3a. Introduction to the covalent bond: What is a covalent bond?

An introduction to bonding in small molecules

Covalent bonds are formed by atoms sharing electrons to form bonds that hold the atoms together in a molecule.

This type of bond usually formed between two non–metallic elements. The molecules might be that of an element i.e. one type of atom only OR from different elements chemically combined to form a compound.

Note: The molecular formula is the summary of all the atoms in a molecule.

The covalent bonding is caused by the mutual electrical attraction between the two positive nuclei of the two atoms forming the bond, and the SHARING of a pair of negative electrons between them.

A COVALENT BOND IS THE SHARING OF A PAIR ELECTRONS BETWEEN TWO ATOMS

It only involves electrons in the outer shell i.e. the outermost energy level containing 1–7 electrons, which can be shared between atoms to form a covalent bond.

One single covalent bond is a sharing of 1 pair of electrons, two pairs of shared electrons between the same two atoms gives a double bond and it is possible for two atoms to share 3 pairs of electrons and give a triple bond.

From the diagram above, and as already stated, a covalent bond is formed by the mutual electrical charge attraction of two positive nuclei and the negative electrons in between.

The two nuclei are the centres of the two atoms bonded together.

The electrons reside where the two energy levels overlap so the electrons are shared in the formation of a covalent bond.

Note: In the examples of covalent bonding it is assumed YOU can work out the electron configuration (arrangement in shells or energy levels) given the atomic number from the Periodic Table.

This kind of bond or electronic linkage, does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why covalent molecules have a particular shape.

In the case of ionic or metallic bonding, the electrical attractive forces act in all directions around the particles involved.

Which electronic structures are the most stable?

because is this what atoms will try to get to electronically!

        symbol, (atomic number) and electron arrangement

When atoms SHARE ELECTRONS in a covalent bond, they try to attain the electron structure (electron configuration) of the electronically very stable atoms of the Group 0 Noble Gases.

The electron structures of helium neon and argon are shown above.

eg helium (2), neon (2.8) or argon (2.8.8), that is, a full outer shell of electrons (full highest energy level).

Hydrogen (1 proton, 1 electron) needs one shared electron to become electronically stable like helium.

Many other elements become stable by sharing electrons to give an outer shell of 8 electrons, like Ne and Ar.

Quite simply, this is because these are the most stable electron arrangements and have a full outer shell of electrons (full highest energy level).

The number of bonds formed depends on the number of electrons that needs to be shared so that any pair of atoms in a molecule forming a covalent bond attain the electron arrangement of a noble gas (i.e. 2, 2.8 or 2.8.8 etc.)

Note that hydrogen and helium only have one shell, so when referring to the full outer shell of hydrogen, it is the one and only shell, but the descriptive word 'outer' is much more crucial when describing the electronic structures of any element with at least two shells e.g. when describing covalent bonding in molecules containing carbon, oxygen, nitrogen and chlorine etc.

In advanced level chemistry you will encounter examples of electronic structures of atoms in covalent molecules that are NOT those of a Noble Gas.

An example of COVALENT BONDING – a covalent bond is formed by two atoms sharing electrons so that the atoms combine to form molecules.

The bond is usually formed between two non–metallic elements combine to form a molecular compound. The two positive nuclei (due to the positive protons in them) of both atoms are mutually attracted to the shared negative electrons between them forming the covalent bond in the molecule. They share the electrons in a way that gives a stable Noble Gas electron arrangement like helium (2) or neon (2.8) etc..

• This kind of bond or electronic linkage does act in a particular direction i.e. along the 'line' between the two nuclei of the atoms bonded together, this is why molecules have a particular shape.

• Hydrogen and oxygen atoms share electrons to give covalent O–H bonds to form molecules of the covalent compound water

• which has a 'bent' shape due to the mutual repulsion of 2 bond pairs and 2 non-bonding lone pairs of electrons

• More on how to draw to draw electronic diagrams for simple small covalent molecules.

Some key points on covalent bonding

Covalent Bond – Core Concept

• Definition: A covalent bond is a chemical bond formed when two non-metal atoms share one or more pairs of electrons.
• Bonding principle: Each atom achieves a stable outer shell (usually a full octet).
• Bond strength: Covalent bonds themselves are strong; intermolecular forces between molecules are often weak.

Student Exam Tips

• Diagrams:
  • Draw dot-and-cross diagrams with correct electron counts.
  • Label atoms clearly.
• Properties:
  • Link bonding type to physical properties (melting point, conductivity, solubility).
  • Distinguish between bond strength and intermolecular forces.
• Comparisons:
  • Be ready to contrast covalent bonding with ionic and metallic bonding.
• Examples:
  • Memorise key examples: H₂, O₂, H₂O, CH₄, CO₂, diamond, graphite, SiO_2.
  • _{These are covered on other pages in Parts 3 and 4}

  • Part 3 Covalent Bonding: small molecules & properties

  • Part 4 Giant covalent structures and other big molecules

• Exam technique:
  • Use precise language: “electrons are shared” (not transferred).
  • Practice past paper questions on bonding diagrams and property explanations.

Typical Misconceptions

• “Covalent bonds are weak” → Wrong. The bonds are strong; it’s the intermolecular forces that are weak in simple molecules.
• “Electrons are transferred” → That’s ionic bonding. Covalent bonding involves sharing.
• “All covalent substances are gases/liquids” → Incorrect. Giant covalent structures are solids with very high melting points.
• “Graphite is metallic” → False. Graphite is covalent but conducts electricity due to delocalised electrons between layers.
• “Covalent compounds dissolve easily in water” → Not always; solubility depends on polarity.

Quick Revision Overlay

• Bond type: Non-metals, shared electrons.
• Diagrams: Dot-and-cross, displayed formulae.
• Properties:
  • Simple molecules → low mp/bp, poor conductivity.
  • Giant covalent → high mp/bp, hardness, conductivity in graphite.
• Exam focus: Diagrams, explanations, property links, comparisons.