Successive ionisation energies: deciding between carbon, silicon and a metal
A student is given the successive ionisation energies of an unknown Period 3 element X, in kJ mol-1: 786, 1577, 3232, 4356, 16 091, 19 805, 23 780, 29 287, 33 878, 38 726, 45 962, 50 502, 235 196, 257 923. The student is also told that X forms a dioxide, XO2, which is a hard solid melting above 1700 °C and is insoluble in water, and a tetrachloride, XCl4, which is a colourless fuming liquid (boiling point 58 °C) that is hydrolysed violently by water. The student concludes that X is carbon, since carbon also has four valence electrons and forms CO2 and CCl4. A second student argues that X must be a metal because XO2 is a solid with a very high melting point. The teacher asks for a decision based on the ionisation energy data, not on the formulae alone, and for an explanation of the two physical properties that the formulae do not reveal.
1. Deduce the group of X from the data. State which feature of the data is decisive and explain it in terms of electron shells and shielding.
Read the worked answer
X is in Group 4 (Group 14).
The decisive feature is the jump between the fourth and fifth ionisation energies: 4356 to 16 091 kJ mol-1, roughly a factor of four, breaking a sequence of fairly steady increases.
Explanation: the first four electrons are removed from the same outer shell (n = 3). Each is shielded by the inner shells and by the other outer electrons, so the effective nuclear charge they feel rises only gradually and the energies increase smoothly. The fifth electron must be taken from the n = 2 shell, which is much closer to the nucleus and is poorly shielded by the outer electrons. Removing it costs far more energy. Four electrons in the valence shell means four valence electrons, so Group 4.
A second, much larger jump appears between the 12th and 13th ionisations (50 502 to 235 196 kJ mol-1), showing that a shell of eight is complete at twelve electrons. The configuration is therefore 2,8,4 - silicon.
2. Use the data to refute the claim that X is carbon, and explain why the properties of XO2 do not support the second student's claim that X is a metal.
Read the worked answer
The ionisation energies are a fingerprint of the element. Carbon's first ionisation energy is 1086 kJ mol-1, well above the observed 786, and carbon's fifth ionisation energy is about 37 800 kJ mol-1, more than double the 16 091 observed here. Carbon's valence electrons are in n = 2, so its fourth ionisation energy is already very high (about 6223 kJ mol-1), whereas X gives up four electrons relatively cheaply. X is therefore a Period 3 element, not carbon.
The properties also contradict both suggestions. CO2 is a simple molecular gas that sublimes at -78 °C; XO2 melts above 1700 °C and is insoluble, which points to a three-dimensional covalent network, not discrete molecules.
A metal would give a conducting solid and a basic oxide that reacts with acids. A hard, insoluble, very high-melting dioxide is characteristic of a network covalent solid, and its acidic behaviour (it reacts with hot concentrated alkali) is consistent with a non-metal. The student is confusing 'solid with a high melting point' with 'ionic or metallic'.
3. Identify X, describe the bonding in XO2 and XCl4, and use that description to explain the difference in melting point and the hydrolysis of XCl4 but not CCl4.
Read the worked answer
X is silicon.
SiO2: giant covalent network. Each silicon atom is covalently bonded tetrahedrally to four oxygen atoms and each oxygen bridges two silicons, giving an extended lattice. Melting requires breaking many strong covalent bonds, hence above 1700 °C and insolubility in water.
SiCl4: simple molecular. Discrete tetrahedral molecules held together only by weak London dispersion forces, so it is a volatile liquid (boiling point 58 °C) despite polar Si-Cl bonds.
Hydrolysis: Si-Cl bonds are polar, leaving silicon electron-poor. Silicon is larger than carbon and has accessible low-energy d orbitals, so it can accept a lone pair from water and expand its coordination number to five, forming a reactive intermediate from which Cl- leaves. This low-energy route makes hydrolysis fast, giving SiO2 and HCl fumes.
CCl4 is inert to water: carbon is small, has no low-energy d orbitals, and its four chlorines shield the carbon, so water cannot approach and no low-energy pathway exists. The C-Cl bond, though polar, is strong and not easily broken, so hydrolysis has a very high activation energy.