Atomic radius

Atomic radiusThe trendThe diagram shows how the atomic radius changes as you go across Period 3.

The figures used to construct this diagram are based on: metallic radii for Na, Mg and Al;

covalent radii for Si, P, S and Cl;

the van der Waals radius for Ar because it doesn't form any strong bonds.

It is fair to compare metallic and covalent radii because they are both being measured in tightly bonded circumstances. It isn't fair to compare these with a van der Waals radius, though.The general trend towards smaller atoms across the period is NOT broken at argon. You aren't comparing like with like. The outer shell is now filled, so there is repulsion between the shells, so the atomic radius increases.

Ionic Radius

Decreases up to Si, increases for P, decreases. For cations, outer shell is gone, but nuclear charge is increasing, so the attraction is greater and radius is smaller. For P, it is an anion, so the electron is added and the radius increases. Going up to Cl it decreases because less electrons need to be added, so less repulsion. Ar doesnt form ions. ElectronegativityElectronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons. Notice that argon isn't included. Electronegativity is about the tendency of an atom to attract a bonding pair of electrons. Since argon doesn't form covalent bonds, you obviously can't assign it an electronegativity.

Explaining the trendThe trend is explained in exactly the same way as the trend in atomic radii.

As you go across the period, the bonding electrons are always in the same level - the 3-level. They are always being screened by the same inner electrons. All that differs is the number of protons in the nucleus. As you go from sodium to chlorine, the number of protons steadily increases and so attracts the bonding pair more closely.

Melting and boiling points

Melting and boiling points rise across the three metals because of the increasing strength of the metallic bonds. The number of electrons which each atom can contribute to the delocalised "sea of electrons" increases. The atoms also get smaller and have more protons as you go from sodium to magnesium to aluminium.

The attractions and therefore the melting and boiling points increase because:

The nuclei of the atoms are getting more positively charged.

The "sea" is getting more negatively charged.

The "sea" is getting progressively nearer to the nuclei and so more strongly attracted.

Silicon has high melting and boiling points because it is a giant covalent structure. You have to break strong covalent bonds before it will melt or boil.Because you are talking about a different type of bond, it isn't profitable to try to directly compare silicon's melting and boiling points with aluminium's.Phosphorus, sulphur, chlorine and argon are simple molecular substances with only van der Waals attractions between the molecules. Their melting or boiling points will be lower than those of the first four members of the period which have giant structures.The sizes of the melting and boiling points are governed entirely by the sizes of the molecules. Remember the structures of the molecules:PhosphorusPhosphorus contains P4 molecules. To melt phosphorus you don't have to break any covalent bonds - just the much weaker van der Waals forces between the molecules.SulphurSulphur consists of S8 rings of atoms. The molecules are bigger than phosphorus molecules, and so the van der Waals attractions will be stronger, leading to a higher melting and boiling point.ChlorineChlorine, Cl2, is a much smaller molecule with comparatively weak van der Waals attractions, and so chlorine will have a lower melting and boiling point than sulphur or phosphorus.ArgonArgon molecules are just single argon atoms, Ar. The scope for van der Waals attractions between these is very limited and so the melting and boiling points of argon are lower again.

Sodium, magnesium and aluminium are all good conductors of electricity.

Silicon is a semiconductor.

None of the rest conduct electricity.

The three metals, of course, conduct electricity because the delocalized electrons (the "sea of electrons") are free to move throughout the solid or the liquid metal.

In the silicon case, explaining how semiconductors conduct electricity is beyond the scope of A level chemistry courses. With a diamond structure, you mightn't expect it to conduct electricity, but it does!

The rest don't conduct electricity because they are simple molecular substances. There are no electrons free to move around.

Reactions with water\

SodiumSodium has a very exothermic reaction with cold water producing hydrogen and a colourless solution of sodium hydroxide.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/nah2o.gif" \* MERGEFORMATINET Magnesium

Magnesium has a very slight reaction with cold water, but burns in steam.

A very clean coil of magnesium dropped into cold water eventually gets covered in small bubbles of hydrogen which float it to the surface. Magnesium hydroxide is formed as a very thin layer on the magnesium and this tends to stop the reaction.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgh2ol.gif" \* MERGEFORMATINET Magnesium burns in steam with its typical white flame to produce white magnesium oxide and hydrogen.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgh2og.gif" \* MERGEFORMATINET Aluminium

Aluminium powder heated in steam produces hydrogen and aluminium oxide. The reaction is relatively slow because of the existing strong aluminium oxide layer on the metal, and the build-up of even more oxide during the reaction.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/alh2o.gif" \* MERGEFORMATINET Silicon

There is a fair amount of disagreement in the books and on the web about what silicon does with water or steam. The truth seems to depend on the precise form of silicon you are using.

The common shiny grey lumps of silicon with a rather metal-like appearance are fairly unreactive. Most sources suggest that this form of silicon will react with steam at red heat to produce silicon dioxide and hydrogen.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/sih2o.gif" \* MERGEFORMATINET But it is also possible to make much more reactive forms of silicon which will react with cold water to give the same products.

Phosphorus and sulphur

These have no reaction with water.

Chlorine

Chlorine dissolves in water to some extent to give a green solution. A reversible reaction takes place to produce a mixture of hydrochloric acid and chloric(I) acid (hypochlorous acid).

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/cl2h2o.gif" \* MERGEFORMATINET In the presence of sunlight, the chloric(I) acid slowly decomposes to produce more hydrochloric acid, releasing oxygen gas, and you may come across an equation showing the overall change:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/cl2h2osun.gif" \* MERGEFORMATINET Argon

There is no reaction between argon and water.Reactions with oxygen

Sodium

Sodium burns in oxygen with an orange flame to produce a white solid mixture of sodium oxide and sodium peroxide.

For the simple oxide:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/nao2a.gif" \* MERGEFORMATINET For the peroxide:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/nao2b.gif" \* MERGEFORMATINET Magnesium

Magnesium burns in oxygen with an intense white flame to give white solid magnesium oxide.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgo2.gif" \* MERGEFORMATINET Aluminium

Aluminium will burn in oxygen if it is powdered; otherwise the strong oxide layer on the aluminium tends to inhibit the reaction. If you sprinkle aluminium powder into a Bunsen flame, you get white sparkles. White aluminium oxide is formed.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/alo2.gif" \* MERGEFORMATINET Silicon

Silicon will burn in oxygen if heated strongly enough. Silicon dioxide is produced.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/sio2.gif" \* MERGEFORMATINET Phosphorus

White phosphorus catches fire spontaneously in air, burning with a white flame and producing clouds of white smoke - a mixture of phosphorus(III) oxide and phosphorus(V) oxide.

The proportions of these depend on the amount of oxygen available. In an excess of oxygen, the product will be almost entirely phosphorus(V) oxide.

For the phosphorus(III) oxide:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/po2tri.gif" \* MERGEFORMATINET For the phosphorus(V) oxide:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/po2pent.gif" \* MERGEFORMATINET Sulphur

Sulphur burns in air or oxygen on gentle heating with a pale blue flame. It produces colourless sulphur dioxide gas.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/so2.gif" \* MERGEFORMATINET Chlorine and argon

Despite having several oxides, chlorine won't react directly with oxygen. Argon doesn't react either.

Reactions with chlorine

Sodium

Sodium burns in chlorine with a bright orange flame. White solid sodium chloride is produced.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/nacl2.gif" \* MERGEFORMATINET Magnesium

Magnesium burns with its usual intense white flame to give white magnesium chloride.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgcl2.gif" \* MERGEFORMATINET Aluminium

Aluminium is often reacted with chlorine by passing dry chlorine over aluminium foil heated in a long tube. The aluminium burns in the stream of chlorine to produce very pale yellow aluminium chloride. This sublimes (turns straight from solid to vapour and back again) and collects further down the tube where it is cooler.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/alcl2a.gif" \* MERGEFORMATINET Silicon

If chlorine is passed over silicon powder heated in a tube, it reacts to produce silicon tetrachloride. This is a colourless liquid which vaporises and can be condensed further along the apparatus.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/sicl2.gif" \* MERGEFORMATINET Phosphorus

White phosphorus burns in chlorine to produce a mixture of two chlorides, phosphorus(III) chloride and phosphorus(V) chloride (phosphorus trichloride and phosphorus pentachloride).

Phosphorus(III) chloride is a colourless fuming liquid.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/pcl2tri.gif" \* MERGEFORMATINET Phosphorus(V) chloride is an off-white (going towards yellow) solid.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/pcl2pent.gif" \* MERGEFORMATINET Sulphur

If a stream of chlorine is passed over some heated sulphur, it reacts to form an orange, evil-smelling liquid, disulphur dichloride, S2Cl2.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/scl2.gif" \* MERGEFORMATINET Chlorine and argon

It obviously doesn't make sense to talk about chlorine reacting with itself, and argon doesn't react with chlorine.

ACID-BASE BEHAVIOUR OF THE PERIOD 3 OXIDES

This page looks at the reactions of the oxides of Period 3 elements (sodium to chlorine) with water, and with acids or bases where relevant. Argon is obviously omitted because it doesn't form an oxide.

A quick summary of the trend

The oxides

The oxides we'll be looking at are:

Na2OMgOAl2O3SiO2P4O10SO3Cl2O7

P4O6SO2Cl2O

Note: If you haven't already been there, you might be interested in looking at the page about the structures and physical properties of the Period 3 oxides as a useful introduction before you go any further.

Use the BACK button on your browser to return quickly to this page later if you choose to follow this link.

The trend in acid-base behaviour

The trend in acid-base behaviour is shown in various reactions, but as a simple summary:

The trend is from strongly basic oxides on the left-hand side to strongly acidic ones on the right, via an amphoteric oxide (aluminium oxide) in the middle. An amphoteric oxide is one which shows both acidic and basic properties.

For this simple trend, you have to be looking only at the highest oxides of the individual elements. Those are the ones on the top row above, and are where the element is in its highest possible oxidation state. The pattern isn't so simple if you include the other oxides as well.

These reactions are all explored in detail on the rest of this page.

Chemistry of the individual oxides

Sodium oxide

Sodium oxide is a simple strongly basic oxide. It is basic because it contains the oxide ion, O2-, which is a very strong base with a high tendency to combine with hydrogen ions.

Reaction with water

Sodium oxide reacts exothermically with cold water to produce sodium hydroxide solution. Depending on its concentration, this will have a pH around 14.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/na2oh2ofull.gif" \* MERGEFORMATINET Reaction with acids

As a strong base, sodium oxide also reacts with acids. For example, it would react with dilute hydrochloric acid to produce sodium chloride solution.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/na2ohclfull.gif" \* MERGEFORMATINET Magnesium oxide

Magnesium oxide is again a simple basic oxide, because it also contains oxide ions. However, it isn't as strongly basic as sodium oxide because the oxide ions aren't so free.

In the sodium oxide case, the solid is held together by attractions between 1+ and 2- ions. In the magnesium oxide case, the attractions are between 2+ and 2-. It takes more energy to break these.

Even allowing for other factors (like the energy released when the positive ions form attractions with water in the solution formed), the net effect of this is that reactions involving magnesium oxide will always be less exothermic than those of sodium oxide.

Reaction with water

If you shake some white magnesium oxide powder with water, nothing seems to happen - it doesn't look as if it reacts. However, if you test the pH of the liquid, you find that it is somewhere around pH 9 - showing that it is slightly alkaline.

There must have been some slight reaction with the water to produce hydroxide ions in solution. Some magnesium hydroxide is formed in the reaction, but this is almost insoluble - and so not many hydroxide ions actually get into solution.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgoh2o.gif" \* MERGEFORMATINET Reaction with acids

Magnesium oxide reacts with acids as you would expect any simple metal oxide to react. For example, it reacts with warm dilute hydrochloric acid to give magnesium chloride solution.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/mgohcl.gif" \* MERGEFORMATINET Aluminium oxide

Describing the properties of aluminium oxide can be confusing because it exists in a number of different forms. One of those forms is very unreactive. It is known chemically as alpha-Al2O3 and is produced at high temperatures.

In what follows we are assuming one of the more reactive forms.

Aluminium oxide is amphoteric. It has reactions as both a base and an acid.

Reaction with water

Aluminium oxide doesn't react in a simple way with water in the sense that sodium oxide and magnesium oxide do, and doesn't dissolve in it. Although it still contains oxide ions, they are held too strongly in the solid lattice to react with the water.

Reaction with acids

Aluminium oxide contains oxide ions and so reacts with acids in the same way as sodium or magnesium oxides. That means, for example, that aluminium oxide will react with hot dilute hydrochloric acid to give aluminium chloride solution.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/al2o3hcl.gif" \* MERGEFORMATINET In this (and similar reactions with other acids), aluminium oxide is showing the basic side of its amphoteric nature.

Reaction with bases

Aluminium oxide has also got an acidic side to its nature, and it shows this by reacting with bases such as sodium hydroxide solution.

Various aluminates are formed - compounds where the aluminium is found in the negative ion. This is possible because aluminium has the ability to form covalent bonds with oxygen.

In the case of sodium, there is too much electronegativity difference between sodium and oxygen to form anything other than an ionic bond. But electronegativity increases as you go across the period - and the electronegativity difference between aluminium and oxygen is smaller. That allows the formation of covalent bonds between the two.

With hot, concentrated sodium hydroxide solution, aluminium oxide reacts to give a colourless solution of sodium tetrahydroxoaluminate.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/al2o3naoh.gif" \* MERGEFORMATINET Silicon dioxide (silicon(IV) oxide)

By the time you get to silicon as you go across the period, electronegativity has increased so much that there is no longer enough electronegativity difference between silicon and oxygen to form ionic bonds.

Silicon dioxide has no basic properties - it doesn't contain oxide ions and it doesn't react with acids. Instead, it is very weakly acidic, reacting with strong bases.

Reaction with water

Silicon dioxide doesn't react with water, because of the difficulty of breaking up the giant covalent structure.

Reaction with bases

Silicon dioxide reacts with sodium hydroxide solution, but only if it is hot and concentrated. A colourless solution of sodium silicate is formed.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/sio2naoh.gif" \* MERGEFORMATINET You may also be familiar with one of the reactions happening in the Blast Furnace extraction of iron - in which calcium oxide (from the limestone which is one of the raw materials) reacts with silicon dioxide to produce a liquid slag, calcium silicate. This is also an example of the acidic silicon dioxide reacting with a base.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/sio2cao.gif" \* MERGEFORMATINET The phosphorus oxides

We are going to be looking at two phosphorus oxides, phosphorus(III) oxide, P4O6, and phosphorus(V) oxide, P4O10.

Phosphorus(III) oxide

Phosphorus(III) oxide reacts with cold water to give a solution of the weak acid, H3PO3 - known variously as phosphorous acid, orthophosphorous acid or phosphonic acid. Its reaction with hot water is much more complicated.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/p4o6h2o.gif" \* MERGEFORMATINET The pure un-ionised acid has the structure:

The hydrogens aren't released as ions until you add water to the acid, and even then not many are released because phosphorous acid is only a weak acid.

Phosphorous acid has a pKa of 2.00 which makes it stronger than common organic acids like ethanoic acid (pKa = 4.76).

It is pretty unlikely that you would ever react phosphorus(III) oxide directly with a base, but you might need to know what happens if you react the phosphorous acid formed with a base.

In phosphorous acid, the two hydrogen atoms in the -OH groups are acidic, but the other one isn't. That means that you can get two possible reactions with, for example, sodium hydroxide solution depending on the proportions used.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh3po3a.gif" \* MERGEFORMATINET

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh3po3b.gif" \* MERGEFORMATINET In the first case, only one of the acidic hydrogens has reacted with the hydroxide ions from the base. In the second case (using twice as much sodium hydroxide), both have reacted.

Phosphorus(V) oxide

Phosphorus(V) oxide reacts violently with water to give a solution containing a mixture of acids, the nature of which depends on the conditions. We usually just consider one of these, phosphoric(V) acid, H3PO4 - also known just as phosphoric acid or as orthophosphoric acid.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/p4o10h2o.gif" \* MERGEFORMATINET This time the pure un-ionised acid has the structure:

Phosphoric(V) acid is also a weak acid with a pKa of 2.15. That makes it fractionally weaker than phosphorous acid. Solutions of both of these acids of concentrations around 1 mol dm-3 will have a pH of about 1.

Once again, you are unlikely ever to react this oxide with a base, but you may well be expected to know how phosphoric(V) acid reacts with something like sodium hydroxide solution.

If you look back at the structure, you will see that it has three -OH groups, and each of these has an acidic hydrogen atom. You can get a reaction with sodium hydroxide in three stages, with one after another of these hydrogens reacting with the hydroxide ions.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh3po4a.gif" \* MERGEFORMATINET

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh3po4b.gif" \* MERGEFORMATINET

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh3po4c.gif" \* MERGEFORMATINET The sulphur oxides

We are going to be looking at sulphur dioxide, SO2, and sulphur trioxide, SO3.

Sulphur dioxide

Sulphur dioxide is fairly soluble in water, reacting with it to give a solution of sulphurous acid (sulphuric (IV) acid), H2SO3. This only exists in solution, and any attempt to isolate it just causes sulphur dioxide to be given off again.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/so2h2o.gif" \* MERGEFORMATINET The un-ionised acid has the structure:

Sulphurous acid is also a weak acid with a pKa of around 1.8 - very slightly stronger than the two phosphorus-containing acids above. A reasonably concentrated solution of sulphurous acid will again have a pH of about 1.

Sulphur dioxide will also react directly with bases such as sodium hydroxide solution. If sulphur dioxide is bubbled through sodium hydroxide solution, sodium sulphite solution is formed first followed by sodium hydrogensulphite solution when the sulphur dioxide is in excess.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/so2naoh1.gif" \* MERGEFORMATINET

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/so2naoh2.gif" \* MERGEFORMATINET Another important reaction of sulphur dioxide is with the base calcium oxide to form calcium sulphite (calcium sulphate(IV)). This is at the heart of one of the methods of removing sulphur dioxide from flue gases in power stations.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/caoso2.gif" \* MERGEFORMATINET Sulphur trioxide

Sulphur trioxide reacts violently with water to produce a fog of concentrated sulphuric acid droplets.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/so3h2o.gif" \* MERGEFORMATINET Pure un-ionised sulphuric acid has the structure:

Sulphuric acid is a strong acid, and solutions will typically have pH's of around 0.

The acid reacts with water to give a hydroxonium ion (a hydrogen ion in solution, if you like) and a hydrogensulphate ion. This reaction is virtually 100% complete.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/h2so4h2oa.gif" \* MERGEFORMATINET The second hydrogen is more difficult to remove. In fact the hydrogensulphate ion is a relatively weak acid - similar in strength to the acids we have already discussed on this page. This time you get an equilibrium:

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/h2so4h2ob.gif" \* MERGEFORMATINET You probably won't need this for the purposes of UK A level (or its equivalents), but it is useful if you understand the reason that sulphuric acid is a stronger acid than sulphurous acid. You can apply the same reasoning to other acids that you find on this page as well.

When a hydrogen ion is lost from one of the -OH groups, the negative charge left on the oxygen is spread out (delocalised) over the ion by interacting with the doubly-bonded oxygens.

It follows that the more of these you have, the more delocalisation you can get - and the more delocalisation, the more stable the ion becomes. The more stable the ion, the less likely it is to recombine with a hydrogen ion and revert to the un-ionised acid.

Sulphurous acid only has one doubly-bonded oxygen, whereas sulphuric acid has two - that makes for a much more effective delocalisation, a much more stable ion, and so for a stronger acid.

Sulphuric acid, of course, has all the reactions of a strong acid that you are familiar with from introductory chemistry courses. For example, the normal reaction with sodium hydroxide solution is to form sodium sulphate solution - in which both of the acidic hydrogens react with hydroxide ions.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohh2so4.gif" \* MERGEFORMATINET In principle, you can also get sodium hydrogensulphate solution by using half as much sodium hydroxide and just reacting with one of the two acidic hydrogens in the acid. In practice, I personally have never ever done it - I can't at the moment see much point!

Sulphur trioxide itself will also react directly with bases to form sulphates. For example, it will react with calcium oxide to form calcium sulphate. This is just like the reaction with sulphur dioxide described above.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/caoso3.gif" \* MERGEFORMATINET The chlorine oxides

Chlorine forms several oxides, but the only two mentioned by any of the UK A level syllabuses are chlorine(VII) oxide, Cl2O7, and chlorine(I)oxide, Cl2O. Chlorine(VII) oxide is also known as dichlorine heptoxide, and chlorine(I) oxide as dichlorine monoxide.

Chlorine(VII) oxide

Chlorine(VII) oxide is the highest oxide of chlorine - the chlorine is in its maximum oxidation state of +7. It continues the trend of the highest oxides of the Period 3 elements towards being stronger acids.

Chlorine(VII) oxide reacts with water to give the very strong acid, chloric(VII) acid - also known as perchloric acid. The pH of typical solutions will, like sulphuric acid, be around 0.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/cl2o7h2o.gif" \* MERGEFORMATINET Un-ionised chloric(VII) acid has the structure:

When the chlorate(VII) ion (perchlorate ion) forms by loss of a hydrogen ion (when it reacts with water, for example), the charge can be delocalised over every oxygen atom in the ion. That makes it very stable, and means that chloric(VII) acid is very strong.

Chloric(VII) acid reacts with sodium hydroxide solution to form a solution of sodium chlorate(VII).

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohhclo4.gif" \* MERGEFORMATINET Chlorine(VII) oxide itself also reacts with sodium hydroxide solution to give the same product.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohcl2o7.gif" \* MERGEFORMATINET Chlorine(I) oxide

Chlorine(I) oxide is far less acidic than chlorine(VII) oxide. It reacts with water to some extent to give chloric(I) acid, HOCl - also known as hypochlorous acid.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/cl2oh2o.gif" \* MERGEFORMATINET The structure of chloric(I) acid is exactly as shown by its formula, HOCl. It has no doubly-bonded oxygens, and no way of delocalising the charge over the negative ion formed by loss of the hydrogen.

That means that the negative ion formed isn't very stable, and readily reclaims its hydrogen to revert to the acid. Chloric(I) acid is very weak (pKa = 7.43).

Chloric(I) acid reacts with sodium hydroxide solution to give a solution of sodium chlorate(I) (sodium hypochlorite).

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohhocl.gif" \* MERGEFORMATINET Chlorine(I) oxide also reacts directly with sodium hydroxide to give the same product.

INCLUDEPICTURE "http://www.chemguide.co.uk/inorganic/period3/naohcl2o.gif" \* MERGEFORMATINET