Period 3 Oxides

Bonding in period 3 oxides

To begin with, let’s look at the bonding within period 3 oxides. This will help with our understanding when we investigate the properties and reactions of period 3 oxides later on.

In general, as you move from left to right across the period, the bonding in period 3 oxides changes from ionic to covalent. This has to do with the electronegativity difference between the period 3 element and oxygen.

Metal oxides

Na₂O and MgO are ionic compounds. This is because there is a large electronegativity difference between the metal and oxygen. They consist of a giant lattice of alternating positive metal ions and negative oxygen ions.

Al₂O₃ is also ionic but displays a covalent character. Although the electronegativity difference between aluminium and oxygen is large enough for a transfer of electrons, forming an ionic bond, the aluminium ion is quite small and has a high charge density. This means that it is partially able to attract one of the oxygen ion’s pairs of electrons, distorting the oxygen ion’s electron cloud. The electron pair starts acting a little like a shared pair of electrons.

Non-metal oxides

SiO₂ is a giant covalent macromolecule. The electronegativity difference between silicon and oxygen isn’t that large, and so SiO₂ bonds covalently. It consists of a giant lattice of silicon and oxygen atoms joined by covalent bonds.

P₄O₁₀, SO₂ and SO₃ also bond covalently. However, they form simple covalent molecules instead of a giant covalent macromolecule.

Fig. 1 - Electronegativity in the periodic table

The properties of period 3 oxides

Next, let’s see how the structure and bonding of period 3 oxides affect their properties. In particular, we’ll focus on the melting point and electrical conductivity. We’ll also look at the oxidation state.

Melting point of period 3 oxides

The period 3 metal oxides have high melting points, while the simple covalent oxides have low melting points. However, the giant macromolecule SiO₂ has a very high melting point.

Metal oxides

Na₂O, MgO and Al₂O₃ have high melting points. This is because they are ionic compounds, held together as a solid by strong electrostatic attraction between their positive metal ions and the negative oxygen ions. MgO and Al₂O₃ have higher melting points than Na₂O because they contain metal ions with a higher charge.

Non-metal oxides

SiO₂ has a very high melting point because it is a giant covalent macromolecule. It consists of a lattice of silicon and oxygen atoms stretching in all directions, held together by strong covalent bonds. Melting SiO₂ involves overcoming these covalent bonds, which requires a lot of energy.

P₄O₁₀, SO₂ and SO₃ have low melting points. This is because they are simple covalent molecules. Although there are strong covalent bonds within the molecules, the only forces holding the molecules together as a solid are weak intermolecular forces, which don’t require much energy to overcome. P₄O₁₀ has a higher melting point than SO₃, which in turn has a higher melting point than SO₂ due to the fact that it is a larger molecule.

Electrical conductivity

In their solid state, none of the period 3 oxides can conduct electricity. However, this changes in other states.

Metal oxides

Period 3 metal oxides (Na₂O, MgO and Al₂O₃) can conduct electricity when molten or aqueous. Remember that metal oxides are made up of an ionic lattice of positive metal ions and negative oxygen ions. As a solid, these ions are fixed firmly in place by strong electrostatic attraction, and so there are no charged particles free to move about. However, when molten or aqueous, some of the electrostatic attraction is overcome, and the ions are able to move around and carry a charge.

Non-metal oxides

Period 3 non-metal oxides (SiO₂, P₄O₁₀, SO₂ and SO₃) can’t conduct electricity in any state. This is because they don’t contain any charged particles that can carry a charge.

Oxidation state

Now let’s have a go at working out the oxidation states of period 3 oxides.

Earlier in this article, we looked at the relative electronegativities of oxygen and the period 3 elements. Oxygen is more electronegative than all of them. This means that when it comes to period 3 oxides, oxygen always takes the lower oxidation state. In particular, it always takes an oxidation state of -2. The sum of the oxidation states in a neutral compound is always zero, and so using this knowledge, we can work out the oxidation states of the period 3 element involved. This table should help:

Fig. 2 - The oxidation states of period 3 oxides

Reactions of period 3 oxides

Period 3 oxides have one thing in common: they all contain oxygen. However, this doesn’t mean that they react in the same way. In this next section, we’ll look at how period 3 oxides react with oxygen, acids, and bases. This involves considering their acid-base nature.

Metal oxides on the left of period 3 tend to be basic in nature, while non-metal oxides are acidic. Al₂O₃ sits in the middle and is amphoteric.

Period 3 oxide reactions with water

In general, period 3 metal oxides are basic in nature. This means that they react with water to produce a hydroxide, forming a basic solution. On the other hand, period 3 non-metal oxides are acidic in nature. They react with water to form an acid.

Metal oxides

Na₂O and MgO react with water to form hydroxides. They do this because of their ionic bonding. They contain strongly basic oxide ions (O₂^-) thanks to the large electronegativity difference between the metal and oxygen. The oxygen ion can accept a hydrogen ion from the solution, acting as a base.

• Na₂O reacts with water to form NaOH, which dissociates into Na⁺ and OH^- ions.
• MgO reacts with water to form Mg(OH)₂, which is sparingly soluble and partially dissociates into Mg²⁺ and OH^- ions.

Here are the equations:

$$ Na_2O(s)+H_2O(l)\rightarrow 2NaOH(aq) $$

$$ MgO(s)+H_2O(l)\rightarrow Mg(OH)_2(aq) $$

On the other hand, Al₂O₃ is insoluble in water and thus won’t react at all.

Non-metal oxides

P₄O₁₀, SO₂ and SO₃ react with water to form acids. Because they bond covalently, they don’t contain any oxygen ions and so can’t act as bases. Instead, they’re able to donate a hydrogen ion in solution, meaning they act as an acid.

• P₄O₁₀ reacts with water to form H₃PO₄, which partially dissociates into H⁺ and H₂PO₄^- ions.
• SO₂ reacts with water to form H₂SO₃ (sulphurous acid), which partially dissociates into H⁺ and HSO₃^- ions.
• SO₃ reacts with water to form H₂SO₄ (sulphuric acid), which fully dissociates into H⁺ and HSO₄^- ions.

You’ll need to know the equations:

$$ P_4O_{10}(s)+6H_2O(l)\rightarrow 4H_3PO_4(aq) $$

$$ SO_2(g)+H_2O(l)\rightarrow H_2SO_3(aq) $$

$$ SO_3(g) + H_2O(l)\rightarrow H_2SO_4(aq) $$

Like Al₂O₃, SiO₂ is insoluble in water. It won’t react in water at all.

Fig. 3 - Sulphurous and sulphuric acid

Period 3 oxides’ reaction with acids and bases

We now know that two of the three period 3 metal oxides are basic in nature. They, therefore, react with acids. In contrast, period 3 non-metal oxides are acidic in nature and therefore react with bases. Al₂O₃ lies between the two groups and is amphoteric in nature.

Metal oxides

Na₂O and MgO act as bases by reacting with acids to form a salt and water. This is because they bond ionically. For example:

• Reacting Na₂O with HCl produces NaCl and H₂O.
• Reacting MgO with HCl produces MgCl₂ and H₂O.

The equations are shown below:

$$ Na_2O(S+2HCl(aq)\rightarrow 2NaCl(aq)+H_2O(l) $$

$$ MgO(s)+2HCl(aq)\rightarrow MgCl_2(aq)+H_2O(l) $$

Al₂O₃ is a little different – it is amphoteric. This means that it can behave as both an acid and a base. As with the other period 3 metal oxides, it acts as a base by reacting with an acid to form a salt and water, thanks to its ionic bonding. Here, the aluminium ion has a positive charge. But it can also act as an acid by reacting with bases, thanks to its covalent character. In this case, we form an aluminate, a compound in which the aluminium ion has a negative charge. For example:

• Reacting Al₂O₃ with HCl produces AlCl₃ and H₂O.
• Reacting Al₂O₃ with NaOH produces a variety of aluminates, depending on the conditions, one of which could be NaAl(OH)₄.

Here are the equations:

$$ Al_2O_3(s) + 6HCl(aq)\rightarrow 2AlCl_3(aq)+3H_2O(l) $$

$$ Al_2O_3(s)+2NaOH(aq)+3H_2O(l)+2NaAl(OH)_4(aq) $$

Non-metal oxides

SiO₂, P₄O₁₀, SO₂, and SO₃ act as acids by reacting with bases to form a salt and water. This is because they bond covalently. For example:

• Reacting SiO₂ with NaOH produces NaSiO₃ and H₂O.
• Reacting P₄O₁₀ with NaOH is like reacting phosphoric acid with NaOH. It produces a mixture of salts, including Na₃PO₄, as well as H₂O.
• Reacting SO₂ with NaOH is like reacting sulphurous acid with NaOH. Overall, it produces Na₂SO₃ and H₂O.
• Reacting SO₃ with NaOH is like reacting sulphuric acid with NaOH. Overall, it produces Na₂SO₄ and H₂O.

Once again, we’ve provided you with the equations:

$$ SiO_2(s)+2NaOH(aq)\rightarrow Na_2SiO_3(aq)+H_2O(l) $$

$$ P_4O_{10}(s)+12NaOH(aq)\rightarrow 4Na_3PO_4(aq)+6H_2O $$

$$ SO_2(g)+2NaOH(aq)\rightarrow Na_2SO_3(aq)+H_2O(l) $$

$$ SO_3(g)+2NaOH(aq)\rightarrow Na_2SO_4(aq)+H_2O(l) $$

Summary of reactions of period 3 oxides

To round up this section, here’s a useful table summarising the reactions of period 3 oxides and their acid-base natures.

Fig. 4 - Reactions of period 3 oxides

Period 3 hydroxides

Another type of period 3 compound that you might need to know about is period 3 hydroxides. In this section, we’ll look at the reactions of three of the period 3 hydroxides – NaOH, Mg(OH)₂ and Al(OH)₃ – with acids and bases.

Reaction with acids and bases

Both NaOH and Mg(OH)₂ act as typical bases. They react with acids to give a salt and water. For example:

• Reacting NaOH with HCl produces NaCl and H₂O.
• Reacting Mg(OH)₂ with HCl produces MgCl₂ and H₂O.

Here are the equations:

$$ NaOH(aq)+HCl(aq)\rightarrow NaCl(aq)+H_2O(l) $$

$$ Mg(OH)_2(aq)+2HCl(aq)\rightarrow MgCl_2(aq)+2H_2O(l) $$

Al(OH)₃ behaves a little differently – it is amphoteric. It acts as a base, reacting with an acid to produce a salt and water. But it also acts as an acid, reacting with bases to produce an aluminate. For example:

• Reacting Al(OH)₃ with HCl produces AlCl₃ and H₂O.
• Reacting Al(OH)₃ with NaOH produces NaAl(OH)₄.

The equations for these reactions are below:

$$ Al(OH)_3(aq)+2HCl(aq)\rightarrow AlCl_3(aq)+ 3H_2O(l) $$

$$ Al(OH)_3(aq)+NaOH_(aq)\rightarrow NaAl(OH)_4(aq) $$

Period 3 chlorides

The last thing on our agenda today is period 3 chlorides. We’ll look at their melting points, oxidation number and their reaction with water.

Melting point

In general, the period 3 metal chlorides have high melting points, while the period 3 non-metal chlorides have low melting points. AlCl₃ is an anomaly – despite being a metal chloride, it has a low melting point.

Metal chlorides

Both NaCl and MgCl₂ have high melting points. This is because they are ionic compounds. Thanks to the large difference in electronegativity between the metal and chlorine, they’re able to bond ionically, and the solid is held together by strong electrostatic attraction between the oppositely charged ions.

AlCl₃ is a little different. At room temperature and pressure, it bonds ionically, and so you’d expect it to have a high melting point. But as you increase the temperature, it turns from an ionic lattice into simple Al₂Cl₆ molecules, which eventually break up into smaller AlCl₃ molecules. These are both examples of simple covalent molecules. They’re held together as a solid by weak intermolecular forces, which don’t require much energy to overcome, and so AlCl₃ actually has a low melting point.

Non-metal chlorides

SiCl₄, PCl₅, SCl₂, and S₂Cl₂ all have low melting points. This is because they are simple covalent molecules. The only forces holding them together as a solid are weak intermolecular forces, which don’t require much energy to overcome.

Oxidation state

In all period 3 chlorides, chlorine takes a negative oxidation state of -1. This is because it is more electronegative than the period 3 element it bonds with. The sum of the oxidation states of the atoms in a neutral compound is always zero, and from this, you can work out the oxidation state of the period 3 element. It is quite simple to remember – with the exception of SCl₂ and S₂Cl₂, the oxidation state matches the element’s group number. The following table summarises the information for you.

Fig. 5 - The oxidation states of period 3 chlorides

Reaction with water

Finally, we’ll explore the reactions of period 3 chlorides with water, including taking a look at the pH of the solution formed. In general, the metal chlorides simply dissolve in water, while the non-metal chlorides react more vigorously. Once again, AlCl₃ bucks the trend by reacting like a non-metal chloride.

Metal chlorides

NaCl and MgCl₂ both dissolve in water. They do this thanks to their ionic bonding. NaCl forms a neutral solution of Na⁺ and Cl^- ions with a pH of 7, while MgCl₂ forms a slightly acidic solution of Mg²⁺ and Cl^- ions with a pH of around 6.

AlCl₃ acts differently, thanks to its covalent character. It reacts with water to form an acidic solution with a pH of around 3, giving off steamy fumes of HCl.

Non-metal chlorides

The other non-metal chlorides react much like AlCl₃, thanks to their covalent bonding. They react with water to form acidic solutions, each with a pH of around 2. All of the reactions give off steamy fumes of HCl.

There are clear trends in all of the different properties and reactions. Knowing some key facts, such as the type of bonding present in each compound, should help you when it comes to period 3 oxides, hydroxides and chlorides.