Oxides

Oxygen constitutes about 21 per cent of atmospheric gases and is very reactive. All elements on 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 large electronegativity than oxygen. So, all binary compounds of oxygen except with that of fluorine are deemed to be oxides.

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

What is Oxide?

An oxide is a category of chemical compound that has one or more oxygen atoms as well as another element in its composition such as Li2O, CO2, H2O, etc.

Classification of Oxides

Oxides can be classified in terms of:

  • 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 non-metallic 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 air to produce carbon dioxide.

2C + O2→ CO2

  • Sulphur burns in air to produce sulphur dioxide.

2S + O2→ SO2

  • Other examples of non-metallic 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 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 as 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, osmium tetroxide, depolymerize 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 Fe2O3

Surface and Bulk Oxides

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

Peroxides and Superoxide’s

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 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 on reaction with acid.

Superoxides have a formula of O2. Super oxides also have 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, electron belongs to both the oxygen atoms. So, an average oxidation state for both the 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 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 character.

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. Some of the characteristics of this oxide include.

  • 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.
  • Covalent in nature and so exists as individual molecules.
  • Have low melting and boiling point.
  • Do not conduct electricity even in a molten state.

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

  • 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 from salt.
  • They are ionic in nature and exhibit a polymeric structure.
  • Have high melting and boiling points.
  • Conduct electricity in a molten state.

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

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

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

The properties are summarized in the table.

Property Acidic oxide Basic oxide Amphoteric oxide
Reacting

element

Nonmetal-

electronegative

Metal-

electronegative

Metalloid
With water Forms acid

SO3 + H2O → H2SO4

Anhydride of acid

Forms a Base

Na2O + H2O → 2NaOH
Anhydride of base

Insoluble -do not react
With Base

or acid

Salt and water

CO2 + Ba(OH)→ BaCO3

+ H2O

Salt and water

MgO + H2SO4 → MgSO4

+ H2O

Salt and water

ZnO + 2NaOH → Na2ZnO2 + H2O

ZnO + H2SO→ ZnSO4 + H2O

With basic/

Acidic oxide

Na2O + CO2 → Na2CO3 CaO + CO2 → CaCO3 Salt
Example CO2, SO3, P2O5, Cl2O Alkali and alkaline

metal oxides

BeO, Al2O3, PbO, SnO, Bi2O5

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.

Periodic table - Trends in the Properties of Oxides

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 change over to elements with increasing electronegative character. The metalloids, placed in between the electropositive metals and electronegative non-metals 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 gases 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. Basic nature of the oxides increases down the column in any group. The solubility of oxides increases down the column.

Trend in the Oxide Properties with the Oxidation States

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

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

Chromium oxide Oxidation state of Chromium Nature of oxide
CrO +2 Basic
Cr2O3 +3 Amphoteric
CrO3 +6 Acidic