We have seen previously that elements are substances made up of just one type of atom, but if you combine two or more elements you can end up with either a mixture or compound. As you will see later, mixing iron filings and powdered sulphur will give you a mixture of iron and sulphur, it will not give you iron sulphide. If the two elements are chemically reacted together you will end up with iron sulphide which will have totally different properties from its component elements iron and sulphur. So what exactly is a compound?


When elements react together, and atoms combine chemically by the formation of chemical bonds, we produce chemical compounds. There are a number of ways in which the element atoms can combine, they can bond covalently which means that one atom of each element "shares" one or more electrons to form the bond. Elements may create ionic bonds, where one element has given up one or more electrons and the other element has received them. This produces entities called ions which are either single atoms or groups of atoms carrying one or more overall positive or negative charges as a result of this electron loss/electron again. We will come to metallic bonding and coordinates (dative) bonding subsequently.


If we mix together carbon and oxygen, by placing some powdered carbon into a gas jar full of oxygen and giving it a good shake, we will not make carbon monoxide or carbon dioxide. These two elements can easily be separated by simply allowing the oxygen to escape from the gas jar. However, if we burn carbon in oxygen then depending on the amount of available oxygen we will produce carbon monoxide or carbon dioxide (this depends on whether or not we have complete or incomplete combustion).


Covalent bonds are usually made between nonmetals, for example in the complete combustion of carbon in oxygen will produce covalently bonded carbon dioxide. In the case of ionic bonds, these are usually made between a metal and a nonmetal, for example sodium and chlorine will combine to produce sodium chloride. Sodium is a bright, soft silvery metal which tarnishes rapidly in air and reacts readily with water to produce sodium hydroxide solution and hydrogen gas. Chlorine is a yellowy green highly poisonous gas which is an irritant and can cause severe lung damage. The product sodium chloride on the other hand is essential in our diet, strange when you consider what the effects would be of consuming its respective elements! This serves to prove that a chemical compound does not display the properties of its component elements.


Q. Sulphuric acid is composed of hydrogen sulphur and oxygen, is sulphuric acid a mixture or compound?


A. Sulphuric acid is most definitely a compound, it is a highly corrosive substance combined by the chemical combination of hydrogen gas, oxygen gas and solid sulphur in the proportions of 2 to 4 to 1 respectively.


Q. Do metals form ionic bonds or covalent bonds, and in compounds containing metals how are the respective elements held together?


A..Metals give up electrons to form positively charged ions, they form ionic bonds with (usually) non-metals and the positively and negatively charged ions are held together by electrostatic attraction.