Period 3 Elements and Oxides

A LevelAQA

Period 3 Elements and Oxides Revision

Period 3 Elements and Oxides

Period 3 elements and their oxides can undergo reactions with molecules like water and oxygen as well as acids and bases to produce various products. The products produced and nature of these reactions depend upon the type of bond within the oxides.

Reactions of Sodium and Magnesium with Water

Sodium and magnesium are both elements with their outer electrons in the s subshell. They both form ionic bonds and positive ions with sodium forming a +1 ion and magnesium forming +2.

When reacting with water, sodium and magnesium undergo slightly different reactions. This is because sodium is more reactive than magnesium and more readily ionises.

Sodium can react with cold water to form sodium hydroxide and hydrogen.


Magnesium reacts very slowly with cold water because it is less reactive than sodium. However, it reacts more readily with steam to form magnesium oxide and hydrogen since the steam provides energy for the reaction to take place.


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Trends in Period 3 Reactions with Oxygen

Apart from argon, which is inert, all of the elements in period 3 react with oxygen to form oxides. When period 3 elements form these oxides, they usually take on their highest possible oxidation state. For example, aluminium would react to form \text{Al}_2\text{O}_3 with the oxidation state of aluminium being +3, the highest it can be. The exception is sulfur which forms an oxide where the oxidation state of sulfur is +4.

When sodium reacts with oxygen, it burns with a yellow flame, and \text{Na}_2\text{O} is formed, a white solid.


Magnesium, aluminium, silicon, and phosphorus all burn with a white flame.

2\text{Mg} + \text{O}_2\rarr 2\text{MgO}
4\text{Al} + 3\text{O}_2 \rarr  2\text{Al}_2\text{O}_3
\text{Si} + \text{O}_2 \rarr \text{SiO}_2
4\text{P} + 5\text{O}_2 \rarr \text{P}_4\text{O}_{10}

Sulfur burns with a blue flame and forms sulfur dioxide an acidic, choking gas.


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Melting Points of Period 3 Oxides

Period 3 oxides have three different structures. \text{Na}_2\text{O}, \text{MgO} and \text{Al}_2\text{O}_3 all have a giant ionic lattice structure. \text{SiO}_2 has a macromolecular structure. \text{P}_4\text{O}_{10} and \text{SO}_2 both have a simple molecular structures. These different structures lead to period 3 metal oxides having different melting points.

The ionic metal oxides have strong forces of attraction between the ions and therefore have high melting points. The melting point from \text{Na}_2\text{O} to \text{Al}_2\text{O}_3 increases due to the increased charge of the cation, which makes the electrostatic forces even stronger.

\text{SiO}_2 is a macromolecular oxide, so it has a lot of very strong covalent bonds. These bonds need a lot of energy to break and therefore have very high melting points.

Simple molecular oxides have weak intermolecular forces between molecules and, therefore, tend to have low melting points.

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Reactions of Period 3 Oxides with Water

When period 3 oxides react with water, they can form alkaline or acidic solutions.

Ionic Oxides
Ionic oxides, namely \text{Na}_2\text{O} and \text{MgO} both react with water to form hydroxides. Sodium hydroxide will have a higher \text{pH} than magnesium hydroxide.

\text{Na}_2\text{O} + \text{H}_2\text{O} \rarr 2\text{NaOH}

Magnesium hydroxide is only slightly soluble in solution so only slightly dissociates making the\text{pH} lower. This difference in solubility is due to the sizes of the magnesium and sodium ion. The magnesium ion is smaller than the sodium ion so has a stronger interaction with hydroxide ions.

\text{MgO} + \text{H}_2\text{O} \rarr \text{Mg(OH)}_2

Simple Molecular Oxides
Simple molecular oxides react with water to form acids. Apart from \text{H}_2\text{SO}_3, all of the acids formed are very strong and have a low \text{pH} of around 0. \text{H}_2\text{SO}_3 is a weaker acid and usually has a\text{ pH} between 2.5 and 3.5.

\text{P}_4\text{O}_{10} + 6\text{H}_2\text{O} \rarr 4\text{H}_3\text{PO}_4
\text{SO}_2 + \text{H}_2\text{O} \rarr \text{H}_2\text{SO}_3
\text{SO}_3 + \text{H}_2\text{O} \rarr \text{H}_2\text{SO}_4

\text{Al}_2\text{O}_3 and \text{SiO}_2 do not react with water because the bonds within each of the molecules are very strong.

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Reactions of Period 3 Oxides with Acids and Bases

Oxides can be basic, acidic, or amphoteric. When these react, they undergo a neutralisation reaction and form a salt and water.

Basic oxides react with acids to form a salt and water. \text{Na}_2\text{O} and \text{MgO} are examples of basic period 3 oxides.

\text{Na}_2\text{O} + 2\text{HCl} \rarr 2\text{NaCl} + \text{H}_2\text{O}
\text{Na}_2\text{O} + \text{H}_2\text{SO}_4 \rarr \text{Na}_2\text{SO}_4 + \text{H}_2\text{O}
\text{MgO} + 2\text{HCl} \rarr \text{MgCl}_2 + \text{H}_2\text{O}

Acidic oxides react with bases to form salts. \text{P}_4\text{O}_{10}, \text{SO}_2 and \text{SO}_3 are both examples of acidic period 3 oxides.

\text{P}_4\text{O}_{10} + 12\text{NaOH} → 4\text{Na}_3\text{PO}_4 + 6\text{H}_2\text{O}
\text{P}_4\text{O}_{10} + 6\text{Na}_2\text{O} \rarr 4\text{Na}_3\text{PO}_4
\text{SO}_2 + 2\text{NaOH}  \rarr \text{Na}_2\text{SO}_3 + \text{H}_2\text{O}
\text{SO}_3 + 2\text{NaOH} \rarr \text{Na}_2\text{SO}_4+ \text{H}_2\text{O}

\text{SiO}_2 is also classed as an acidic period 3 oxide because it only reacts with very concentrated sodium hydroxide. This is because of the strong covalent bonds in \text{SiO}_2.

\text{SiO}_2 + 2\text{NaOH} \rarr \text{Na}_2\text{SiO}_3 + \text{H}_2\text{O}

Amphoteric oxides can act as both an acid and an alkali and can react with acids or bases. \text{Al}_2\text{O}_3 is an amphoteric period 3 oxide.

Below are some reactions where aluminium oxide acts as a base.

\text{Al}_2\text{O}_3 + 3\text{H}_2\text{SO}_4 \rarr \text{Al}_2\left(\text{SO}_4\right)_3  + 3\text{H}_2\text{O}
\text{Al}_2\text{O}_3 + 6\text{HCl} \rarr 2\text{AlCl}_3 + 3\text{H}_2\text{O}

Below is an example of a reaction where aluminium oxide acts as an acid.

\text{Al}_2\text{O}_3 + 2\text{NaOH} + 3\text{H}_2\text{O} → 2\text{NaAl}\left(\text{OH}\right)_4

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Period 3 Elements and Oxides Example Questions

\text{P}_4\text{O}_{10} is a bigger molecule (/ has more electrons/ has a higher \text{M}_r)

The Van der Waals forces between molecules are stronger/ require more energy to break

\text{Al}_2\text{O}_3 + 6\text{HCl} \rarr 2\text{AlCl}_3 + 3\text{H}_2\text{O}

Silicon dioxide reacts with bases.

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