3.7. Covalent Bonds

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Ions in ionic compounds bind to each other with ionc bonds. Metal atoms in metals bind to each other with metal bonds. But there are other things in this world: Water, gases, plants, animals, humans – you and me. How do the rest of the atoms bind to each other?

A few of the elements form diatomic molecules (they come "two by two", like H2, O2 or Cl2). Why is that? The short answer is: Because how they form covalent bonds.

These elements form diatomic molecules (although it is uncertain if astatine and tennessine really do).These elements form diatomic molecules (although it is uncertain if astatine and tennessine really do).

How covalent bonds form

In chemical reactions: Atoms often "strive" to get noble gas structure.

For hydrogen, three alternatives:

  1. Take up an electron, form a hydride ion, H:

    H + e → H
  2. Give off an electron, form a hydrogen ion, H+:

    H → H+ + e
  3. "Borrow" an electron from another atom: Form a covalent bond.

How a hydrogen molecule forms. Energy is always released when bonds form.How a hydrogen molecule forms. Energy is always released when bonds form.

One hydrogen atom may "borrow" an electron from another hydrogen atom. The two hydrogen atoms then share an electron pair ⇒ they both get noble gas structure ("full" outer shell, like helium, He).

  • The bond is covalent, which means the two atoms share the electrons equally between them.

The hydrogen molecule, H2

H2 Molecular formula

H:H

Lewis dot structure

  • The dots between the atoms represent the two electrons shared by the two atoms.

H–H

Structural formula

  • The dash between the atoms represent the electron pair bond between the two atoms.

Single, double and triple bonds

The chlorine molecule, Cl2

Lewis dot structure of a chlorine molecule.Lewis dot structure of a chlorine molecule.

The chlorine atom has seven valence electrons ⇒ it needs to "borrow" one to get noble gas structure (a "full" outer shell, with eight valence electrons).

Two chlorine atoms may share an electron pair, each sharing one electron with the other.

  • An electron pair bond (single bond) forms between the chlorine atoms.
Cl2 Molecular formula

Cl–Cl

Structural formula

  • A single bond.

The oxygen molecule, O2

Lewis dot structure of an oxygen molecule.Lewis dot structure of an oxygen molecule.

The oxygen atom has six valence electrons ⇒ it needs to "borrow" two to get noble gas structure (a "full" outer shell, with eight valence electrons).

Two oxygen atoms may share two electron pairs, each sharing two electrons with the other.

  • Two electron pair bonds (a double bond) form between the oxygen atoms.
O2 Molecular formula

O=O

Structural formula

The nitrogen molecule, N2

Lewis dot structure of a nitrogen molecule.Lewis dot structure of a nitrogen molecule.

The nitrogen atom has five valence electrons ⇒ it needs to "borrow" three to get noble gas structure (a "full" outer shell, with eight valence electrons).

Two nitrogen atoms may share three electron pairs, each sharing three electrons with the other.

  • Three electron pair bonds (a triple bond) form between the oxygen atoms.
N2 Molecular formula

N≡N

Structural formula

Bonds in polyatomic ions

Atomic ions – consist of only one charged atom (ion).

Polyatomic ions – consist of two or more atoms, forming a charged particle.

  • Atoms linked with covalent bonds.

The sulfate ion, \(\text{SO}_4^{2-}\)

Lewis dot structure of a sulfate ion. Two extra electrons are needed for all the atoms to get noble gas structure.Lewis dot structure of a sulfate ion. Two extra electrons are needed for all the atoms to get noble gas structure.

Both the sulfur and the oxygen atoms each have six valence electrons ⇒ they need to "borrow" two electrons from another atom to get noble gas structure. For all the atoms to get noble gas structure, two extra electrons must be taken up when the sulfate ion forms. This is why the sulfate ion is 2–.

The carbonate ion, \(\text{CO}_3^{2-}\)

Lewis dot structure of a carbonate ion. Two extra electrons are needed for all the atoms to get noble gas structure.Lewis dot structure of a carbonate ion. Two extra electrons are needed for all the atoms to get noble gas structure.

The oxygen atoms have six valence electrons, and need to "borrow" two to get noble gas structure. Two of the oxyten atoms may borrow two electrons each from the carbon atom.

The carbon atom has four valence electrons, and need to borrow another four from the third oxygen atom to get noble gas structure.

The third oxygen atom needs to take up an additional two electrons to get noble gas structure. This is why the carbonate ion is 2–.