The Periodic Table and Group Trends: Question 5

Worked solution

Part 1(a): Identifying the trend

Reading down the table: beryllium shows no reaction, magnesium reacts very slowly, calcium fizzes steadily, and strontium fizzes even more vigorously than calcium. Each element reacts more readily with cold water than the one above it.

Reactivity with cold water increases going down Group II.

Part 1(b): Predicting barium’s reaction

Barium sits directly below strontium in Group II. Since reactivity increases going down the group, and strontium already reacts more vigorously than calcium, barium is expected to react even more vigorously than strontium, likely fizzing rapidly, and possibly reacting fast enough to be noticeably more energetic than any of the metals above it.

Part 1(c): The general rule behind the prediction

This kind of prediction is possible because of a general rule of the Periodic Table: the position of an element in the Periodic Table can be used to predict its properties. Elements in the same group show similar chemical behaviour and a consistent trend down the group, so once the trend is known from several members of a group, the properties of a further, less familiar member can be predicted even without testing it directly.

Part 2(d)(i): Oxidation numbers of iron

In $\text{FeCl}_2$, chloride ions each carry a charge of $-1$, and there are 2 of them, giving a total negative charge of $-2$. For the compound to be electrically neutral, the iron ion must have a charge of $+2$.

In $\text{FeCl}_3$, there are 3 chloride ions, giving a total negative charge of $-3$, so the iron ion must have a charge of $+3$.

$\text{FeCl}_2: \text{Fe} = +2 \qquad \text{FeCl}_3: \text{Fe} = +3$

Part 2(d)(ii): What this shows about transition elements

Iron forms two different chlorides with two different oxidation numbers ($+2$ and $+3$) from the same pair of elements, showing that transition elements can form ions with variable oxidation numbers. In addition, the two iron compounds have different colours (pale green for iron(II), yellow-brown for iron(III)), showing that transition elements form coloured compounds.

Final answers

• (a) Reactivity with cold water increases going down Group II.
• (b) Barium would react more vigorously than strontium.
• (c) An element’s position in the Periodic Table can be used to predict its properties.
• (d)(i) Iron is $\boxed{+2}$ in $\text{FeCl}_2$ and $\boxed{+3}$ in $\text{FeCl}_3$.
• (d)(ii) Transition elements have variable oxidation numbers and form coloured compounds.