Simple Molecular Substances

Any pair of electrons in the outer shell that is actively shared is called a bonding pair, while a pair that is not involved in bonding is known as a lone pair.

Arrangement and Structure of Simple Molecules

Water is composed of distinct, individual units rather than a giant continuous lattice. These units are called simple molecules, which consist of a small, fixed number of non-metal atoms joined together by covalent bonds.

The arrangement of these bonds determines the 3D shape of the molecule because the electron pairs repel each other. For example, four bonds create a tetrahedral shape (e.g., methane, $CH_4$), three bonds and one lone pair make a pyramidal shape (e.g., ammonia, $NH_3$), and two bonds with two lone pairs result in an angular or bent shape (e.g., water, $H_2O$). Sometimes, these molecules pack together into a regular 3D simple molecular lattice, such as in solid ice.

Chemists use different models to represent these arrangements, but each has limitations. A 2D displayed formula uses lines for bonds but completely ignores 3D angles. Dot-and-cross diagrams show where electrons originated but fail to show relative atom sizes. Meanwhile, ball-and-stick models show 3D arrangements well, but falsely depict bonds as physical sticks rather than invisible electrostatic forces.

Comparing Bonds and Forces

Every time you boil a kettle, you are separating liquid water molecules from each other to form a gas, but you are not splitting the water molecules into hydrogen and oxygen atoms. This perfectly illustrates the crucial difference between the strong bonds inside a molecule and the weak forces between molecules.

Feature	Intramolecular (Covalent Bonds)	Intermolecular Forces
Location	Within the molecule (between atoms)	Between separate molecules
Nature	Strong electrostatic attraction to a shared pair of electrons	Weak electrostatic attraction between molecules
Relative Strength	Very Strong	Very Weak (approximately 1/10th the strength)
Effect of Heating	Do NOT break during melting or boiling	ARE overcome during melting or boiling
Role	Determines chemical properties and stability	Determines physical properties (state, melting/boiling point)

Physical Properties of Simple Molecules

The weak intermolecular forces between molecules dictate the bulk properties of the substance as a whole. Because these forces require very little energy to overcome, simple molecular substances typically have low melting and boiling points, and are usually gases or liquids at room temperature.

As the size (relative molecular mass) of a molecule increases, it has more electrons. This leads to stronger intermolecular forces, which require more energy to overcome, resulting in higher melting and boiling points. For instance, some solids like iodine ($I_2$) undergo sublimation, turning directly from a solid into a gas when these weak forces are overcome.

Crucially, simple molecules do not conduct electricity in any state. This is because the molecules have no overall electric charge, and they contain absolutely no free ions or delocalised electrons to carry an electrical current.

Worked Example

Substance Y has a melting point of $-114^{\circ}C$ and a boiling point of $78^{\circ}C$. Predict its physical state at room temperature ($20^{\circ}C$) and explain this in terms of its structure and bonding.

Step 1: Identify the physical state based on the given temperatures.

• Room temperature ($20^{\circ}C$) is higher than the melting point ($-114^{\circ}C$) but lower than the boiling point ($78^{\circ}C$).

Step 2: Explain the properties using structural terminology.

• The low melting and boiling points indicate that Substance Y is made of simple molecules.

Step 3: Explain the energy required to change state.

• It has weak intermolecular forces between its molecules that require very little heat energy to overcome.

Step 4: Differentiate this from the internal bonding.

• The atoms within the molecules are held together by strong covalent bonds, which are not broken during these physical state changes.