The table shows some properties of four Group I elements.
(a) (i) Complete the table by predicting:
● the boiling point of rubidium
● the atomic radius of potassium.
(ii) Describe the trend in the melting point of the Group I elements down the group.
(iii) Deduce the physical state of potassium at 60°C.
Explain your answer.
(b) Caesium is a radioactive element with a proton number of 55.
(i) Define proton number.
(ii) State one industrial use of radioactive isotopes.
(c) Sodium hydride, NaH, reacts with iron(III) oxide.
(i) Balance the equation for this reaction.
\(Fe_2O_3 + 3NaH → …..Fe + …..NaOH\)
(ii) Explain how this equation shows that iron(III) oxide is reduced.
▶️ Answer/Explanation
(a) (i) Boiling point of Rb: 700°C (any value between 675-755°C acceptable). Atomic radius of K: 0.22nm (any value between 0.195-0.245nm acceptable). These values follow the observed trends in Group I elements.
(a) (ii) The melting point decreases down Group I because the metallic bonds weaken as atomic size increases, requiring less energy to overcome.
(a) (iii) Potassium is solid at 60°C because its melting point (63°C) is higher than 60°C.
(b) (i) Proton number is the number of protons in an atom’s nucleus, determining its chemical identity.
(b) (ii) Radioactive isotopes are used in industrial radiography to detect flaws in metal castings and welds.
(c) (i) Balanced equation: \(Fe_2O_3 + 3NaH → 2Fe + 3NaOH\)
(c) (ii) Iron(III) oxide is reduced as it loses oxygen atoms to form elemental iron, characteristic of reduction in redox reactions.
The table shows the mass of some of the ions in a 1000 \(cm^3\) sample of sea water.
(a) Answer these questions using only the information in the table.
(i) State which negative ion has the lowest mass in 1000 \(cm^3\) of sea water.
(ii) Give the formulae of the ions in potassium sulfate.
(iii) Calculate the mass of calcium ions in 200cm3 of this sample of sea water.
mass = ………………………… mg
(iv) A sample of this sea water is evaporated.
State the name of the compound which is present in the greatest quantity when this sample is evaporated.
(v) Give the name of the ion which reacts with aqueous silver nitrate to give a cream precipitate.
(b) The \(B_3O_6^{3–}\) ion can be converted to boric acid, \(H_3BO_3\).
Boric acid is also produced when boron trichloride, \(BCl_3\), reacts with water.
Complete the equation for this reaction.
\(BCl_3 + …..H_2O → H_3BO_3 + …..HCl\)
(c) The symbol of a strontium ion is shown.
\(^{87}_{38} Sr^{2+}\)
Deduce the number of electrons, protons and neutrons in one atom of this strontium ion.
number of electrons …………………………………………………………………………………………………….
number of protons ………………………………………………………………………………………………………
number of neutrons …………………………………………………………………………………………………….
(d) Some isotopes of strontium are radioactive.
(i) Give one medical use of radioactive isotopes.
(ii) The isotope \(^{235}\)U is also radioactive.
State the major use of this isotope of uranium.
▶️ Answer/Explanation
(a) (i) Ans: \(B_3O_6^{3-}\) / metaborate
From the table, metaborate has the smallest mass (5.1 mg) among negative ions.
(ii) Ans: \(K^+\) AND \(SO_4^{2-}\)
Potassium sulfate is \(K_2SO_4\), which dissociates into \(K^+\) cations and \(SO_4^{2-}\) anions.
(iii) Ans: 80 mg
From table, 1000cm³ contains 400mg Ca²⁺. For 200cm³: (400 × 200)/1000 = 80mg.
(iv) Ans: sodium chloride
Na⁺ and Cl⁻ have the highest masses (10,500mg and 19,000mg respectively) in the table.
(v) Ans: bromide
Br⁻ reacts with AgNO₃ to form cream-colored AgBr precipitate (distinct from white AgCl or yellow AgI).
(b) Ans: \(BCl_3 + 3H_2O → H_3BO_3 + 3HCl\)
Balancing the equation: 3 water molecules provide enough H atoms for boric acid and 3 HCl molecules.
(c) Ans:
Electrons: 36 (38 protons – 2 charge)
Protons: 38 (atomic number)
Neutrons: 49 (87 nucleons – 38 protons)
(d) (i) Ans: cancer treatment / tracer
Radioisotopes like Sr-89 are used in bone cancer treatment or as tracers in medical imaging.
(ii) Ans: source of energy / energy production
U-235 is primarily used as fuel in nuclear reactors due to its fissionable properties.