Covalent Bonding (SQA National 5 Chemistry): Revision Note

Covalent bonds

• Non-metal atoms can share electrons with other non-metal atoms to obtain a full outer shell of electrons

• A covalent bond forms when two positive nuclei are held together by their common attraction for a shared pair of electrons

• Covalent bonds between atoms are very strong

• When two or more atoms are covalently bonded together, they form molecules

Formation of a covalent bond

Single bonds

• A single bond is formed when two atoms share one pair of electrons (a total of two electrons)

• For example, chlorine (Cl₂)

  • A chlorine atom is in Group 7, so it has 7 outer electrons

  • To get a full outer shell of 8, it needs to gain 1 electron

  • When two chlorine atoms bond, each atom shares one of its electrons with the other

  • This creates one shared pair in the middle

  • By sharing one pair, each chlorine atom can now 'count' both shared electrons as its own

  • This means each atom has a full outer shell of 8 (2 shared electrons + 6 unshared electrons)

Double bonds

• A double bond is formed when two atoms share two pairs of electrons (a total of four electrons).

• For example, oxygen (O₂)

  • An oxygen atom is in Group 6, so it has 6 outer electrons

  • To get a full outer shell of 8, it needs to gain 2 electrons

  • When two oxygen atoms bond, each atom shares two of its electrons with the other

  • This creates two shared pairs in the middle

  • By sharing two pairs, each oxygen atom can now 'count' all four shared electrons as its own

  • This means each atom has a full outer shell of 8 (4 shared electrons + 4 unshared electrons)

Triple bonds

• A triple bond is formed when two atoms share three pairs of electrons (a total of six electrons).

• For example, nitrogen (N₂)

  • A nitrogen atom is in Group 5, so it has 5 outer electrons

  • To get a full outer shell of 8, it needs to gain 3 electrons

  • When two nitrogen atoms bond, each atom shares three of its electrons with the other

  • This creates three shared pairs in the middle

  • By sharing three pairs, each nitrogen atom can now 'count' all six shared electrons as its own

  • This means each atom has a full outer shell of 8 (6 shared electrons + 2 unshared electrons)

When 2 nitrogen atoms combine they form a molecule and share three pairs of electrons. This is a triple bond.

Covalent bonding diagrams

• A key skill is being able to draw dot-and-cross diagrams to show how the outer electrons are shared in simple molecules

Hydrogen (H₂)

• A hydrogen atom has one outer electron and needs one more to get a full shell of two

• The two hydrogen atoms each share their single electron, forming one shared pair

• This is a single covalent bond

Fluorine (F₂)

• A fluorine atom (Group 7) has seven outer electrons and needs one more for a full shell of eight

• The two fluorine atoms each share one electron, forming one shared pair

• This is a single covalent bond

• All the other elements of Group 7 do exactly the same because they also have 7 electrons in their outer shell

Oxygen (O₂)

• An oxygen atom (Group 6) has six outer electrons and needs two more for a full shell of eight

• The two oxygen atoms each share two electrons, forming two shared pairs

• This is a double covalent bond

Nitrogen (N₂)

• A nitrogen atom (Group 5) has five outer electrons and needs three more for a full shell of eight

• The two nitrogen atoms each share three electrons, forming three shared pairs

• This is a triple covalent bond