The Periodic Table and Group Trends: Chemistry 0620 (Cambridge O Level / IGCSE)

A chemistry outreach stand at a science fair displays a card for a newly-discovered element, T. The card gives only one piece of information: element T is in Group II of the Periodic Table.

Visitors are asked to predict the charge on the ion formed when element T reacts with a non-metal.

Which charge is correct?

A school science club investigates trends down Group I, the alkali metals, using samples of lithium and sodium.

(a) A sample of lithium has a mass of $0.87\ \text{g}$ and a volume of $1.63\ \text{cm}^3$. Calculate the density of this lithium sample, giving your answer to 2 significant figures. [2]

(b) A sample of sodium has a mass of $1.45\ \text{g}$ and a volume of $1.50\ \text{cm}^3$. Calculate the density of this sodium sample, giving your answer to 2 significant figures, and state how density changes going down Group I based on your two results. [3]

(c) The club also times how long an identical-sized cube of each metal takes to react completely with excess water. A lithium cube takes 84 s to react completely, and a sodium cube takes 12 s to react completely.

(i) Calculate how many times faster the sodium cube reacts than the lithium cube. [2]

(ii) Predict whether a same-sized cube of potassium would react faster or slower than the sodium cube. Give a reason for your answer. [2]

In a school laboratory, a technician bubbles excess chlorine gas through $100\ \text{cm}^3$ of aqueous potassium bromide solution containing $23.8\ \text{g}$ of dissolved potassium bromide. The symbol equation for the reaction is:

$\text{Cl}_2 + 2\text{KBr} \rightarrow 2\text{KCl} + \text{Br}_2$

(a) State the type of reaction occurring, and describe the colour change observed in the solution. [2]

(b) Explain why chlorine is able to displace bromide ions from the solution, but iodine could not displace bromide ions from an identical solution. [2]

(c) Calculate the maximum mass of bromine that can be produced from $23.8\ \text{g}$ of potassium bromide.

[$A_r$: K = 39, Cl = 35.5, Br = 80] [4]

A materials science teacher gives the class four unlabelled element samples, W, X, Y and Z, each with a short property card, and asks the class to identify which part of the Periodic Table each element belongs to.

• W: density $8.9\ \text{g/cm}^3$, melting point $1455\,^{\circ}\text{C}$; forms green and blue compounds; used as a catalyst in some industrial reactions.
• X: a soft metal with a low density; reacts vigorously with cold water, forming an alkaline solution and floating on the surface as it reacts.
• Y: a colourless gas that exists as single, separate atoms; does not react with other substances.
• Z: a diatomic non-metal that is a dark grey-black solid at room temperature; reacts with iron wool when gently heated.

(a) State which element, W, X, Y or Z, is a transition element. Give one reason for your answer. [2]

(b) State which element is a Group I (alkali) metal. Give one reason for your answer. [2]

(c) State which element is a Group VII (halogen) element. Give one reason for your answer. [2]

(d) State which element is a noble gas. Give one reason for your answer. [2]

A student is completing a project on patterns in the Periodic Table that go beyond what is covered in class.

Part 1: Group II metals

The student finds these observations for Group II metals reacting with cold water:

(a) Identify the trend in reactivity with cold water going down Group II. [1]

(b) Barium is the next element below strontium in Group II. Predict how barium would react with cold water compared with strontium, giving a reason for your answer. [2]

(c) State the general rule connecting an element's position in the Periodic Table to its properties, which allows this kind of prediction to be made for an element like barium. [1]

Part 2: Iron compounds

The student also examines two iron compounds from the school stock cupboard: iron(II) chloride, $\text{FeCl}_2$, and iron(III) chloride, $\text{FeCl}_3$. A solution of $\text{FeCl}_2$ is pale green, and a solution of $\text{FeCl}_3$ is yellow-brown.

(d) (i) State the oxidation number (charge) of the iron ion in $\text{FeCl}_2$ and in $\text{FeCl}_3$. [2]

(ii) State what these two facts about iron compounds show about the properties of transition elements. [2]

The modern Periodic Table arranges every known element into a single continuous sequence, starting with hydrogen.

Which quantity fixes an element's exact position in this sequence?

A teacher gives four students the electron configurations of four elements, J, K, L and M, and asks each of them to identify that element's position in the Periodic Table.

• J: 2,8,2
• K: 2,8,7
• L: 2,8,8
• M: 2,8,1

(a) State the period and the group of element J, explaining how each is found from its electron configuration. [2]

(b) State the period and the group of element K, explaining how each is found from its electron configuration. [2]

(c) Element L has a full outer electron shell. State the group that L belongs to, and identify whether elements in this group are reactive or unreactive. [2]

(d) State the period and the group of element M, and predict whether M is a metal or a non-metal, giving a reason based on its number of outer-shell electrons. [2]

The reactivity of Group I metals increases going down the group, while the reactivity of Group VII non-metals decreases going down the group. Both trends can be explained by looking at atomic structure.

(a) State what happens to atomic radius (atomic size) going down Group I, from lithium to potassium. [1]

(b) A Group I atom reacts by losing its single outer-shell electron to form a $1+$ ion. Using the ideas of atomic radius and the outer-shell electron's distance from the nucleus, explain why this outer electron becomes easier to lose going down Group I, causing reactivity to increase. [3]

(c) A Group VII atom reacts by gaining one electron into its outer shell to form a $1-$ ion. Using the same ideas, explain why an incoming electron becomes harder to gain going down Group VII, causing reactivity to decrease. [3]

(d) State how the reactivity trend down Group I compares with the reactivity trend down Group VII. [1]

Period 3 of the Periodic Table contains eight elements, arranged left to right in order of increasing proton number: sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorus (P), sulfur (S), chlorine (Cl) and argon (Ar).

(a) State how metallic character changes going across Period 3, from sodium to chlorine. [1]

(b) Sodium (11 protons) has the electron configuration 2,8,1 and chlorine (17 protons) has the electron configuration 2,8,7. Using the number of protons and the number of outer-shell electrons, explain why sodium readily loses its outer electron to form an ion, while chlorine readily gains one electron rather than losing seven. [3]

(c) Explain, in terms of electron configuration, why chlorine shows much stronger non-metallic character than sodium. [2]

(d) Argon, the final element of Period 3, does not readily form ions and is neither classed as a metal nor a non-metal in the usual reactive sense. Suggest why, in terms of its electron configuration. [2]

Neon has the electron configuration 2,8 and helium has the electron configuration 2. Both are noble gases in Group VIII (Group 0), and both are extremely unreactive. Even though a helium atom's outer shell holds only 2 electrons rather than 8.

What is the best explanation for why both neon and helium are unreactive?