Dot-and-cross diagrams

· Dot-and-cross diagrams show the outer-shell electrons involved in bonding.
· Use dots for electrons from one atom and crosses for electrons from another atom.
· Only the valence electrons usually need to be shown unless the question asks otherwise.
· Diagrams must show the difference between ionic, covalent and coordinate bonding.
· CIE may ask for dot-and-cross diagrams of any compounds stated in Topics 3.4 and 3.5.
· Diagrams may include expanded octets or species with an odd number of electrons.

Ionic dot-and-cross diagrams

· Ionic bonding involves electron transfer from a metal atom to a non-metal atom.
· The metal forms a positive ion / cation by losing electron(s).
· The non-metal forms a negative ion / anion by gaining electron(s).
· Show ions in square brackets with the charge outside the bracket, e.g. [Na]⁺ and [Cl]⁻.
· The outer shell of the anion should usually show a full octet.
· Common examples: NaCl, MgO, CaF₂.
· For MgO, Mg loses 2 electrons and O gains 2 electrons.
· For CaF₂, Ca loses 2 electrons and each F gains 1 electron.

Sodium loses one electron to form Na⁺, while chlorine gains one electron to form Cl⁻. The final ions are shown with charges, which is the key convention for ionic dot-and-cross diagrams. Source

Covalent dot-and-cross diagrams

· Covalent bonding involves a shared pair of electrons between two atoms.
· Each single covalent bond = 1 shared pair of electrons.
· A double bond = 2 shared pairs of electrons.
· A triple bond = 3 shared pairs of electrons.
· Show lone pairs as pairs of electrons not used in bonding.
· Hydrogen needs 2 electrons in its outer shell.
· Most Period 2 atoms aim for an octet: C, N, O, F.
· Common examples: H₂, O₂, N₂, Cl₂, HCl, CO₂, NH₃, CH₄, C₂H₆, C₂H₄.
· In CO₂, carbon forms two double bonds: O=C=O.
· In N₂, the nitrogen atoms form a triple bond, with one lone pair on each N.
· In C₂H₄, the two carbon atoms share two pairs of electrons in the C=C bond.

Carbon dioxide contains two C=O double bonds. Each double bond contains two shared pairs of electrons, allowing carbon and oxygen atoms to reach stable outer shells. Source

Lone pairs and exam accuracy

· A lone pair is a pair of outer-shell electrons not involved in bonding.
· Lone pairs must be shown when they affect the final electron arrangement.
· NH₃ has 3 bonding pairs and 1 lone pair on nitrogen.
· H₂O has 2 bonding pairs and 2 lone pairs on oxygen.
· Missing lone pairs is a common reason for losing marks in dot-and-cross questions.
· Check that the total number of outer-shell electrons shown matches the atoms and any charge.

Ammonia has three N–H covalent bonds and one lone pair on nitrogen. This is a useful model for recognising how lone pairs are shown in dot-and-cross diagrams. Source

Coordinate bonding

· A coordinate bond / dative covalent bond is a covalent bond where both electrons in the shared pair come from the same atom.
· Show a coordinate bond using a shared pair where both symbols are from the donor atom, or use an arrow if allowed: donor → acceptor.
· The atom donating the electron pair must have a lone pair.
· The receiving species must be able to accept an electron pair.
· Key CIE examples: NH₄⁺ and Al₂Cl₆.
· In NH₄⁺, NH₃ donates its lone pair on nitrogen to H⁺.
· After formation, all N–H bonds in NH₄⁺ are equivalent, but the diagram should show the origin of the coordinate bond if asked.
· In Al₂Cl₆, chloride atoms can donate lone pairs to electron-deficient aluminium atoms, forming coordinate bonds.

Ammonium forms when ammonia donates a lone pair to H⁺. In a dot-and-cross diagram, the coordinate bond should show both electrons coming from nitrogen. Source

Expanded octets and odd-electron species

· Period 3 elements can sometimes have an expanded octet.
· CIE examples include SO₂, PCl₅ and SF₆.
· PCl₅ has phosphorus bonded to 5 chlorine atoms, so phosphorus has more than 8 electrons around it.
· SF₆ has sulfur bonded to 6 fluorine atoms, so sulfur has an expanded outer shell.
· In expanded-octet diagrams, make sure surrounding halogen atoms still show complete octets.
· CIE may also include species with an odd number of electrons, so not every atom may achieve a perfect octet.
· For odd-electron species, show the unpaired electron clearly.

Sulfur hexafluoride is an important expanded-octet example. Sulfur forms six S–F bonds, so it has more than eight electrons in its outer-shell bonding diagram. Source

Compounds you should be ready to draw

· Simple covalent molecules: H₂, O₂, N₂, Cl₂, HCl.
· Molecules with multiple bonds: CO₂, C₂H₄, N₂.
· Molecules with lone pairs: NH₃, H₂O, HCl, Cl₂.
· Organic covalent examples: CH₄, C₂H₆, C₂H₄.
· Ionic examples: NaCl, MgO, CaF₂.
· Coordinate bonding examples: NH₄⁺, Al₂Cl₆.
· Expanded octet examples: SO₂, PCl₅, SF₆.
· Shape-linked examples from Topic 3.5: BF₃, CO₂, CH₄, NH₃, H₂O, SF₆, PF₅.

How to draw a high-mark dot-and-cross diagram

· Count the outer-shell electrons for each atom.
· Decide whether bonding is ionic, covalent or coordinate.
· For ionic diagrams, show electron transfer, then draw bracketed ions with charges.
· For covalent diagrams, show shared pairs between atoms and lone pairs where needed.
· Use different symbols, usually dots and crosses, to show which atom each electron came from.
· Check charges, octets, duplets for hydrogen, and total electron count.
· For coordinate bonds, make clear that both bonding electrons come from the same atom.
· For expanded octets, do not force the central Period 3 atom to have only 8 electrons.