This page describes the reactions of the Period 3 elements from sodium to argon with water, oxygen and chlorine.

Reactions with water




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




Notice also that the oxide is produced on heating in steam. Hydroxides are only ever produced using liquid water.


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.


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.

But it is also possible to make much more reactive forms of silicon which will react with cold water to give the same products.

Note:  These more reactive forms are produced as powders. Cotton and Wilkinson's Advanced Inorganic Chemistry (third edition - page 316) suggests that the reactivity of one of these could be due to a very high surface area, or perhaps because the silicon exists in a graphite-like structure.

A correspondent from the silicon industry tells me that when silicon is cut into slices, the silicon dust formed reacts with water at room temperature - producing hydrogen and getting very hot. He says

"The silicon is cut in a glycol slurry [. . .] The powdered Si is protected somewhat from moisture in the glycol slurry, but when we clean the slurry in aqueous solutions the reaction with water takes off."

This is probably the effect of the high surface area of the dust produced, combined with the fact that you are exposing uncontaminated silicon to the water. One source suggests that the lack of reactivity of silicon is due to a layer of silicon dioxide on its surface. If you expose a new surface by cutting the silicon, that layer won't, of course, exist.

Phosphorus and sulphur

These have no reaction with water.



Note:  You may also find the chloric(I) acid written as HClO. The form I have used more accurately reflects the way the atoms are joined up. It doesn't matter which you use.



There is no reaction between argon and water.

Reactions with oxygen



For the simple oxide:

For the peroxide:


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

Note:  If magnesium is burns in air rather than in pure oxygen, it also reacts with the nitrogen in the air. You get a mixture of magnesium oxide and magnesium nitride formed.


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.


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

Note:  There is disagreement between various web or textbook sources about the temperature needed to ignite the silicon, varying from 400°C to well over 1000°C. In fact, there isn't a "right" answer to this. It depends on what sort of silicon you are talking about and how finely divided it is. For example, one of the amorphous (non-crystalline powder) forms of silicon even catches fire spontaneously in air at room temperature. Other forms need higher temperatures and a richer oxygen supply.


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:

For the phosphorus(V) oxide:

Note:  You may come across these oxides written as P2O3 and P2O5. Don't use these forms! They are as logical as writing, say, ethene as CH2 and ethane as CH3.


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

Note:  Sulphur dioxide can, of course, be converted further into sulphur trioxide in the presence of oxygen, but it needs the presence of a catalyst and fairly carefully controlled conditions. If you are interested in this, see the page on the Contact Process.

This isn't particularly relevant to the current topic, but if you do feel the urge to follow this link, use the BACK button on your browser to return to this page.

Chlorine and argon

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

Reactions with chlorine


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


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



Note:  You may find versions of this equation showing the aluminium chloride as Al2Cl6. In fact, this exists in the vapour at temperatures not too far above the sublimation temperature - not in the solid. The structure of aluminium chloride is discussed on the page about Period 3 chlorides.






Phosphorus(III) chloride is a colourless fuming liquid.


Note:  These equations are often given starting from P rather than P4. It depends which form of phosphorus you are talking about.

If you are talking about white phosphorus (as I am here), P4 is the correct version. If you are talking about red phosphorus, then P is correct. Red phosphorus has a different (polymeric) structure, and P4 would be wrong for it.

In my experience, red phosphorus is less commonly used in labs at this level (it isn't as excitingly reactive as white phosphorus!) - which is why I am concentrating on the white form.



Chlorine and argon


Questions to test your understanding


questions on the reactions of Period 3 elements


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© Jim Clark 2005 (updated October 2014)