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1.7Unit 1. Structure, Bonding and Introduction to Organic Chemistry

Covalent Bonding & Structure — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

15 min read 21 parts Lesson 9 of 36 1 quiz

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1.7 Covalent Bonding & Structure: covalent, dative and octet exceptions — IAL EDEXCEL LINEAR (CHEMISTRY) notes: Describe covalent and dative covalent bonding, draw dot-and-cross diagrams, explain octet exceptions and compare simple molecular with giant covalent structures.Covalen…

  • Double bond
  • Triple bond
  • Dative (coordinate) covalent bond
  • Ammonium ion
  • Aluminium chloride
  • Non-polar bond

Part 1 of 6Covalent bonding

Covalent bond
  • Covalent bond the electrostatic attraction between the nuclei of two atoms and the shared pair of electrons between them.
Covalent bonding occurs between two non-metal atoms.
  • No electrons are transferred; they are shared.
  • Each atom usually provides one electron, and sharing gives each atom a noble-gas-like outer shell.
Bonding happens because electrons are more stable when attracted by two nuclei than by one.
  • When the bond forms, two atomic orbitals overlap to form a molecular orbital.
A single covalent bond is drawn as a short straight line, e.g. H-H.
Shared electrons are not fixed in position; they move constantly and are best treated as a charge cloud.

Part 2 of 6Dot-and-cross diagrams and multiple bonds

Dot-and-cross diagram
  • Dot-and-cross diagram shows the outer (valence) electrons of each atom, with dots for one atom and crosses for the other,
    • and the shared pairs in the overlap.
Double bond
  • Double bond two shared pairs between two atoms.
  • Examples are O2, CO2 and ethene, C2H4.
Triple bond
  • Triple bond three shared pairs between two atoms.
  • An example is N2.
Use the Periodic Table group number to find how many outer electrons a non-metal atom has.

Part 3 of 6Dative covalent bonding

1Dative (coordinate) covalent bond
  • Dative (coordinate) covalent bond a covalent bond in which both shared electrons come from the same atom.
2
It forms when a lone pair is donated to an electron-deficient atom that has an unfilled outer orbital.
  • Once formed, a dative bond is identical to any other covalent bond.
3Ammonium ion
  • NH3 donates its nitrogen lone pair to the electron-deficient H+ to form NH4+.
  • H+ has space for two electrons in its shell.
4Aluminium chloride
  • AlCl3 is electron deficient and needs two more electrons to complete the outer shell of aluminium.
  • At high temperature it exists as the monomer AlCl3.
  • At lower temperature two molecules join as the dimer Al2Cl6, with a lone pair on a chlorine atom of each molecule forming a dative bond to the other aluminium atom.

Part 4 of 6Polarity and the octet rule

Non-polar bond
  • Non-polar bond atoms with the same electronegativity share the electrons equally, as in Cl-Cl.
Polar bond
  • Polar bond atoms with different electronegativities share the electrons unequally.
  • The electrons are drawn towards the more electronegative atom, which becomes δ−, and the other atom becomes δ+.
  • The positive and negative charge centres do not coincide, so the electron distribution is asymmetric.
  • The larger the electronegativity difference, the more polar the bond, until it becomes ionic.
  • In HCl, chlorine is more electronegative, so Cl is δ− and H is δ+.
Expanded octet
  • Expanded octet atoms from Period 3 onwards can hold more than eight outer electrons because their d-orbitals are accessible.
  • PCl5 has 10 electrons around phosphorus.
  • SF6 has 12 electrons around sulfur.
Electron deficient
  • Electron deficient the central atom holds fewer than eight outer electrons.
  • Boron has the outer configuration 2s22p1, so its three electrons are all paired in bonds.
  • BF3 has 6 electrons around boron.

Part 5 of 6Simple molecular structures

Simple molecular structures
Simple molecules such as H2O and N2 contain a fixed number of covalently bonded atoms.
Low melting and boiling points
  • Low melting and boiling points only weak intermolecular forces between molecules are overcome, so little energy is needed.
Mostly insoluble in water
  • Mostly insoluble in water the exception is a polar molecule that forms hydrogen bonds, such as sucrose.
No conduction in the solid or liquid state
  • No conduction in the solid or liquid state there are no charged particles.
  • Some, such as HCl, conduct in solution because they react with water to form ions (H+ and Cl−).
Buckminsterfullerene, C60
  • Buckminsterfullerene, C60 a spherical molecule that is not a giant covalent structure.
  • It melts and boils at higher temperatures than small molecules because its many electrons give larger intermolecular forces.
  • It is an insulator because its delocalised electrons cannot pass between molecules.

Part 6 of 6Giant covalent structures

Giant covalent structures
Giant covalent lattice
  • Giant covalent lattice a huge number of non-metal atoms joined by strong covalent bonds in a fixed ratio.
Very high melting and boiling points
  • Very high melting and boiling points a large number of covalent bonds must be broken, which needs a lot of energy.
Hardness varies.
  • Diamond and silicon(IV) oxide are hard because their 3D network of strong bonds is difficult to break.
  • Graphite is soft because the forces between its layers are weak.
Most are insoluble in water.
Most do not conduct electricity.
  • Diamond and silicon(IV) oxide have all four outer electrons on each atom in bonds, so no electrons are free to move.
  • Graphite has one delocalised electron per carbon, which moves along the layers when a voltage is applied.

Key terms

TermMeaning
Covalent bondThe electrostatic attraction between the nuclei of two atoms and the shared pair of electrons between them.
Dative covalent bondA covalent bond in which both shared electrons come from one atom.
Electron deficientHaving fewer than eight electrons in the outer shell of the central atom.
Expanded octetMore than eight electrons in the outer shell of a central atom from Period 3 or beyond.
Giant covalent latticeA huge network of atoms joined by strong covalent bonds.
Polar bondA covalent bond with unequal sharing of electrons because the atoms differ in electronegativity.

Avoid these mistakes

1
Describing a covalent bond as the attraction between atoms — it is the attraction of both nuclei for the shared electron pair.
2
Thinking all covalent substances have low melting points — giant covalent lattices melt at very high temperatures.
3
Saying C60 is a giant covalent structure — it is a simple molecule with larger intermolecular forces.
4
Saying a dative bond differs from other covalent bonds once formed — it is identical; only the source of the electrons differs.

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