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wise otter.Chemistry
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Chemical Bonding

Topic 1 of 6

What holds particles together?

Ionic, covalent, metallic and dative bonding, with complete electron diagrams.

A-Level 9476 (2026-2027)

All chemical bonds involve attraction between charges

Identify the positive and negative particles before naming the bond.

The electrostatic explanation
BondAttracting particlesWhat not to confuse it with
IonicOppositely charged ions in an extended structure.Electron transfer creates the ions; transfer itself is not the attractive force.
CovalentA shared electron pair and the positively charged nuclei of both bonded atoms.It is not an attraction between two negative electrons.
MetallicA lattice of positive ions and delocalised electrons.The electrons are not confined to individual two-atom bonds.

In NaCl, sodium loses one electron and chlorine gains one, forming Na+ and Cl-. In MgO, magnesium loses two and oxygen gains two, forming Mg2+ and O2-. Both products have full outer shells and zero overall charge. The formula gives an ion ratio; neither solid consists of isolated two-ion molecules.

NaCl and MgO: ions after transfer

The upper row shows Na+ and Cl-; the lower row Mg2+ and O2-. Each bracketed ion has eight outer-shell electrons. Chloride shows seven dots and one transferred cross; oxide six dots and two transferred crosses. Metal ions show eight remaining electrons, all crosses, from the formerly inner shell.

Dots come from the non-metal; crosses come from the metal. Metal cations expose a previously inner shell after losing electrons. The nuclei do not change.

An ionic lattice is held by many attractions in all directions. Larger charge magnitudes and smaller interionic distances strengthen electrostatic attraction. MgO therefore has much stronger lattice attraction than NaCl: its ions have charges of magnitude two rather than one. Compare the whole structure rather than imagining a single Na-Cl covalent bond.

Count shared pairs and every lone pair

A single, double or triple bond contains one, two or three shared pairs.

Dots and crosses track which atom supplied each electron; they do not identify different kinds of electron. A shared pair contributes to the outer-shell count of both atoms. Hydrogen needs a duet in its first shell; the C, N, O and Cl examples below reach octets. The diagram counts electrons and does not necessarily show molecular shape.

Each dot or cross is one outer-shell electron. A pair between atoms is shared; a pair beside one atom is a lone pair.

Hydrogen (H2)

Hydrogen dot-and-cross diagramTwo hydrogen atoms share one pair of electrons: one dot from the left hydrogen and one cross from the right. Each hydrogen has two electrons in its shared first shell.HH
  • Left H
  • Right H
One shared pair. Each hydrogen has a full first shell of two electrons.

Oxygen (O2)

Oxygen dot-and-cross diagramTwo oxygen atoms share two electron pairs, a double bond. The left oxygen contributes six dots and the right six crosses. Each oxygen has two lone pairs as well as the two shared pairs, giving eight electrons around each atom.OO
  • Left O
  • Right O
Two shared pairs form a double bond. Each oxygen also has two lone pairs.

Water (H2O)

Water dot-and-cross diagramOxygen contributes six dots: one in each of two bonds and four in two lone pairs. Each hydrogen contributes one cross. Oxygen has an octet and each hydrogen a duet. The flat layout shows electron accounting; a water molecule is bent.HOH
  • Oxygen
  • Hydrogens
Two shared pairs and two lone pairs around oxygen. Water is bent; this layout only counts electrons.

Methane (CH4)

Methane dot-and-cross diagramCarbon contributes four dots, one to each of four shared pairs. Each of four hydrogens contributes one cross. Carbon has an octet and each hydrogen a duet. Methane is tetrahedral; the flat cross layout does not show its shape.CHHHH
  • Carbon
  • Hydrogens
Four shared pairs, with no lone pair on carbon. Methane is tetrahedral, not flat.

Carbon dioxide (CO2)

Carbon dioxide dot-and-cross diagramCarbon shares two pairs with each oxygen, making two double bonds. Carbon contributes four dots in total. Each oxygen contributes six crosses: two in the shared pairs and four in two lone pairs. All three atoms have octets.OCO
  • Carbon
  • Oxygens
Two double bonds. Each oxygen has two lone pairs; carbon has none.

Ammonia (NH3)

Ammonia dot-and-cross diagramNitrogen contributes five dots: three in shared pairs and two in one lone pair. Each hydrogen contributes one cross. Nitrogen has an octet and every hydrogen a duet. Ammonia is pyramidal, not flat.NHHH
  • Nitrogen
  • Hydrogens
Deduce three shared pairs and one lone pair from nitrogen's five outer electrons. Ammonia is pyramidal.

Only outer-shell electrons are shown. Shared electrons count towards both bonded atoms' outer shells. Dots and crosses identify the source atom, not different types of electron.

Required examples H2, O2, CO2 and CH4, with H2O and NH3 to support the shape discussion. Count lone pairs as well as bonding pairs.

Nitrogen: three shared pairs

The two nitrogen atoms share three electron pairs. Each has one lone pair. Five dots come from the left nitrogen and five crosses from the right nitrogen, giving ten outer electrons in total and an octet around each atom.

N2: a triple bond and one lone pair on each nitrogen.

Chlorine and hydrogen chloride

The first row shows Cl2 with one shared pair and three lone pairs on each chlorine. The second shows HCl with one shared pair and three lone pairs on chlorine. Hydrogen has only two electrons around it.

Cl2 has 14 outer electrons; HCl has 8. Hydrogen does not need an octet.

Ethene: account for twelve outer electrons

Two carbons share two electron pairs. Each carbon also shares one pair with each of two hydrogens. All carbons have octets and all hydrogens duets. Dots come from the left carbon and the two right-hand hydrogens. Crosses come from the right carbon and the two left-hand hydrogens.

Dots: left C and right-hand H atoms. Crosses: right C and left-hand H atoms. The C=C bond contains two shared pairs; each C-H bond contains one.

A dative bond uses two electrons from one donor

Formation differs; after formation it is a covalent shared pair.

In a co-ordinate (dative covalent) bond, one atom supplies both electrons of the shared pair. The donor must have a lone pair, and the acceptor must have an available orbital. An arrow points from donor to acceptor when showing bond formation. It does not mean that the finished bond contains a different kind of electron.

NH3 donates its lone pair to H+

The ammonium ion has four N-H shared pairs and no nitrogen lone pair. The upper N-H pair contains two dots donated by nitrogen. The other three pairs contain one nitrogen dot and one hydrogen cross. The whole structure is bracketed with charge plus one.

NH3 + H+ → NH4+. Five dots from nitrogen and three crosses from the original hydrogens give eight electrons. The added proton contributes no electron.

AlCl3 is electron deficient in its simple monomer description: Al has three shared pairs, only six electrons around it. Two units can associate to make Al2Cl6. A chlorine lone pair from each unit donates to the other aluminium. Two chlorine atoms bridge the Al centres, and each Al then has four shared pairs.

Al2Cl6: two chlorine bridges

Two aluminium atoms and six chlorine atoms form a dimer. Four chlorines are terminal with three lone pairs each. Two bridging chlorines each have two lone pairs and two shared pairs. Each aluminium supplies three crosses to ordinary shared pairs and receives a two-dot donated pair. The complete drawing contains 48 outer electrons.

Dots are chlorine electrons; crosses are aluminium electrons. Each two-dot bond pair was donated by a bridging Cl. This is a flat connectivity diagram, not the actual three-dimensional shape.
Check your understandingDoes NH4+ keep one unusually weak N-H bond because it was formed by donation?Think it through, then reveal the answer
No. Donation describes where a shared pair came from. In the ammonium ion the four N-H bonds are equivalent; there is no permanently labelled weak dative bond.