🔗 Bonding & Structure

States of matter

Melting and boiling happen when particles gain enough energy to overcome the forces holding them together, so the stronger those forces are, the higher the melting and boiling points. State changes are physical changes, since the particles themselves do not change. This means mass is conserved and the change can be reversed.

In equations, state symbols show the physical state of each substance: (s) means solid, (l) means liquid, (g) means gas, and (aq) means dissolved in water, which is described as aqueous.

Ionic bonding

Atoms bond in order to reach a full outer shell, the stable arrangement that noble gases already have. They can reach this by transferring electrons, which is ionic bonding, by sharing electrons, which is covalent bonding, or by pooling electrons, which is metallic bonding. These are the three strong types of bonding, and it is important to know which substances use which type.

The group number shows the charge on the ion an element is likely to form: elements in Group 1 form 1+ ions, Group 2 form 2+ ions, Group 6 form 2− ions, and Group 7 form 1− ions. Elements on the left side of the table give away electrons, while those on the right side take them.

Ionic bonding occurs between a metal and a non-metal, where the metal atom transfers its outer electron or electrons to the non-metal atom. The compound is held together by the electrostatic attraction between the oppositely charged ions, which acts in all directions and builds up a giant ionic lattice of alternating positive and negative ions.

The common compound ions to learn are hydroxide (OH⁻), nitrate (NO₃⁻), carbonate (CO₃²⁻), sulfate (SO₄²⁻) and ammonium (NH₄⁺). To write the formula of an ionic compound, the charges must balance to zero: for example, Mg²⁺ needs two Cl⁻ ions to balance, giving the formula MgCl₂. A quick way to work this out is to swap each ion's charge number over to become the other ion's subscript.

The properties of ionic compounds follow from this structure: they have high melting points, because many strong bonds must be broken, and they only conduct electricity when molten or dissolved, because the ions must be free to move. In a solid, the ions are locked in the lattice and cannot move, so no conduction occurs.

Covalent & carbon

A covalent bond is a shared pair of electrons between non-metal atoms, and that shared pair counts towards both atoms' outer shells. Seven elements exist as diatomic molecules, meaning their atoms pair up: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.

Simple molecules, such as H₂O, CO₂ and CH₄, have strong covalent bonds within each molecule, but only weak intermolecular forces between molecules. Melting a simple molecular substance breaks only these weak forces, not the covalent bonds themselves, so simple molecular substances have low melting and boiling points. They have no charged particles that are free to move, so they do not conduct electricity.

Every model has limitations. Dot-and-cross diagrams show where each electron comes from but do not show the shape of the molecule, while ball-and-stick models show the molecule in three dimensions, even though the sticks are not real bonds and the atoms are not drawn to scale.

Giant covalent structures

Metals & nanoparticles

In metallic bonding, layers of positive metal ions sit in a sea of delocalised electrons, and the attraction between the ions and these shared electrons forms the bond. Because the electrons can move freely, they carry charge and heat, which is why metals conduct electricity and heat well, and because the layers of ions can slide over one another, metals are malleable.

An alloy is a metal that has been deliberately mixed with small amounts of other elements. Pure metals are often too soft, because their regular layers of atoms can slide easily, whereas an alloy contains atoms of different sizes that distort the layers so they can no longer slide, making the alloy harder and stronger. Steel, brass and bronze are all alloys, and nearly every metal object in everyday use is actually an alloy rather than a pure metal.

Polymers are huge covalent molecules made of long chains. Because these long chains have a lot of contact with each other, the intermolecular forces between them are relatively strong, which is why polymers are solid at room temperature.

Nanoparticles (Triple Science)

Nanoparticles are between 1 and 100 nm across, made of only a few hundred atoms. They are smaller than fine particles, such as PM2.5, which are in turn smaller than coarse particles, such as PM10 dust.

If the side length of a cube is reduced by a factor of 10, its surface area to volume ratio increases by a factor of 10, so a small amount of nanoparticles has an enormous amount of surface available to react or interact with its surroundings.

Nanoparticles are used in sunscreens, where they block ultraviolet light while remaining invisible on the skin, as catalysts, in targeted medicines, in antibacterial silver coatings, and in electronics. However, their effects on health and on the environment are not yet fully understood, so exam questions often ask you to weigh up the risks against the benefits.