reaction. Chlorine displaces both bromine (orange) and iodine (brown); bromine displaces only iodine; iodine displaces neither.

Order: chlorine > bromine > iodine — the same order as the group, so reactivity decreases down Group VII.

(b) Bromine gains electrons from iodide ions; potassium ions are spectators.

(c) Iodine is less reactive than both chlorine and bromine, so it cannot displace them from their halide ions.

(d) Yes — fluorine is above chlorine in Group VII, so it is more reactive and would displace it.

Down Group VII the density increases, the colour of the elements gets darker (pale yellow-green chlorine, red-brown bromine, grey-black iodine) and the reactivity decreases. Iodine is lower than chlorine, so: higher, darker, lower. Answer: C.

D is the most tempting: it has density and colour right but uses the Group I reactivity trend — in Group VII reactivity falls down the group. A reverses the colour trend. B reverses both density and reactivity; only its colour entry is right.

(a) Density increases down the group (from $1.82$ to $9.78\text{ g/cm}^3$).

(b) Metallic character increases down the group: nitrogen and phosphorus are non-metals (do not conduct), arsenic is in between, and antimony and bismuth are shiny and conduct well, like metals.

(c) Continue each trend: moscovium would be a solid, denser than bismuth (more than about $10\text{ g/cm}^3$), a good conductor — a metal.

(d) Basic — bismuth behaves as a metal, and metal oxides are basic (whereas nitrogen, a non-metal, forms acidic oxides).

(a) P and Q: they are silvery (metallic) and react with water — Group I metals react with water to give hydrogen and an alkaline solution. Their melting points and densities are also low, like the alkali metals.

(b) P: it has the lower melting point ($39$ against $63\,^{\circ}\text{C}$), the higher density ($1.53$ against $0.86\text{ g/cm}^3$) and reacts more vigorously — all trends that increase or decrease down Group I in the right direction.

(c) R is a grey-black solid (iodine) and S is a red-brown liquid (bromine). S is higher in Group VII (lower density, lower melting point), and reactivity decreases down the group, so S is more reactive.

(d) Most reactive metal: P. Most reactive halogen: S. $\text{P}^+$ and $\text{S}^-$ give PS; a metal with a non-metal gives ionic bonding.

(e) No: R is iodine, which is less reactive than bromine, so it cannot displace bromide ions.

(a) Convert to $\text{dm}^3$ and use $24\text{ dm}^3$ per mole at r.t.p.

The ratio $\text{M} : \text{H}_2$ is $2:1$.

(b) $A_r = \dfrac{\text{mass}}{\text{moles}}$. $39$ is the $A_r$ of potassium.

(c) The same volume needs the same number of moles of metal: $0.00500$ mol.

(d) The same volume — equal moles of any Group I metal give equal moles of hydrogen (same $2:1$ ratio). The reaction would take longer, because sodium is higher in Group I and so less reactive than potassium.