Atoms form ionic bonds by giving or accepting electrons and covalent bonds by sharing electrons. Oxidation numbers represent electron sharing, loss, or gain. Oxygen is a strong oxidizing agent, but other elements can also oxidize. Elements have different oxidation numbers, and ions have an overall oxidation number.
Atoms combine by giving and accepting negatively charged electrons, forming an ionic bond, or by sharing electrons, forming a covalent bond. Typically, ionic bonds form between metals and nonmetals, while covalent bonds form only between nonmetals. An atom’s oxidation number represents the number of electrons it has given, accepted, or shared with other atoms, and will be positive when it has lost or shared electrons, and negative when it has gained or shared another atom’s electrons.
Originally, oxidation meant combining with oxygen; the opposite process, the removal of oxygen, was known as reduction. For example, when calcium combines with oxygen, the oxygen removes two electrons from the calcium, acquiring a -2 charge, leaving the calcium with a +2 charge. An ionic bond has formed; the calcium has been oxidized and the oxygen has been reduced, giving them oxidation numbers of +2 and -2, respectively. Covalent bonding also involves oxidation and reduction: to form water, two hydrogen atoms combine with an oxygen atom, each sharing an electron with it. The oxidation numbers of the two hydrogen atoms are each +1 and the oxygen atom -2.
Oxidation need not involve oxygen; today oxidation simply means loss of electrons and reduction, gain of electrons. As a rule, elements towards the lower left corner of the periodic table have the greatest tendency to lose electrons, while those towards the upper right corner have the greatest tendency to gain electrons. Oxygen has a strong tendency to remove electrons from other elements and is therefore an oxidizing agent; however, other elements can also oxidize. For example, fluorine is an even stronger oxidizing agent.
The oxidation number of oxygen is usually -2, but many elements can have more than one possible oxidation number. For iron it can be +2 or +3, while for hydrogen it is +1 when combined with non-metals, as in water (H2O), but -1 when combined with metals, as for sodium hydride (Na +H-) , where it accepts an electron. The oxidation number of nitrogen can vary from -3 to +5. The oxidation numbers of the ions in an ionic compound are equivalent to the charges on the ions, while those of the atoms in a compound always add to zero.
Elements can combine to form ions that have an overall oxidation number. For example, one sulfur atom can combine with four oxygen atoms to form a sulfate group (SO42-) with an oxidation number of -2. Ammonia, a compound of nitrogen and hydrogen with the formula NH3, can combine with an additional hydrogen atom to form the ammonium group NH4+, with oxidation number +1. This behaves like a metal and forms ionic compounds such as ammonium sulphate ((NH4+)2SO42-), where two ammonium groups combine with one sulphate group.
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