Oxides

Oxygen constitutes about 21 per cent of atmospheric gases and is very reactive. All elements on the earth react with oxygen to form oxygen-containing compounds. Of the oxygen compounds, binary compounds of oxygen with lesser electronegative elements are called oxides. Fluorine is the only element having larger electronegativity than oxygen. So, all binary compounds of oxygen except fluorine are deemed to be oxides.

Most of the stable oxides have oxygen in their -2 oxidation state. Examples of oxides include Na₂O, CaO, A₂O₃, CO₂, N₂O₃, Cl₂O and XeO₂.

What Is Oxide?

An oxide is a category of a chemical compound that has one or more oxygen atoms as well as another element in its composition, such as Li₂O, CO_2, H₂O, etc.

Classification of Oxides

Oxides can be classified in terms of the following:

• The other combining element as metallic, nonmetallic oxides.
• Structure of the oxide as polymeric, molecular and compound.
• Nature of formation as at the surface or bulk.
• Oxidation state as peroxides and superoxides.
• Nature of oxides as acidic, basic, amphoteric and neutral.

Metallic and Nonmetallic Oxides

Binary oxygen compounds containing electropositive metals are metallic oxides, while those having nonmetals are nonmetallic oxides.

Metallic oxides:  A chemical compound formed by metal and oxygen.

Examples:

• Magnesium reacts with oxygen to form magnesium oxide

2Mg + 2O₂→ 2MgO

• Other examples of metallic oxides – Na₂O, Al₂O₃, FeO, CuO, V₂O₅

Nonmetallic oxides: In the air, nonmetal reacts with oxygen to produce nonmetallic oxides.

Examples:

• Carbon burns in the air to produce carbon dioxide.

2C + O_{2→ CO2}

• Sulphur burns in the air to produce sulphur dioxide.

2S + O₂→ SO₂

• Other examples of nonmetallic oxides – CO, NO₂, Cl₂O, XeO₄

Polymeric, Molecular and Compound Oxides

Polymeric oxides: In oxides with a crystalline structure, oxygen is bonded with several metallic atoms such that the bonding and the structure of the oxide are polymeric. For example, titanium oxide crystal has a rutile structure, where each titanium is surrounded by six oxygen atoms, and each oxygen is bonded to three titanium in a triangular direction. The bonding atom involves other unit cells. This involves the participation of many unit cells, like polymer formation from a monomer.

So, these oxides are considered polymeric oxides.

Molecular oxides: The binary oxides existing as individual molecules are molecular oxides.

Oxides with simple atomic ratios are generally molecular in nature. Oxides of carbon/nitrogen and halogen are examples of molecular oxides.

Some polymeric and crystalline oxides, like phosphorus pentoxide, selenium dioxide, and osmium tetroxide, depolymerise to molecular oxides at higher temperatures.

Compound oxides: These are a mixture of one or more binary oxides. Fe₃O₄ is a mixture of FeO and Fe₂O_3.

Surface and Bulk Oxides

The strong reactive metal reacts completely to form oxides in the entire volume. Less reactive metals like aluminium and noble metals like silver and gold react to form only oxides at the surface. The surface oxide prevents the entry of oxygen inside to react with the atoms in the bulk.

Peroxides and Superoxides

Peroxides are oxides having an oxygen-oxygen bonding, and an oxidation state of -1 are peroxides. H₂O₂ has a molecular structure of H-O-O-H. Here, each oxygen is bonded to one Hydrogen, and there is a -O-O- bonding. Since the oxidation state is the number of the bond that oxygen makes with a hetero-atom, the oxidation state of both oxygen atoms is the same and equal to -1. So, H₂O₂ is a peroxide. Metallic peroxide, like sodium peroxide, releases hydrogen peroxide in reaction with acid.

The formula for superoxide is O₂^–, and it has an O-O bonding. But one oxygen has an extra electron. Hence, unlike peroxides, the oxidation state of one oxygen atom is zero, while that of the other is -1. As both the oxygen can exchange the extra electron, the electron belongs to both the oxygen atoms. So, an average oxidation state for both oxygen atoms becomes – 1/2. In superoxides, oxygen has a fractional oxidation state of – 1/2. Potassium oxide (KO₂) is an example of superoxide.

Acidic/Basic/Amphoteric/Neutral Oxides

Most of the elements on the earth exist in the form of oxides. But the elemental oxides differ much in their chemical properties. Some oxides are acidic in character, while some are basic. Some oxides can behave both as an acid and basic, and some are neutral and do not possess either acidic or basic characteristics.

Neutral oxides: Water is an example of neutral oxides. Water is neither acidic nor basic. Other examples of neutral oxides are carbon monoxide, nitric oxide, and nitrous oxide.

Acidic oxide: Examples include carbon dioxide, sulphur dioxide, phosphorus pentoxide, sulphur trioxide, chlorine oxides, etc. The characteristics of this oxide include the following:

• They are mostly oxides of non-metals.
• They dissolve in water, and the solution behaves like an acid. Because of this property, these oxides are referred to as anhydride of acids.
• Acidic oxides react with basic substances in both the presence and absence of water to form a salt.
• They are covalent in nature and so exist as individual molecules.
• They have low melting and boiling points.
• They do not conduct electricity even in a molten state.

Basic oxides: Examples of basic oxides are Na₂O and CaO. They have the following characteristics.

• They are formed when electropositive metals react with oxygen.
• They dissolve in water to form hydroxide ions, and hence act as bases. Basic oxides are the anhydrides of bases.
• Basic oxides react with acidic substances to form salt.
• They are ionic in nature and exhibit a polymeric structure.
• They have high melting and boiling points.
• They conduct electricity in a molten state.

Amphoteric oxides: Examples include beryllium oxide, aluminium oxide and zinc oxide.

These are oxides formed by elements exhibiting the trans-over of property from the metallic to non-metallic in the periodic table.

• They are oxides of metalloids and some elements close to them.
• These oxides are generally insoluble in water.
• They react with both acids and bases to form salts.

The properties of Amphoteric oxides are summarised in the table.

Trends in the Properties of Oxides

The variation in the properties of the oxides of elements in terms of chemical nature, strength and stability can be judged from their location in the periodic table.

Trends in the Properties of Oxides Along the Period

In the periodic table, electropositive elements are present on the left side. Along the period, the electropositive nature slowly decreases and changes over to elements with increasing electronegative character. The metalloids, placed in between the electropositive metals and electronegative nonmetals, indicate the change over the property.

Electropositive metals form basic oxides. Along the period, as electronegativity increases, the basic nature of the oxides decreases and becomes both acidic and basic (amphoteric) at some transition element, and then becomes increasingly acidic. So, in any period, the first group elements form strongly basic oxides, while the seventh group elements form strongly acidic oxides. Higher the electronegativity of the element bonding with oxygen, higher the acid nature of the oxide.

In the inert gas elements, only xenon forms oxides, and they are acidic.

Trends in the Properties of Oxides Along the Column

Down the column, elements become more electropositive, and hence form stronger basic oxides. The basic nature of the oxides increases down the column in any group. The solubility of oxides increases down the column.

The Trend in the Oxide Properties with the Oxidation States

Most elements exhibit multiple oxidation states, and oxides stabilise the maximum oxidation state of the combining element. So, the higher the oxidation state of the element, higher the stability of the oxide. The higher the oxidation state of the element, the oxide is relatively more acidic.

Chromium, for example, forms multiple oxides with different oxidation states. Chromium oxide with the highest oxidation state is acidic.