Ch 4 · Covalent bonding and coordinate (dative covalent) bonding · AS & A Level Chemistry 9701
Topic 4 of 7 in Chemical bonding — AS & A Level Chemistry 9701 — 68 lessons.
Introduction
Covalent bonding underlies most of chemistry, but Cambridge examiners test it at several levels: the electrostatic definition, structures for named molecules, expanded octets, dative bonding, and the orbital picture of sigma and pi bonds with hybridisation. Each idea must be explained precisely, since marks are awarded for specific wording.
Key ideas — Covalent bonding in simple molecules
Covalent bonding is the electrostatic attraction between two nuclei and a shared pair of electrons, formed when each atom contributes one electron to the pair. In H_2, Cl_2 and N_2, identical atoms share one, one and three pairs of electrons, giving single, single and triple bonds. In O_2, oxygen atoms share two pairs (a double bond); in CO_2, carbon forms two double bonds, one to each oxygen; in NH_3, nitrogen shares three pairs with three hydrogens and keeps one lone pair. Each atom attains a full outer shell, usually a stable octet (or two electrons for hydrogen).
🧑🏫 Teacher's explanation
Imagine two people playing tug-of-war with a rope, but instead of trying to pull the rope away, they both hold on tightly to keep it centered. In chemistry, the 'rope' is a pair of electrons, and the 'people' are the positive nuclei of the atoms. Because both nuclei are pulling on the same negative electrons, they are attracted to each other, creating a stable bond.
📖 Definitions
🔍 Worked example — Easy
Calculate the total bond energy required to break all bonds in one mole of H_2 molecules, given the H-H bond energy is 436 kJ mol^-1.
🔍 Worked example — Standard
In the formation of the ammonium ion, NH_3(g) + H^+(g) arrow NH_4^+(g), the bond energy of the new N-H bond is 391 kJ mol^-1. If the energy required to break the H-H bond is 436 kJ mol^-1, calculate the enthalpy change (Δ H) for this reaction.
🔍 Worked example — Exam-style
Compare the reactivity of N≡ N and N=N. Given the bond enthalpy of N≡ N is 945 kJ mol^-1 and N=N is 418 kJ mol^-1, which is more reactive and why?
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🎯 Exam question
Key ideas — Expanded octets in Period 3 elements
Elements in Period 3 and beyond have accessible 3d orbitals, so they can hold more than eight electrons in their outer shell — an expanded octet. In SF_6, sulfur forms six bonds to fluorine (twelve electrons); in PCl_5, phosphorus forms five bonds to chlorine (ten electrons); in SO_2, sulfur bonds to two oxygens while keeping a lone pair, again exceeding eight electrons. Period 2 elements such as nitrogen and oxygen cannot expand their octet, since they have no accessible d orbitals.
🧑🏫 Teacher's explanation
Imagine you are organizing a dinner party. Most guests (Period 2 elements like Carbon or Nitrogen) have exactly eight chairs around their table; they cannot fit anyone else without making the seating awkward. However, Period 3 elements like Sulfur or Phosphorus are like larger banquet halls. Because they have an extra floor (the 3d subshell) available, they can accommodate more guests than the standard eight-chair rule suggests.
📖 Definitions
🔍 Worked example — Easy
Calculate the total number of valence electrons shared in a molecule of SF_6.
🔍 Worked example — Standard
Explain why NCl_5 does not exist, whereas PCl_5 does.
🔍 Worked example — Exam-style
In SO_2, sulfur is the central atom. If sulfur uses its valence electrons to form two bonds with oxygen and retains one lone pair, calculate the total number of electrons in the valence shell of the sulfur atom.
⚠️ Common errors
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🎯 Exam question
Key ideas — Coordinate (dative covalent) bonding
A coordinate (dative covalent) bond is a covalent bond in which both shared electrons are provided by the same atom. In the ammonium ion, NH_4^+, the lone pair on nitrogen in NH_3 is donated into the empty orbital of a proton, H^+, forming a fourth N-H bond identical to the other three once formed. In Al_2Cl_6, each AlCl_3 unit is electron-deficient at aluminium; a lone pair on a chlorine atom of one unit is donated into the empty orbital on the aluminium of the other, forming two dative bonds that bridge the units into a dimer.
🧑🏫 Teacher's explanation
Imagine you are playing a game of doubles tennis. In a standard covalent bond, both players contribute one ball to a shared basket. In a coordinate (dative) covalent bond, one player brings both balls, and the other player simply provides the basket to hold them. The 'bond' is the shared container, but the 'source' of the electrons is singular.
📖 Definitions
🔍 Worked example — Easy
In the formation of the ammonium ion, NH_3 reacts with H^+. If NH_3 has one lone pair and H^+ has an empty orbital, describe the nature of the resulting N-H bonds in NH_4^+.
🔍 Worked example — Standard
Calculate the total number of electrons involved in the formation of one Al_2 extCl_6 dimer from two AlCl_3 units, assuming each Al-Cl bond is formed by one electron from each atom, and two dative bonds are formed by donating lone pairs from Cl.
🔍 Worked example — Exam-style
A molecule has a bond energy of 410 kJ/mol for a standard covalent bond and 380 kJ/mol for a dative covalent bond. If a reaction involves breaking one dative bond, calculate the energy required for 2.5 moles.
⚠️ Common errors
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🎯 Exam question
Key ideas — Sigma and pi bonds from orbital overlap
A sigma (σ) bond forms by direct, end-on overlap of two atomic orbitals along the axis joining the nuclei, giving electron density concentrated along that axis. Every single bond is a sigma bond. A pi (π) bond forms by sideways overlap of two parallel p orbitals, giving electron density in two lobes above and below the bond axis, with a node along the axis. A double bond is one sigma plus one pi bond; a triple bond is one sigma plus two pi bonds, from two perpendicular pairs of p orbitals.
🧑🏫 Teacher's explanation
Think of a covalent bond like two people holding hands. A sigma (σ) bond is like a firm, direct handshake where your arms are straight and pointing directly at each other; this is the strongest, most direct connection. A pi (π) bond is like two people standing side-by-side and linking arms; it is a secondary connection that happens 'above and below' the main line of the handshake.
📖 Definitions
🔍 Worked example — Easy
Identify the number of sigma (σ) and pi (π) bonds in an ethene molecule, C_2H_4, given its structure is H_2C=CH_2.
🔍 Worked example — Standard
An organic molecule contains a triple bond between two carbon atoms. How many sigma and pi bonds are present in this specific triple bond?
🔍 Worked example — Exam-style
Compare the bond strength and bond length of a single bond (e.g., Cl-Cl) versus a double bond (e.g., O=O). Which has a shorter bond length and why?
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🎯 Exam question
Key ideas — Hybridisation: sp, sp2 and sp3
Hybridisation is the mixing of atomic orbitals on one atom into a new set of equivalent hybrid orbitals suited to the observed bond angles. An sp3 carbon mixes one s and three p orbitals into four orbitals arranged tetrahedrally (109.5°), as in methane. An sp2 carbon mixes one s and two p orbitals into three orbitals arranged trigonally (120°) in a plane, leaving one unhybridised p orbital for a pi bond, as in ethene. An sp carbon mixes one s and one p orbital into two orbitals arranged linearly (180°), leaving two unhybridised p orbitals for two pi bonds, as in ethyne.
🧑🏫 Teacher's explanation
Imagine you have a set of different colored building blocks: one large red sphere (the s-orbital) and three long blue rods (the p-orbitals). If you want to build a perfectly symmetrical pyramid, you can't just use the different shapes as they are. You have to melt them down and reshape them into four identical, smaller hybrid blocks. This is what an atom does to prepare for bonding.
📖 Definitions
🔍 Worked example — Easy
A carbon atom in methane (CH_4) undergoes sp^3 hybridisation. How many hybrid orbitals are formed and what is the observed bond angle?
🔍 Worked example — Standard
Compare the hybridization of carbon in ethene (C_2H_4) and ethyne (C_2H_2).
🔍 Worked example — Exam-style
Determine the number of σ and π bonds in a molecule where carbon is sp^2 hybridized and forms a double bond with another carbon atom.
⚠️ Common errors
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🎯 Exam question
(a) State the hybridization of each carbon atom. [1] (b) Describe the type of orbital overlap that forms the π component of the triple bond. [2] [3 marks]
Key ideas — Bond energy, bond length and reactivity
Bond energy is the energy needed to break one mole of a given covalent bond in the gas phase. Bond length is the internuclear distance at minimum potential energy. A shorter bond is generally stronger, with a higher bond energy, since the shared electron density sits closer to both nuclei; multiple bonds are shorter and have higher bond energies than single bonds between the same elements. Molecules with a low-energy bond, such as C-I, tend to react more readily, since less energy breaks that bond; molecules with high-energy bonds, such as N_2, are comparatively unreactive.
🧑🏫 Teacher's explanation
Imagine two people holding onto a heavy rope. The closer they stand to each other, the harder it is for someone to pull them apart. In chemistry, the nuclei are the people and the shared electrons are the rope. The distance between the nuclei is the 'bond length', and the effort needed to snap that rope is the 'bond energy'.
📖 Definitions
🔍 Worked example — Easy
Calculate the bond energy of the H-Cl bond using the following data: E(H-H) = 436 kJ mol^-1, E(Cl-Cl) = 243 kJ mol^-1, and E(H-Cl) = 431.5 kJ mol^-1. Use the reaction: H_2(g) + Cl_2(g) arrow 2HCl(g).
🔍 Worked example — Standard
A molecule has a bond length of 1.54 AA and another has 1.34 AA. Which bond is likely to have a higher bond energy?
🔍 Worked example — Exam-style
In the reaction CH_4 + Cl_2 arrow CH_3Cl + HCl, identify the bond broken and the bond formed.
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Describe, in terms of orbital overlap and hybridisation, the bonding between the two carbon atoms in ethene, CH_2=CH_2. [4]
🔍 Worked example — Easy
Calculate the number of electrons involved in the double bond of ethene, C_2H_4.
🔍 Worked example — Standard
Compare the bond lengths of a single C-C bond (e.g., in ethane, C_2H_6) and a double C=C bond (e.g., in ethene, C_2H_4).
🔍 Worked example — Exam-style
Describe the hybridization and orbital arrangement in ethene, CH_2=CH_2.
⚠️ Common errors
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Practice
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