Covalent bonding and dot-and-cross diagramsEdexcel A-Level Chemistry: Flashcards
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Define a covalent bond.
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- Define a covalent bond.
- The strong electrostatic attraction between two nuclei and the shared pair of electrons between them.
- What is a lone pair?
- A pair of outer-shell electrons not involved in bonding.
- How many shared pairs are in N₂?
- Three (a triple bond), with one lone pair on each nitrogen.
- How many lone pairs does each oxygen atom have in CO₂?
- Two, each oxygen being joined to carbon by a double bond.
- What is a dative covalent bond?
- A covalent bond where both shared electrons come from one atom.
- What does an atom need to accept a dative bond?
- An empty orbital: it is electron deficient or is a positive ion.
- How does NH₄⁺ form from NH₃?
- The N lone pair is donated to an H⁺ ion, forming a dative covalent bond.
- Are the four N–H bonds in NH₄⁺ identical?
- Yes: once formed, the dative bond cannot be distinguished from the others.
- How many outer electrons does Al have in AlCl₃?
- Six, so it is electron deficient.
- How does Al₂Cl₆ form?
- A lone pair on a Cl atom of one AlCl₃ is donated to the Al of another, forming a dative bond; this happens at both Al atoms.
- How many dative bonds are in Al₂Cl₆?
- Two, one at each aluminium (eight Al–Cl bonds in total).
- How does bond length change from single to double to triple?
- It decreases, because more shared pairs pull the nuclei closer.
- How does bond strength change from single to triple?
- It increases, because greater electron density gives stronger attraction.
Exam questions on Covalent bonding and dot-and-cross diagrams
- Nitrogen makes up about 78% of the atmosphere and is very unreactive. The two atoms in a nitrogen molecule are joined by a triple bond of length 0.110 nm, whereas a typical nitrogen–nitrogen double bond is 0.125 nm long.Explain why the N≡N bond is shorter and stronger than the N=N bond.2 marks
- Ammonia gas, NH₃, reacts with hydrogen chloride gas to form solid ammonium chloride, NH₄Cl. In the ammonium ion all four N–H bonds are identical in length and strength.Explain how the fourth N–H bond in the ammonium ion forms, and why it is identical to the other three.2 marks
- Aluminium chloride can exist as AlCl₃ molecules in the vapour at high temperature. At lower temperatures the vapour contains Al₂Cl₆ molecules, formed when two AlCl₃ molecules join together. In an AlCl₃ molecule each aluminium atom has only six electrons in its outer shell.Explain how two AlCl₃ molecules join to form Al₂Cl₆.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).