Chemistry » Essential Ideas in Chemistry » Atoms And Compounds

# Defining Compounds

## Compounds

### Definition: Compound

A compound is a group of two or more different atoms that are attracted to each other by relatively strong forces or bonds. The atoms are combined in definite proportions.

Compounds can be divided into molecular compounds (molecules), ionic compounds (salts) and metallic compounds (metals).

• Molecular compounds form as a result of covalent bonding where electrons are shared between non-metal atoms.

• Ionic compounds form as a result of ionic bonding where electrons are transferred from metals to non-metals.

• Metals are formed as a result of metallic bonding where metal atoms lose their outer electrons to form a lattice of regularly spaced positive ions and a “pool” of delocalised electrons that surround the positive ions.

The following diagram illustrates how compounds can be subdivided by the type of bonding and the structure:

## Covalent Molecular Structures

Relatively small molecules are called covalent molecular structures. These exist and interact as separate molecules. Oxygen ($$\text{O}_{2}$$), water ($$\text{H}_{2}\text{O}$$), octane ($$\text{C}_{8}\text{H}_{18}$$), sulfur ($$\text{S}_{8}$$) and buckminsterfullerene ($$\text{C}_{60}$$, buckyballs) are all examples of covalent molecular structures.

Example of covalent molecular structures: buckminsterfullerene

Example of covalent molecular structures: sulfur

## Network Structures

Compounds that exist as giant repeating lattice structures are called network structures. Examples include covalent molecules such as diamond, graphite and silica. Ionic substances are also network structures, for example a sodium chloride crystal is a huge lattice of repeating units made of sodium and chloride ions. All substances formed as a result of ionic bonding are network structures. Metals exist as large continuous lattice structures and are also classified as network structures. For example copper, zinc and iron can be seen as a giant crystals and are therefore considered to be network structures.

Example of network structures: covalent network

Example of network structures: ionic network

Example of network structures: metallic network

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