Silane is the same shape as methane (CH₄); tetrahedral, with bond angles of 109.5°. The Lewis diagram and shape are drawn below. Remember that molecules are three-dimensional, even though we draw the Lewis diagrams in two dimensions on the page.

Phosphine is the same shape as ammonia (NH₃); trigonal pyramidal, with bond angles that might be estimated at approx 107° (although they are lower). The electron domains form a tetrahedral shape, but one bond is missing (it is a lone pair).

The Lewis diagram and shape are drawn below. Remember that molecules are three-dimensional, even though we draw the Lewis diagrams in two dimensions on the page.

Hydrogen sulfide is the same shape as water (H₂O); bent, with bond angles that might be estimated as approx 104.5° (although they are lower). The electron domains form a tetrahedral shape, but two bonds are missing (they are lone pairs).

The Lewis diagram and shape are drawn below. Remember that molecules are three-dimensional, even though we draw the Lewis diagrams in two dimensions on the page.

Boron and beryllium can form stable molecules with incomplete valence shells. Boron in BCl₃ forms only three single bonds with each chlorine; a total of 6 electrons in the valence shell. Beryllium in BeCl₂ forms only two single bonds with each chlorine; a total of 4 electrons in the valence shell.

Boron and beryllium can form stable molecules with incomplete valence shells. Boron in BCl₃ forms only three single bonds with each chlorine; a total of 6 electrons in the valence shell. Beryllium in BeCl₂ forms only two single bonds with each chlorine; a total of 4 electrons in the valence shell.

Thus in BCl₃ and BeCl₂ respectively the shapes and bond angles are trigonal planar, 120° and linear, 180°.

Giant covalent structures are likely to have high melting and boiling points; it takes a lot of energy to break the network of strong covalent bonds. They are also unlikely to conduct electricity in any state (graphite is an exception) as they do not usually have mobile electrons or mobile ions to carry charge.

So Y is the correct answer.

Simple covalent structures are likely to have low melting and boiling points; it does not take much energy to break the relatively weak intermolecular forces between molecules. They are also unlikely to conduct electricity in any state as they do not usually have mobile electrons or mobile ions to carry charge.

So Z is the correct answer.

In diamond the carbon atoms are bonded in a tetrahedral arrangement to 4 other carbon atoms, in a giant covalent structure.

In graphite the carbon atoms are bonded in a trigonal planar arrangement to 3 other carbon atoms, in a giant covalent structure, with carbon hexagon rings, in layers. (The fourth electron from every carbon atom is delocalised between the layers and holds the layers together.)

In buckminsterfullerene the carbon atoms are bonded in a trigonal planar arrangement to 3 other carbon atoms, forming the surface of the C₆₀ 'carbon football' molecule with interlinking carbon hexagons (20) and pentagons (12).

In graphite the carbon atoms are bonded in a trigonal planar arrangement to 3 other carbon atoms, in a giant covalent structure, with carbon hexagon rings, in layers. The fourth electron from every carbon atom is delocalised between the layers and holds the layers together (weakly).

Graphite conducts electricity in the solid state because the delocalised electrons between layers are able to move and carry charge.

Graphite is a good solid state lubricant because the layers of atoms are able to slide over one-another; the London dispersion forces between the layers are weak and easily broken. Individual atoms cannot move independently, but the layers can easily slide off the structure (think of a pencil marking a piece of paper).

The correct answer is therefore: 'The delocalised electrons are free to move between layers: The layers of atoms in the lattice can slide over one-another'.

To be polar, a molecule must have polar bonds (differences in electronegativity) within it and must have asymmetric geometry (molecules will then have an overall dipole moment). If the dipoles of polar bonds cancel out because the molecule is symmetrical, then the molecule will have no overall dipole.

The C–H bonds are not considered to be significantly polar, and in addition, CH₄ and C₂H₆ both have symmetrical geometry.

The C^δ+–Cl^δ– bond is polar. In CCl₄ the dipoles cancel as the molecule is symmetrical, but in CH₃Cl the dipoles will not cancel out and CH₃Cl is therefore polar.

Lewis (dot-cross) diagrams for C₂H₆, OH^– and CO₂ may all be drawn that accurately represent these species in terms of bond length and shape etc. The carbonate ion CO₃^2– is said to exhibit resonance, because the shape of the compound ion with three C–O bonds of equal length cannot be represented by a conventional Lewis diagram. The resonance structures are shown below (showing bonds only for clarity; lone pairs are not shown):

The ozone molecule is said to exhibit resonance, because the shape of the molecule with two O–O bonds of equal length cannot be represented by a conventional Lewis diagram. The resonance structures are shown below (showing bonds only for clarity; lone pairs are not shown). The oxygen atoms form 2: 3: 1 bonds (or vica versa) respectively. The arrow represents a coordinate (dative) bond.