0.5 What are the objects around us made of  (Page 2/3)

We can classify covalent molecules into covalent molecular structures and covalent network structures. Covalent molecular structures are simply individual covalent molecules and include water, oxygen, sulphur ( ${\mathrm{S}}_8$ ) and buckminsterfullerene ( ${\mathrm{C}}_{60}$ ). All covalent molecular structures are simple molecules. Covalent network structures are giant lattices of covalently bonded molecules, similar to the ionic lattice. Examples include diamond, graphite and silica ( ${\mathrm{SiO}}_2$ ). All covalent network structures are giant molecules.

Examples of ionic substances are sodium chloride ( $\mathrm{NaCl}$ ) and potassium permanganate ( ${\mathrm{KMnO}}_4$ ). Examples of metals are copper, zinc, titanium, gold, etc.

Representing molecules

The structure of a molecule can be shown in many different ways. Sometimes it is easiest to show what a molecule looks like byusing different types of diagrams , but at other times, we may decide to simply represent a molecule using its chemical formula or its written name.

1. Using formulae to show the structure of a molecule. A chemical formula is an abbreviated (shortened) way of describing a molecule, or some other chemical substance. Inthe chapter on classification of matter, we saw how chemical compounds can be represented using element symbols from the Periodic Table. A chemical formulacan also tell us the number of atoms of each element that are in a molecule and their ratio in that molecule. For example, the chemical formula for a molecule of carbon dioxide is ${\mathrm{CO}}_2$ The formula above is called the molecular formula of that compound. The formula tells us that in one molecule of carbon dioxide, there is one atom of carbon and two atoms of oxygen. Theratio of carbon atoms to oxygen atoms is 1:2.

   Molecular formula

   This is a concise way of expressing information about the atoms that make up a particular chemical compound. The molecular formula gives the exact number ofeach type of atom in the molecule.

   A molecule of glucose has the molecular formula: ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ . In each glucose molecule, there are six carbon atoms, twelve hydrogen atoms andsix oxygen atoms. The ratio of carbon:hydrogen:oxygen is 6:12:6. We can simplify this ratio to write 1:2:1, or if we were to use the element symbols, the formulawould be written as ${\mathrm{CH}}_2\mathrm{O}$ . This is called the empirical formula of the molecule.

   Empirical formula

   This is a way of expressing the relative number of each type of atom in a chemical compound. In most cases, the empiricalformula does not show the exact number of atoms, but rather the simplest ratio of the atoms in the compound.

   The empirical formula is useful when we want to write the formula for a giant molecule . Since giant molecules may consist of millions of atoms, it is impossible to say exactly howmany atoms are in each molecule. It makes sense then to represent these molecules using their empirical formula. So, in the case of a metal such ascopper, we would simply write Cu, or if we were to represent a molecule of sodium chloride, we would simply write NaCl.Chemical formulae therefore tell us something about the types of atoms that are in a molecule and the ratio in which these atoms occur in the molecule, but they don't give us any idea of what the molecule actually lookslike, in other words its shape . To show the shape of molecules we can represent molecules using diagrams.Another type of formula that can be used to describe a molecule is its structural formula . A structural formula uses a graphical representation to show a molecule's structure( [link] ).

   

2. Using diagrams to show the structure of a molecule Diagrams of molecules are very useful because they help us to picture how the atoms are arranged in the molecule and they help us to see the shape of themolecule. There are two types of diagrams that are commonly used:
   • Ball and stick models This is a 3-dimensional molecular model that uses 'balls' to represent atoms and 'sticks' to represent the bonds between them. The centres of the atoms (theballs) are connected by straight lines which represent the bonds between them. A simplified example is shown in [link] .

     

   • Space-filling model This is also a 3-dimensional molecular model. The atoms are represented by spheres. [link] and [link] are some examples of simple molecules that are represented in different ways.

     

     

   [link] shows the bonds between the carbon atoms in diamond, which is a giant molecule . Each carbon atom is joined to four others, and this pattern repeats itself until a complex lattice structure is formed. Each black ball in the diagram represents a carbon atom, and each line represents the bondbetween two carbon atoms. Note that the carbon atoms on the edges are actually bonded to four carbon atoms, but some of these carbon atoms have been omitted.