Simple molecules and covalent bonds Revision Notes

A covalent bond is a chemical bond formed when two atoms share a pair of electrons. Unlike ionic bonds, where electrons are transferred, covalent bonding involves mutual sharing, allowing each atom to count the shared electrons towards its outer shell.

This sharing creates a strong link between the atoms, holding them together in a molecule. Covalent bonds are common between non-metal atoms, which tend to have similar electronegativities and prefer to share electrons rather than lose or gain them.

• Covalent bonds involve sharing, not transferring, electrons.
• They usually occur between non-metal atoms.
• The shared electrons form a pair that constitutes the bond.

Atoms form covalent bonds to achieve a more stable electronic arrangement. Stability is often associated with having a full outer electron shell, similar to the electronic configuration of noble gases, which are chemically inert due to their complete valence shells.

For example, hydrogen atoms have one electron each and need two electrons to fill their outer shell (which holds 2 electrons). By sharing their single electrons, two hydrogen atoms form a covalent bond, resulting in a hydrogen molecule (H₂) where each atom effectively has two electrons in its outer shell.

This principle applies to other atoms as well: sharing electrons allows each atom in the bond to 'count' the shared electrons towards completing its valence shell, achieving a noble gas-like configuration.

• Atoms share electrons to fill their outer shells.
• Full outer shells correspond to noble gas configurations.
• Sharing electrons leads to greater chemical stability.

Consider the chlorine molecule (Cl₂). Each chlorine atom has seven electrons in its outer shell and needs one more to complete the octet (eight electrons). By sharing one electron each, the two chlorine atoms form a single covalent bond, resulting in both atoms having eight electrons in their outer shells.

In water (H₂O), oxygen shares electrons with two hydrogen atoms. Oxygen has six valence electrons and needs two more to complete its octet. Each hydrogen atom shares one electron with oxygen, forming two covalent bonds and giving oxygen a full outer shell.

Methane (CH₄) is another example where carbon shares electrons with four hydrogen atoms. Carbon has four valence electrons and needs four more to complete its octet. By sharing one electron with each hydrogen atom, carbon achieves a stable electronic configuration.

• H₂: two hydrogen atoms share electrons to fill their shells.
• Cl₂: two chlorine atoms share one electron each to complete octets.
• H₂O: oxygen shares electrons with two hydrogens to fill its octet.
• CH₄: carbon shares electrons with four hydrogens to complete its octet.

The goal of covalent bonding is for atoms to achieve electronic configurations similar to noble gases, which have full outer shells and are chemically stable. For example, neon has a full outer shell of eight electrons, and helium has two electrons filling its only shell.

When atoms share electrons in covalent bonds, they effectively 'count' the shared electrons as part of their own outer shell. This shared arrangement mimics the electron configuration of the nearest noble gas, explaining the stability of molecules formed by covalent bonding.

This concept is fundamental to understanding why molecules form and why certain combinations of atoms are stable.

• Noble gases have full outer electron shells.
• Atoms share electrons to mimic noble gas configurations.
• Shared electrons count towards each atom’s outer shell.

Covalent bonding is a key concept in chemistry that explains how atoms combine to form molecules by sharing electrons. This sharing allows atoms to achieve stable electronic configurations similar to noble gases, which is the driving force behind bond formation.

Understanding covalent bonds helps explain the properties of many substances, including their molecular structure, stability, and reactivity. It also provides a foundation for studying more complex molecules and chemical reactions.

Recognising covalent bonding patterns is essential for predicting molecular shapes, bond strengths, and chemical behaviour in various contexts.

• Covalent bonds form by sharing electron pairs.
• This sharing leads to stable, noble gas-like electron configurations.
• It is fundamental to molecular structure and chemical stability.

Atoms tend to be more stable when they have full outer electron shells, often resembling the electron configuration of noble gases. Covalent bonding occurs when two atoms share one or more pairs of electrons, allowing each atom to count the shared electrons towards completing its outer shell.

Unlike ionic bonding, where electrons are transferred, covalent bonding involves mutual sharing. This sharing creates a strong bond holding the atoms together in a molecule.

Each shared pair of electrons constitutes a single covalent bond. Some atoms can form multiple covalent bonds by sharing more than one pair of electrons.

• Covalent bonds involve sharing electron pairs.
• Shared electrons help atoms achieve full outer shells.
• Single, double, or triple bonds correspond to one, two, or three shared pairs of electrons.

Dot-and-cross diagrams are visual tools to show how electrons are arranged in atoms and how they are shared in covalent bonds. In these diagrams, electrons from one atom are represented by dots, and electrons from the other atom by crosses, making it clear which electrons come from which atom.

The diagrams focus on the outermost electrons (valence electrons) because these are involved in bonding. Inner electrons are not shown as they do not participate in bonding.

By drawing these diagrams, we can see how atoms share electrons to form covalent bonds and how the shared electrons complete the outer shells of both atoms.

• Dots and crosses represent electrons from different atoms.
• Only valence electrons are shown.
• Shared pairs appear between atoms, indicating covalent bonds.

In hydrogen gas (H₂), each hydrogen atom has one electron. By sharing their single electrons, they form one covalent bond, resulting in a molecule where both hydrogens have two electrons in their outer shell, achieving the stable configuration of helium.

In chlorine gas (Cl₂), each chlorine atom has seven valence electrons. They share one pair of electrons to form a single covalent bond. This sharing allows each chlorine atom to have eight electrons in its outer shell, matching the noble gas argon.

Dot-and-cross diagrams for these molecules show one shared pair of electrons between the two atoms, with the remaining electrons shown as lone pairs around each atom.

• H₂: each H shares 1 electron forming 1 single bond.
• Cl₂: each Cl shares 1 electron forming 1 single bond.
• Shared pair completes outer shells for both atoms.

Water (H₂O) consists of one oxygen atom and two hydrogen atoms. Oxygen has six valence electrons and needs two more to complete its outer shell. It forms two single covalent bonds by sharing one electron with each hydrogen atom.

Ammonia (NH₃) has one nitrogen atom with five valence electrons and three hydrogen atoms. Nitrogen shares one electron with each hydrogen, forming three single covalent bonds, completing nitrogen's outer shell with eight electrons.

Dot-and-cross diagrams for these molecules show the shared electron pairs between oxygen or nitrogen and hydrogen atoms, as well as lone pairs on oxygen or nitrogen that are not shared.

• H₂O: oxygen shares electrons with 2 hydrogens forming 2 single bonds.
• NH₃: nitrogen shares electrons with 3 hydrogens forming 3 single bonds.
• Lone pairs remain on oxygen and nitrogen atoms.

Methane (CH₄) has one carbon atom with four valence electrons and four hydrogen atoms. Carbon shares one electron with each hydrogen, forming four single covalent bonds, completing carbon's octet.

Ethene (C₂H₄) contains two carbon atoms and four hydrogen atoms. Each carbon has four valence electrons. The two carbons share two pairs of electrons between them, forming a double bond. Each carbon also forms single bonds with two hydrogen atoms.

Dot-and-cross diagrams for methane show four single bonds, while for ethene they show a double bond between carbons and single bonds to hydrogens. This illustrates how multiple bonds form by sharing more than one pair of electrons.

• CH₄: carbon forms 4 single bonds with hydrogens.
• C₂H₄: carbons share 2 pairs of electrons forming a double bond.
• Multiple bonds involve sharing more than one electron pair.

Simple molecular compounds consist of small molecules held together by covalent bonds within each molecule. These molecules are discrete entities, meaning they are separate from each other rather than forming a continuous network.

The atoms within each molecule share electrons to form strong covalent bonds, but the forces between different molecules are much weaker. These weak forces are called intermolecular forces, such as London dispersion forces or dipole-dipole interactions, depending on the molecule.

• Molecules are small and distinct units.
• Strong covalent bonds hold atoms within molecules.
• Weak intermolecular forces act between molecules.

Because the intermolecular forces between molecules are weak, only a small amount of energy is required to overcome these forces when changing state from solid to liquid (melting) or liquid to gas (boiling).

This results in simple molecular compounds having low melting and boiling points compared to ionic or metallic substances, which have strong ionic or metallic bonds throughout the structure.

For example, water (H₂O) has a melting point of 0°C and boiling point of 100°C, which are relatively low compared to ionic compounds like sodium chloride (NaCl), which melts at 801°C.

• Weak intermolecular forces require little energy to break.
• Low melting and boiling points are typical.
• Example: Water melts at 0°C and boils at 100°C.

Simple molecular compounds do not conduct electricity in solid or liquid states because they do not contain charged particles that are free to move.

Unlike ionic compounds, which dissociate into ions in molten or aqueous states, simple molecular compounds remain as neutral molecules.

Since electrical conductivity requires mobile charged particles (ions or electrons), these compounds are poor conductors under normal conditions.

• No free ions or electrons to carry charge.
• No electrical conductivity in solid or liquid states.
• Example: Sugar (C₁₂H₂₂O₁₁) does not conduct electricity when dissolved in water.

Common examples of simple molecular compounds include hydrogen (H₂), oxygen (O₂), nitrogen (N₂), methane (CH₄), and ammonia (NH₃).

These substances have low melting and boiling points and do not conduct electricity.

For instance, methane boils at −161°C and ammonia boils at −33°C, illustrating the weak intermolecular forces present.

• Hydrogen, oxygen, nitrogen, methane, ammonia are simple molecules.
• All have low melting and boiling points.
• None conduct electricity in any state.

Simple molecular compounds are made of molecules held together by strong covalent bonds internally but weak forces externally.

Their low melting and boiling points arise because only weak intermolecular forces need to be overcome to change state.

They do not conduct electricity because they lack free charged particles to carry current.

• Strong covalent bonds within molecules.
• Weak intermolecular forces between molecules.
• Low melting and boiling points.
• Poor electrical conductivity.

Covalent bonds form when two non-metal atoms share one or more pairs of electrons. Each shared pair constitutes a single covalent bond. This sharing allows each atom to attain a full outer electron shell, often resembling the electron configuration of the nearest noble gas, which is a more stable state.

For example, hydrogen atoms share their single electrons to form H₂ molecules, achieving the stable helium configuration with two electrons in their outer shell. Similarly, other atoms share electrons to complete their outer shells, which typically contain eight electrons (the octet rule).

• Covalent bonds involve sharing electron pairs.
• Atoms share electrons to complete their outer shells.
• Stable electronic configurations resemble noble gases.

Dot-and-cross diagrams visually represent the electrons from different atoms in a molecule. Electrons from one atom are shown as dots, and electrons from the other atom(s) as crosses. Shared pairs of electrons are placed between the atomic symbols to indicate covalent bonds.

These diagrams help illustrate how atoms share electrons and how many bonds form between atoms. They also show lone pairs (non-bonding electrons) that remain on atoms after bonding.

For example, in a water molecule (H₂O), oxygen shares electrons with two hydrogen atoms, and the diagram shows shared pairs between O and each H, plus lone pairs on oxygen.

• Dots and crosses represent electrons from different atoms.
• Shared pairs between atoms indicate covalent bonds.
• Lone pairs are non-bonding electrons shown on atoms.

Methanol consists of one carbon atom bonded to three hydrogen atoms and one hydroxyl group (OH). Carbon shares electrons with each hydrogen atom and the oxygen atom, forming four covalent bonds.

Oxygen in methanol shares one electron with carbon and one with hydrogen, completing its outer shell with two lone pairs remaining. The dot-and-cross diagram shows carbon’s four shared pairs with hydrogens and oxygen, and oxygen’s shared pairs with carbon and hydrogen, plus lone pairs.

This arrangement satisfies the octet rule for carbon and oxygen, and the duet rule for hydrogen.

• Carbon forms four single covalent bonds in CH₃OH.
• Oxygen forms two bonds and has two lone pairs.
• Dot-and-cross diagrams show shared and lone electron pairs.

Ethylene contains two carbon atoms double bonded to each other and each bonded to two hydrogen atoms.

Each carbon atom shares two electrons with the other carbon atom, forming a double bond (two shared pairs). Each carbon also shares one electron with each hydrogen atom, forming single bonds.

The dot-and-cross diagram shows the double bond between carbons as two pairs of shared electrons and single bonds to hydrogens. This sharing allows each carbon to have eight electrons in its outer shell.

• C₂H₄ has a carbon–carbon double bond.
• Each carbon forms two single bonds with hydrogen.
• Double bonds involve sharing two pairs of electrons.

Oxygen (O₂) molecules have two oxygen atoms sharing two pairs of electrons, forming a double bond. Each oxygen atom also has two lone pairs.

Carbon monoxide (CO) has a triple bond between carbon and oxygen, sharing three pairs of electrons, plus a lone pair on oxygen and a lone pair on carbon. The molecule is polar due to the difference in electronegativity.

Nitrogen (N₂) molecules have two nitrogen atoms sharing three pairs of electrons, forming a triple bond. Each nitrogen atom has one lone pair remaining. The triple bond is very strong, making N₂ a very stable molecule.

• O₂ has a double bond with two lone pairs on each oxygen.
• CO has a triple bond and lone pairs on both atoms.
• N₂ has a triple bond and one lone pair on each nitrogen.