Simple molecules and covalent bonds

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What is a Covalent Bond?

Atoms bond to achieve stability. Most atoms seek the stable electronic configuration of a noble gas (Group 0), which typically means having 8 electrons in their outer shell (the octet rule), or 2 electrons for Hydrogen and Helium (duet rule).

A covalent bond is formed when two atoms share a pair of electrons. This sharing allows both atoms to fill their outer shells without transferring electrons completely (which would form an ionic bond).

Why does this happen? The positively charged nuclei of both atoms are attracted to the shared negative electron pair. This electrostatic attraction holds the atoms together.
Core Requirement: You must be able to state that a covalent bond involves the sharing of a pair of electrons leading to noble gas configurations.

Covalent Bond

A covalent bond is the strong electrostatic attraction between a shared pair of electrons and the nuclei of two bonded atoms.

Key phrase for exams: "Attraction between shared pair of electrons and nuclei."
Note: Do not just say "sharing electrons." You must mention the attraction to the nuclei to get full marks.

Drawing Dot-and-Cross Diagrams (Core Molecules)

Rules for Drawing:

Use dots (•) for electrons from one atom and crosses (×) for electrons from the other.
Only draw outer-shell electrons.
Show the shared pair in the overlapping region of the two atoms.
Ensure every atom achieves a full outer shell (8 or 2).

1. Hydrogen (H_2)

Each H has 1 electron. They share 1 pair.
Result: Each H sees 2 electrons (stable duet).
Diagram: Two circles overlapping. One • and one × in the overlap. One • on the left H, one × on the right H.

2. Chlorine (Cl_2)

Each Cl has 7 outer electrons. They share 1 pair.
Result: Each Cl sees 8 electrons (stable octet).
Diagram: Two circles overlapping. One • and one × in the overlap. Six other electrons around each Cl.

3. Water (H_2O)

Oxygen (Group 6) has 6 outer electrons. Hydrogen has 1.
O shares one pair with each of two H atoms.
Result: O has 8 electrons (4 bonding pairs + 2 lone pairs). Each H has 2.
Crucial Detail: Oxygen has two non-bonding pairs (lone pairs) remaining. Do not forget these!

4. Methane (CH_4)

Carbon (Group 4) has 4 outer electrons. Hydrogen has 1.
C shares one pair with each of four H atoms.
Result: C has 8 electrons. Each H has 2.
Crucial Detail: All 4 bonds are single covalent bonds.

Lone Pairs on Central Atom
None
3 pairs on each Cl
2 pairs on O
None
1 pair on N

⚠︎ Dot-and-Cross Diagram Errors

Mistake 1: Forgetting Lone Pairs

Error: Drawing H_2O with only the bonding electrons.
Correction: Oxygen has 6 valence electrons. 2 are shared. 4 remain as two lone pairs. You must draw these non-bonding electrons on the Oxygen atom.

Mistake 2: Wrong Number of Electrons

Error: Drawing Carbon in CH_4 with only 6 outer electrons.
Correction: Carbon starts with 4. It shares 4 pairs (8 total). Ensure the central atom has 8 electrons around it.

Mistake 3: Confusing Ionic and Covalent

Error: Drawing arrows showing electron transfer in covalent molecules.
Correction: Covalent bonds involve sharing, not transfer. Use overlapping circles or lines to show sharing.

Drawing Dot-and-Cross Diagrams Correctly

When to use: Whenever asked to 'describe' or 'draw' the bonding in a molecule.

Why examiners accept this: Examiners look for specific visual evidence of understanding:

Correct count: The total number of dots/crosses must match the valence electrons of the atoms.
Full shells: Every atom (except H) must have 8 electrons around it; H must have 2.
Lone pairs: Non-bonding electrons must be shown on the correct atoms.

Example of correct phrasing/drawing:
For Ammonia (NH_3):

Draw N in the center with 5 valence electrons (e.g., 5 crosses).
Draw three H atoms around it, each with 1 electron (e.g., 1 dot).
Show three overlapping regions where one cross and one dot meet (3 bonding pairs).
Show one lone pair (2 crosses) remaining on the Nitrogen atom.
Tip: Clearly label 'lone pair' or ensure it is visually distinct from bonding pairs.

Properties of Simple Molecular Compounds

1. Low Melting and Boiling Points

Structure: Simple molecular substances consist of distinct, small molecules held together by covalent bonds.
Explanation: The covalent bonds within the molecule are strong, but the forces between the molecules (intermolecular forces) are weak.
Result: Little energy is needed to overcome these weak intermolecular forces to melt or boil the substance. The covalent bonds themselves do not break during melting/boiling.

2. Poor Electrical Conductivity

Structure: Electrons in covalent bonds are localized (fixed) between specific atoms.
Explanation: There are no free-moving charged particles (no delocalized electrons and no ions).
Result: Electricity cannot flow. This applies to both solid and liquid states (unlike ionic compounds, which conduct when molten).

Explaining Properties (Melting Point & Conductivity)

For Low Melting/Boiling Points:

Context: When asked to explain physical properties.
Key Phrase: "Weak intermolecular forces between molecules require little energy to overcome."
Why this works: It distinguishes between the strong intramolecular bonds (which don't break) and the weak intermolecular forces (which do). Examiners reject answers that say "bonds are broken" during melting.

For Poor Conductivity:

Context: When asked why a covalent substance does not conduct electricity.
Key Phrase: "There are no free-moving charged particles / ions / delocalized electrons."
Why this works: It directly addresses the requirement for charge carriers in conduction. Be specific: do not just say "no electrons." Say "no free or delocalized electrons" and "no ions."

Supplement Molecules: Double and Triple Bonds

For supplement molecules, you must handle multiple bonds (sharing more than one pair).

1. Oxygen (O_2) - Double Bond

Each O has 6 outer electrons.
To get 8, each needs to share 2 pairs.
Diagram: Two overlapping circles. In the overlap: two dots and two crosses (4 electrons total). Each O also has two lone pairs (4 non-bonding electrons).

2. Carbon Dioxide (CO_2) - Double Bonds

C has 4 outer electrons. Each O has 6.
C shares 2 pairs with the left O and 2 pairs with the right O.
Diagram: O=C=O structure. Two double bonds (two shared pairs each). Each O has two lone pairs. C has no lone pairs.

3. Nitrogen (N_2) - Triple Bond

Each N has 5 outer electrons.
To get 8, each needs to share 3 pairs.
Diagram: Two overlapping circles. In the overlap: three dots and three crosses (6 electrons total). Each N also has one lone pair (2 non-bonding electrons).

4. Ethene (C_2H_4) - Double Bond between Carbons

Each C shares 2 pairs with the other C.
Each C also shares 1 pair with two H atoms.
Diagram: H₂C=CH₂. The central C-C bond is a double bond (two shared pairs). Each C has no lone pairs.

⚠︎ Supplement Bonding Errors

Mistake 1: Incorrect Electron Count in CO_2

Error: Drawing single bonds between C and O.
Correction: Carbon needs 4 bonds to be stable. Oxygen needs 2. You must draw double bonds (two shared pairs) for both C-O connections.

Mistake 2: Forgetting Lone Pairs in N_2 or O_2

Error: Drawing only the bonding electrons.
Correction: In N_2, each N has 1 lone pair. In O_2, each O has 2 lone pairs. These are essential for the octet rule.

Mistake 3: Confusing Intermolecular Forces with Bonds

Error: Saying "weak bonds between molecules."
Correction: Use the term intermolecular forces. They are not bonds.

Drawing Double/Triple Bonds Correctly

When to use: When drawing O_2, N_2, CO_2, or C_2H_4.

Why examiners accept this: They verify that you understand valency requirements:

Carbon forms 4 bonds.
Nitrogen forms 3 bonds (plus 1 lone pair).
Oxygen forms 2 bonds (plus 2 lone pairs).

Example for N_2:

Draw two N atoms.
Show three shared pairs (6 electrons) between them.
Show one lone pair on each N atom.
Check: Does each N have 8 electrons? Yes (6 bonding + 2 lone).

Example for CO_2:

Draw C in the middle, O on sides.
Show two shared pairs between C and left O.
Show two shared pairs between C and right O.
Show two lone pairs on each O.
Check: Does C have 8 electrons? Yes (4 bonding pairs). Do Os have 8? Yes (2 bonding pairs + 2 lone pairs).