Ionic bonding and the formation of ions, covalent bonding and shared electron pairs, metallic bonding and the sea of delocalised electrons, dot and cross diagrams, and how the type of bonding is decided by the elements involved.

What this topic is asking

OCR wants you to describe ionic, covalent and metallic bonding, explain how ions form, draw dot and cross diagrams, describe the sea of delocalised electrons in metals, and explain how the elements involved decide the type of bonding.

Ionic bonding

For example, when sodium reacts with chlorine, the sodium atom (outer shell $1$) transfers its outer electron to the chlorine atom (outer shell $7$). Sodium becomes $\text{Na}^+$ and chlorine becomes $\text{Cl}^-$, and the attraction between them forms the compound sodium chloride. The number of electrons transferred follows the group: Group 1 metals form $1+$ ions, Group 2 form $2+$, Group 7 non-metals form $1-$ ions, and Group 6 form $2-$. Ionic compounds form giant lattices of millions of ions, not single molecules.

Covalent and metallic bonding

Covalent bonds are found in non-metal elements (such as $\text{H}_2$, $\text{O}_2$, $\text{Cl}_2$) and in compounds of non-metals (such as $\text{H}_2\text{O}$, $\text{CO}_2$ and methane $\text{CH}_4$). A dot and cross diagram shows the outer electrons of each atom using dots for one atom and crosses for the other, so you can see which electrons are transferred (ionic) or shared (covalent). In metals, the delocalised electrons are the key to explaining why metals conduct electricity and are malleable.

Choosing the type of bonding

You can decide the bonding type from the elements involved:

• Metal + non-metal produces ionic bonding (electrons transferred), for example sodium chloride or magnesium oxide.
• Non-metal + non-metal produces covalent bonding (electrons shared), for example water or carbon dioxide.
• Metal only (an element or alloy) has metallic bonding, for example copper or steel.

This simple rule, combined with the group numbers, lets you predict the formula of many compounds and is the foundation for explaining their properties in the next topic.