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8873 / 2027
Chemical Bonding

Lesson 1 of 3

Bonding and electrons

Build ionic, covalent and donor-pair diagrams.

A-Level 8873, revised syllabus (2026-2027)

  • I can explain the electrostatic basis of bonding.Syllabus

    Syllabus 8873, 2(a). Explain the electrostatic basis of bonding.

  • I can explain bonding using electron diagrams.Syllabus

    Syllabus 8873, 2(b). Explain bonding using electron diagrams.

One idea behind three strong bonds

Identify the positive and negative charges that attract.

All three bond types are electrostatic
BondAttracting particlesExample
IonicPositive and negative ions throughout a latticeNaCl; MgO
CovalentA shared electron pair and the positive nuclei of the bonded atomsH2; Cl2
MetallicPositive metal ions and delocalised electronsCopper

Electron transfer explains ion formation; attraction between the resulting ions is the ionic bond. Sodium transfers one outer electron to chlorine, making Na+ and Cl-. Magnesium transfers two to oxygen, making Mg2+ and O2-. Each compound is neutral overall and extends as a lattice, not a collection of NaCl or MgO molecules.

Sodium chloride

Na (2,8,1) loses one electron. Cl (2,8,7) gains that electron.

Sodium chloride: ions after electron transferA sodium ion with charge plus one and a chloride ion with charge minus one. Each has a full outer shell of eight electrons. The chloride has seven dots from chlorine and one cross transferred from sodium. Sodium's remaining outer shell has eight crosses, which were previously in an inner shell.Na+Cl-
One Na+ for each Cl-. The total charge is zero.

Magnesium chloride

Mg (2,8,2) loses two electrons. Two Cl atoms each gain one.

Magnesium chloride: ions after electron transferOne magnesium ion with charge plus two and two chloride ions, each with charge minus one. All three ions have full outer octets. Each chloride shows seven dots from chlorine and one cross transferred from magnesium. Magnesium's eight remaining outer-shell electrons were previously in an inner shell.Mg2+Cl-Cl-
One Mg2+ for every two Cl- ions: +2 - 1 - 1 = 0.
  • Electron from chlorine
  • Electron from the metal

Each bracket shows the ion's full outer shell. The eight electrons on Na+ and Mg2+ were previously in an inner shell. Dots and crosses track origin; all electrons are the same kind of particle.

These groups show the ion ratio, not separate NaCl or MgCl2 molecules. The ions form an extended lattice.

NaCl shows the one-electron transfer. The additional MgCl2 comparison shows why a 2+ ion needs two singly charged negative ions.

Magnesium oxide: two electrons transferred

Mg2+ has eight outer-shell crosses. O2- has six original oxygen dots and two transferred crosses. Brackets and ion charges distinguish electron transfer from sharing.

The Mg2+ outer shell was previously an inner shell. The 1:1 ion ratio balances +2 and -2.

In a covalent bond, electron density between nuclei attracts both nuclei. A shared pair can come one electron from each atom or both from one atom. Dots and crosses record origin; electrons themselves are identical. In a metal, electrons are delocalised through the structure, so attractions are not restricted to one fixed atom pair.

Check your understanding

Which statement best describes the covalent bond in a chlorine molecule, Cl2?

  1. The attraction between Cl+ and Cl- ions formed by electron transfer
  2. The attraction between a shared pair of electrons and the two Cl nuclei
  3. The attraction between one Cl2 molecule and the Cl2 molecules next to it
  4. The attraction between a lattice of positive ions and delocalised electrons
Show the answer

The attraction between a shared pair of electrons and the two Cl nuclei

All bonds are electrostatic. The shared pair lies between the nuclei and attracts both, holding the atoms together.

Make every electron count

Use shared pairs for bonds and keep the remaining electrons as lone pairs.

Count the total outer electrons first. A single bond uses one shared pair, a double bond two pairs, and a triple bond three. Hydrogen has a duet; the other simple molecules below achieve octets. An electron shared in a bond counts towards both bonded atoms, but is counted only once in the total-electron audit.

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.

The H2, O2, CO2 and CH4 panels cover the required patterns. Water and ammonia additionally prepare the shape discussion.

Nitrogen: three shared pairs

Dots belong to the left atom and crosses to the right. Each mark is one electron. Shared pairs are between atoms; the other pairs are lone pairs.

Outer electrons only. Shapes and spacing are not molecular geometry.

Chlorine: one shared pair

Dots belong to the left atom and crosses to the right. Each mark is one electron. Shared pairs are between atoms; the other pairs are lone pairs.

Outer electrons only. Shapes and spacing are not molecular geometry.

Hydrogen chloride: one shared pair

Dots belong to the left atom and crosses to the right. Each mark is one electron. Shared pairs are between atoms; the other pairs are lone pairs.

Outer electrons only. Shapes and spacing are not molecular geometry.

Ethene: a double bond between the carbons

Each carbon has two C-H shared pairs and two C-C shared pairs. The left carbon supplies four dots and the right carbon four crosses. The two left hydrogens supply crosses and the two right hydrogens dots.

Dots: left carbon and right hydrogens. Crosses: right carbon and left hydrogens. Ethene has 12 outer electrons, six shared pairs, and no lone pairs on carbon.
Check the electron ledger
MoleculeBonding pairsLone pairsTotal outer electrons
H2102
O222 per O12
N231 per N10
Cl213 per Cl14
HCl13 on Cl8
CO242 per O16
CH4408
C2H46012

Check your understanding

How many lone pairs are on each oxygen atom in a correct dot-and-cross diagram of CO2?

  1. Three on each oxygen, beside a C-O single bond
  2. One on each oxygen, beside a C=O triple bond
  3. Two on each oxygen, beside a C=O double bond
Show the answer

Two on each oxygen, beside a C=O double bond

CO2 has 16 outer electrons. Two double bonds use 8. The other 8 form two lone pairs on each oxygen, giving every atom an octet.

When one atom supplies the shared pair

A lone-pair donor needs an electron-pair acceptor.

A co-ordinate or dative covalent bond contains a shared pair supplied entirely by one atom. NH3 has a lone pair on N; H+ has an empty orbital and no electron to contribute. Donation forms NH4+. After formation all four N-H bonds are equivalent; the origin of the pair does not create a permanently weaker bond.

Ammonium: nitrogen donates its lone pair

Five dots represent the nitrogen electrons and three crosses come from the original hydrogens. The upper N-H pair contains two dots because its hydrogen arrived as H+. The whole ion is bracketed with a single positive charge.

NH3 + H+ → NH4+. Eight outer electrons form four shared pairs; the ion has no lone pair on nitrogen.

In gaseous AlCl3 under suitable conditions, an Al atom has only six electrons in its three covalent bonds. Two units join to form Al2Cl6: a Cl from each unit donates a lone pair to the electron-deficient Al in the other unit. Two bridging Cl atoms result; each Al is surrounded by four shared pairs.

Al2Cl6: two chlorine bridges

Two Al atoms each make four bonds. Each has two terminal chlorines and two shared bridging chlorines. Six mixed dot-cross pairs are original bonds; two cross-cross pairs are donated chlorine lone pairs. Terminal chlorines have three lone pairs each; bridging chlorines have two.

Dots: Al electrons. Crosses: Cl electrons. Count 48 outer electrons: 16 in eight bonds and 32 in lone pairs. The flat arrangement shows connectivity, not bond angles.

Check your understanding

Why does NH4+ have eight outer electrons around nitrogen, not nine?

  1. N and four H atoms supply 9, and the 1+ charge means one electron fewer.
  2. Nitrogen loses one of its own 5 electrons before it forms any bonds.
  3. One H atom does not share its electron, so that H is left out of the count.
Show the answer

N and four H atoms supply 9, and the 1+ charge means one electron fewer.

5 + 4 = 9 electrons from the neutral atoms, minus one for the positive charge, gives 8. Or: NH3 supplies 8 and H+ supplies none.