Simple atomic structures

All of the atoms are held together by weak forces only. The substances are always gases at room temperature, liquifying and solidifying at temperatures far below zero celsius (0ºC). As the only known cases of simple atomic structure are the noble gases (Group 0), they are of little interest as regards moles calculations.

Xenon - simple atomic The individual atoms are not held together except by weak forces of attraction. 1 mole of xenon contains 1 mole of separate xenon atoms. When xenon solidifies (at very low temperatures) the atoms all align into a regular arrangement, giving a weak crystalline structure. The solid structure does not have enough energy for vibrations to overcome the weak forces of attraction holding the atoms together.

1 mole of any noble gas contains 1 mole of atoms = 6.02 x 10²³ atoms of that gas.

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Simple molecular substances

1 mole of a simple molecular substance contains 1 mole of molecules of that substance = 6 x 10²³ molecules

In the case of water, where 1 molecule is made up of two hydrogen atoms and 1 oxygen atom bonded together, it is clear that each molecule contains a total of three atoms. We can talk about these atoms as if they were not chemically bonded.

Water - simple molecular The individual molecules are not held together except by weak forces of attraction. 1 mole of water contains 1 mole of separate water molecules. When water freezes and becomes ice, the water molecules all align into a regular arrangement giving a crystalline structure. The structure now does not have enough energy for the water molecules to overcome the forces of attraction between them.

It is valid to say that 1 mole of water contains 2 moles of hydrogen atoms and 1 mole of oxygen atoms.

Notice that it is important to specify that you are talking about atoms. To say that 1 mole of water contains 1 mole of oxygen is incorrect. 1 mole of oxygen implies 1 mole of oxygen 0₂ molecules as this is the usual state of oxygen under standard conditions.

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Giant molecular structures

These structures have a giant lattice in which the atoms are all covalently bonded together, effectively making the entire structure 1 molecule. The atoms may be all the same, in the case of a giant molecular element such as diamond, or they may be different as in silicon dioxide.

Diamond	Silicon dioxide

The elemental giant structures may be discussed as if the atoms were not bonded together, eg 1 mole of carbon contains 6 x 10²³ carbon atoms.

In the case of the giant covalent compounds the simplest ratio of atoms withn the structure is considered to be the "formula unit". 1 mole of silicon dioxide means 1 mole of silicon dioxide formula units.

The simplest ratio of silicon to oxygen within the giant structure is 1 silicon atom to every 2 oxygen atoms; the formula unit is SiO_2. 1 mole of silicon dioxide contains 1 mole of silicon atoms and 2 moles of oxygen atoms.

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Giant ionic structures

Ionic compounds are giant structures comprising a lattice of oppositely charged ions. The simplest ratio of the ions is called the formula unit. 1 mole of an ionic compound is understood to contain 1 mole of formula units of that substance.

Sodium chloride - NaCl In the ionic lattice there is a repeating structure of sodium ions and chloride ions in a near-infinite array. Each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions. Overall there is a ratio of one sodium ion per chloride ion - the formula is NaCl In one mole of sodium chloride there is one mole of the formula unit, i.e. there are 6.02 x 1023 sodium ions and 6.02 x 1023 chloride ions.

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Giant metallic structures

Metallic structures comprise a giant structure of metal atoms within which the valence electrons are delocalised. In terms of discussing the number of moles the structure can be considered to be made up of associated atoms.

Metallic lattice In the lattice there is a repeating structure of metal ions held together by a sea of delocalised electrons. Metals have giant structures where the metal ions are regularly arranged in close packed patterns. In one mole of any metal there is one mole of metal atoms, i.e. there are 6.02 x 1023 atoms.

Therefore 1 mole of any metal contains 6 x 10²³ metal atoms.

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