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10.5Chapter 10: Structure, Bonding, and Properties

Covalent bonding — Alhussan Chemistry - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 5 of 9 2 quizzes

📚 Alhussan Chemistry - Stage 9 Hub / Topical Revision
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10.5 Covalent bonding — Sharing Electron Pairs in Molecules — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define a covalent bond and draw dot-and-cross diagrams to show shared electron pairs in simple molecules.Covalent bond = a shared pair of electrons that jo…

  • Dots and crosses
  • Outer shells only
  • Double bond
  • Number of bonds
  • Structural formulae
  • Compounds of non-metals

Part 1 of 6What a covalent bond is

Covalent bond
Covalent bond a shared pair of electrons that joins two atoms together.
  • It forms when two non-metal atoms share electrons so that each gains a full outer shell.
  • The shared pair is attracted to both nuclei, which holds the atoms together.
Molecule
Molecule a particle made up of two or more atoms strongly joined together.
  • Examples include water, carbon dioxide, methane, ammonia and the diatomic elements H2, Cl2 and O2.

Part 2 of 6Why atoms form covalent bonds

Nitrogen
  • Nitrogen its atom has five outer electrons and needs three more to fill the outer shell with eight, which is stable.
Hydrogen
  • Hydrogen its atom has one outer electron and needs one more to fill its shell.
Ammonia
  • Ammonia nitrogen and hydrogen reach full outer shells by sharing electrons, and each shared pair is one covalent bond.
  • Its formula is NH3, with three hydrogen atoms for every nitrogen atom.
  • Fish release ammonia as a waste product of digestion, as a gas through the gills, and it has a bad smell.

Part 3 of 6Drawing dot-and-cross diagrams

1
Write the electron configuration of each atom to find how many outer electrons it has.
2
Decide how many electrons each atom needs to complete its outer shell.
3
Draw the atoms with outer shells overlapping where a bond forms, and put one dot and one cross in each overlap.
4
Add the remaining outer electrons as pairs on each atom.
5
Check every atom except hydrogen has eight electrons around it.
Dots and crosses
  • Dots and crosses electrons from one atom are shown as dots and from the other as crosses, but all the electrons are the same.
Outer shells only
  • Outer shells only inner shells are not drawn.
MoleculeFormulaBonds
HydrogenH21 single bond
WaterH2O2 single bonds, 2 lone pairs on O
AmmoniaNH33 single bonds, 1 lone pair on N
MethaneCH44 single bonds
OxygenO21 double bond
Carbon dioxideCO22 double bonds
Double bond
Double bond two shared pairs of electrons.
Lone pair
Lone pair a pair of outer electrons not used in bonding.
Hydrogen
  • Hydrogen it needs only two electrons in its outer shell, whereas the others need eight.

Part 4 of 6Predicting formulae

Number of bonds
  • Number of bonds an atom makes as many covalent bonds as the electrons it needs to complete its outer shell.
  • Hydrogen and chlorine make one bond, oxygen two, nitrogen three and carbon four.
Examples
  • Examples carbon with hydrogen gives CH4, nitrogen with hydrogen gives NH3, and water has oxygen bonded to two hydrogens.
Structural formulae
  • Structural formulae a line represents a shared pair, so H-O-H is water and O=C=O is carbon dioxide.

Part 5 of 6Which substances have covalent bonds

Compounds of non-metals
  • Compounds of non-metals most exist as molecules in which every atom has a share in a full outer shell,
    • such as methane (CH4) and hydrogen sulfide (H2S).
Non-metal elements
  • Non-metal elements helium, neon, argon, krypton and xenon exist as single atoms.
  • Most other non-metal elements exist as molecules,
    • such as hydrogen (H2) and chlorine (Cl2),
    • where each atom contributes one electron to the shared pair.
Ionic or covalent
  • Ionic or covalent ionic bonds form in compounds of a metal with a non-metal,
    • and covalent bonds form in compounds of non-metals and in non-metal elements.

Part 6 of 6Properties of simple covalent substances

Properties of simple covalent substances
Bonds
  • Bonds the covalent bonds inside molecules are strong, but the forces between molecules are weak.
Melting and boiling points
  • Melting and boiling points little energy is needed to separate the molecules, so they are low.
Conductivity
  • Conductivity they do not conduct electricity because they have no free charged particles.

Key terms

TermMeaning
Covalent bonda shared pair of electrons between two atoms.
Moleculea small group of atoms joined by covalent bonds.
Double bondtwo pairs of electrons shared between two atoms.
Lone paira pair of outer electrons not used in bonding.
Dot-and-cross diagrama diagram showing outer electrons as dots and crosses.
Diatomicmade of molecules containing two atoms.

Avoid these mistakes

1
Saying covalent bonds form between metals and non-metals — that is ionic bonding.
2
Forgetting the lone pairs on oxygen in water — oxygen has two lone pairs.
3
Thinking the weak forces between molecules are the covalent bonds — the covalent bonds are inside the molecules.
4
Drawing eight electrons around hydrogen — hydrogen has only two.

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