Elements And Compounds << Learners Hobby

The fact that matter can be classified into elements. compounds and mixtures is not new to you. This classification is based on what matter is made up of. Elements and compounds are made up of one kind of particle - either atoms or molecules. Mixture. on the other hand, contain more than one kind of particle, since they are composed of two or more elements or compounds mixed up loosely. We call elements and compounds pure substances because their composition is always fixed. This is not true of mixture. A mixture of salt and sugar, for example, can contain the two substances in any proportion. If you mix 100 gm of sugar and 100 gm of salt, it will be a salt-and-sugar mixture, and if you mix 100 gm of sugar and 50 gm of salt, it will still be a salt-and-sugar mixture.
ELEMENTS
    All matter is made up of elements. There are more than 114 elements, of which 92 occur in nature. The rest have been synthesised (made) by man. Iron, copper, hydrogen and oxygen are all elements.
    The smallest particle of an element is an atom. An element is a pure substance that cannot be broken up into simpler substances and contains only one king of atom. Different elements are made up of different kinds of atoms. But all the atoms in one element are identical. For instance, hydrogen contains hydrogen atoms, which are all identical. Oxygen contains oxygen atoms, which are identical. But hydrogen atoms are different from oxygen atoms. It is the kind of atom an element contains that determines its properties. The atoms of hydrogen are very different from those of oxygen, so hydrogen and oxygen behave differently, or have different properties.
    Though the atom is the smallest particle of an element, the atoms of many elements cannot exist on their own. They need to link with other atoms to form molecules. An oxygen atom, for example, cannot exist on its own. It pairs up with another atom to form an oxygen molecule. Mind you, an oxygen atom is still the most basic particle of oxygen-it has all the properties characteristic of oxygen. Also, it is the atom of an element which takes part in a reaction. That is how a water molecule is made up of two hydrogen atoms and one oxygen atom. If only a molecule of oxygen could take part in a reaction, this would not have been so. We can now say the following about atoms and molecules.
    An atom is the smallest particle of an element. It is not always capable of existing independently. It takes part in a chemical reaction.
    A molecule is the smallest particle is an element or a compound that can exist independently.
    The atomicity of an element is the number of atoms contained in a molecule of the element. The automicity of oxygen is two, while that of phosphorus is four. The atoms of some elements can exist independently, for example, helium, gold and silver.
COMPOUNDS
    A compound is a pure substance formed by the chemical combination of two or more elements in a fixed proportion. Common salt and water are two compounds that we use every day.
    Let us now see what exactly the definition of a compound means. The  smallest particle of a compound, as you already know, is a molecule. Every molecule of a compound is identical. Each contains the same number of atoms of the elements which constitute the compound. In other words, the composition of a compound is always the same, no matter how you prepare it. That is why we call a compound a pure substance. We could also say that a compound always contains the same elements chemically combined in the same ratio. A molecule of water will always contain two atoms of hydrogen and one atom of oxygen. Do you now see why we say that a compound is formed by the chemical combination of elements in a 'fixed proportion? And we specify 'chemical combination'  because just mixing together two elements does not form a compound.

    A compound, like an element, is a pure substance. However, an element cannot be broken down into simpler substances, while a compound can be broken down into the elements it is made up of. Remember though, that chemical processes have to be used to break down a compound. You cannot use physical methods to separate the elements that form a compound. For example, you cannot break down water into oxygen and hydrogen, unless you pass an electric current though it.

    A compound is made up of elements. However, the properties of a compound are very different from those of the elements that constitute it. This is because the atoms of the constituent elements combine, or link up together, to form different molecules, and the properties of a substance, as you know, depend on the kind of molecules it contains. Salt, for instance, is a white, crystalline solid formed by sodium and chlorine. But sodium is a soft, yellow metal, while chlorine is a greenish yellow gas with a choking smell.