Identifying a Period 3 Metal from Ionization Energy Data
During a practical session at a Castilla y León university, students receive a sealed vial labelled X. The label states only that X is a period-3 element. The successive ionization energies of X, in kJ mol^-1, are IE1 = 578, IE2 = 1817, IE3 = 2745, IE4 = 11578, IE5 = 14830 and IE6 = 18376. A 0.270 g sample of X reacts completely with hydrochloric acid, and the resulting chloride is titrated with AgNO3; the titration accounts for 0.0300 mol of chloride ions. The chloride XCl3 is a white solid that sublimes at about 180 °C and dissolves in hexane. The oxide X2O3 is insoluble in water but dissolves in both 2 M HCl and 2 M NaOH. The students must decide whether X behaves as a typical metal and whether its chloride is ionic. They also have samples of Mg and Si for comparison.
1. Use the ionization-energy data and the titration result to identify X. State its group and period, and explain why there is a large jump between IE3 and IE4.
Read the worked answer
The jump from IE3 = 2745 kJ mol^-1 to IE4 = 11578 kJ mol^-1 is about 8833 kJ mol^-1. Such a jump means that the fourth electron must be removed from a much less shielded inner shell, so X has only three valence electrons. A period-3 element with three valence electrons is in group 13, so X is aluminium.
The titration supports this: 0.0300 mol of Cl^- corresponds to 0.0100 mol of XCl3, because each formula unit contains three chloride ions. Hence n(X) = 0.0100 mol and M(X) = 0.270 g / 0.0100 mol = 27.0 g mol^-1, which is the molar mass of Al.
Electron configuration: [Ne] 3s^2 3p^1. The three valence electrons are removed relatively easily; the fourth would come from the n = 2 shell, much closer to the nucleus, so IE4 is far larger.
2. Explain why AlCl3 sublimes at a relatively low temperature and dissolves in hexane, whereas Al2O3 is an insoluble, high-melting amphoteric oxide.
Read the worked answer
AlCl3 is not a simple ionic lattice. The small, highly charged Al^3+ ion polarizes the large Cl^- ion strongly, so the bonding has substantial covalent character. In the solid and vapour, AlCl3 exists as molecular species (often the dimer Al2Cl6), held together by weak London dispersion forces. These weak intermolecular forces explain the low sublimation temperature and the solubility in nonpolar hexane.
Al2O3 has a giant lattice with strong ionic/covalent interactions throughout, so it is hard and high-melting and does not dissolve in water or hexane. Its amphoteric behaviour comes from its ability to react with both acids and bases: with HCl it gives Al^3+ (or [Al(H2O)6]^3+), and with NaOH it gives aluminate, [Al(OH)4]^-.
3. Arrange Mg, Al and Si in order of atomic radius and in order of first ionization energy. Explain any order that is not a simple trend across the period.
Read the worked answer
Atomic radius: Mg > Al > Si. Across period 3, the nuclear charge increases while electrons enter the same n = 3 shell, so the effective nuclear charge rises and the radius contracts.
First ionization energy: Si > Mg > Al. The expected smooth increase is broken at Al. Al has the configuration [Ne] 3s^2 3p^1; its first electron is removed from the higher-energy, more shielded 3p orbital, so IE1(Al) is lower than IE1(Mg), where a 3s electron is removed from a filled subshell. Si has a higher IE1 than both because its 3p electron is less shielded than Al's and the nuclear charge is greater.