Question 2 of 7: Chemical Bonding

Question II: Chemical Bonding (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Part (a) — metallic vs. van der Waals bonds. A metallic bond is a PRIMARY bond: the metal's valence electrons are delocalized into a shared "electron gas" that permeates the whole lattice of positive ion cores, non-directionally binding every ion to every neighbour; typical bonding energies are $68$–$850$ kJ/mol (e.g. copper $\approx 323$ kJ/mol, tungsten $\approx 850$ kJ/mol). A van der Waals bond is a SECONDARY bond: a weak electrostatic attraction between induced or permanent dipoles on otherwise electrically neutral atoms/molecules (fluctuating-dipole, polar-molecule, or hydrogen-bond type); typical energies are only $\approx 2$–$10$ kJ/mol (e.g. condensed argon $\approx 7.7$ kJ/mol; between graphite sheets). The metallic bond has the much higher bonding energy of the two. Because melting requires supplying roughly the bond energy to pull the structure apart, the WEAKER bond — van der Waals — is the one that correlates with a lower melting point (solid argon melts at $84$ K, versus copper at $1358$ K).

Part (b) — silica's covalent network. In 100%-covalent SiO₂, every silicon atom is covalently bonded to FOUR oxygen atoms, arranged tetrahedrally around it (the $\text{SiO}_4^{4-}$ tetrahedron, Si–O bond $\approx0.16$ nm); every oxygen atom, in turn, BRIDGES exactly two silicon atoms, so each tetrahedron shares each of its four corners with a neighbouring tetrahedron. This corner-sharing repeats indefinitely in three dimensions, building a continuous, open, three-dimensional covalent network (with an $\text{Si:O}$ atom ratio of $1:2$, since each of the 4 O per tetrahedron is shared between 2 Si, contributing $4\times\tfrac12 =2$ oxygens per silicon). The 2-D schematic below simplifies the true 3-D tetrahedral network to triangles for clarity.

Part (c) — bonding type identification.

• a. NaCl — ionic bonding. Na (electronegativity $\approx0.9$) transfers its single valence electron to Cl (electronegativity $\approx3.0$); the resulting Na⁺/Cl⁻ ions attract electrostatically, non-directionally, in a rock-salt lattice.
• b. CH₄ (methane) — covalent bonding. Each of the four C–H bonds is a shared, directional electron pair (small electronegativity difference, C $\approx2.5$, H $\approx2.1$); the molecules themselves then interact only weakly, by secondary (van der Waals, induced-dipole) forces, which is why methane is a gas at room temperature despite its strong intramolecular covalent bonds.
• c. Polymer chains — covalent bonding (chain backbone) with secondary bonding between chains. The C–C (or C–Si, etc.) backbone of the chain itself is covalent; adjacent chains are held to each other only by much weaker secondary (van der Waals or, in polymers like nylon, hydrogen) bonds, which is why thermoplastics soften well below any covalent-bond-breaking temperature.