7th Class Science Related to Competitive Exam NCERT Summary - Elements, Compounds and Mixtures

Elements, Compounds and Mixtures

• Matter can be classified into elements, compounds and mixtures. These, in turn, are made of small particles such as molecules and atoms.
• Elements: When a substance is made of only one kind of atoms, it is called an element. For example, copper, gold, silver, iron, are all elements. Similarly, oxygen is a gaseous element made of oxygen molecules and each oxygen molecule is made of two oxygen atoms. Hydrogen, nitrogen, chlorine are other examples of gaseous elements.
• Compounds: When two or more atoms combine in a fixed ratio, they produce compounds. They can be broken down into the elements from which they are made. Water is an example of a compound. Each water molecule is made of two atoms of hydrogen and one atom of oxygen. Thus on analysing water, we find two different types of atoms, those of hydrogen and oxygen. Sugar is yet another example of a compound. A molecule of sugar is made of 12 atoms of carbon, 22 atoms of hydrogen and 11 atoms of oxygen. Each compound contains its constituent elements in a fixed proportion.

Note: Properties of a compound are different from the properties of its constituent elements. For example, hydrogen bums, oxygen supports burning and both are gases but water is a liquid and it puts off fire.

• Mixtures: Many substances we come across in our daily lives are neither pure elements nor compounds; they are mixtures. A mixture could contain several elements and compounds. For example, air is a mixture of several gases. It contains elements such as nitrogen, oxygen and compounds such as carbon dioxide and water vapour. It also contains some other gases and dust particles.

SYMBOLS AND FORMULAE

• Till date, more than hundred chemical elements are known to man. All the matter in the entire universe is made from them. These elements include metals like zinc, copper, silver, gold, iron, sodium, potassium. And non-metals like carbon, silicon, oxygen, hydrogen, sulphur. Nitrogen and iodine. Most of them occur on the Earth in various amounts. Ninety per cent by mass of the Earth's crust is composed of only five elements—oxygen, silicon, aluminium, iron and calcium.
• When referring to these elements and their many compounds, it would be very inconvenient to use their full names all the time. Thus scientists use symbols and groups of symbols as "chemical shorthand" to represent elements, compounds and chemical reactions conveniently and accurately.
• Each element is symbolised by a single letter or two letters of the English alphabet. In most cases, the first letter of the name of an element is taken as a symbol and is written in capitals. For example, H stands for hydrogen, S for sulphur, 0 for oxygen and C for carbon.
• In some cases, two or more elements have names beginning with the same letter. To avoid confusion, more letters from an element's name are added to its symbol. For example, C1 is used for chlorine, Ca for calcium, Co for cobalt, Cr for chromium. In such symbols, only the first letter is written in capitals.
• Symbols of some elements have been derived from their Latin names.

For example:

• The molecules of an element or of a compound can be denoted by a group of atomic symbols. This grouping is known as the chemical formula of the molecule. For example, oxygen exists in groups of two atoms called molecules. The number of atoms in a single molecule of an element is known as atomicity.

• The molecules of a compound consist of the atoms of its constituting elements in a definite proportion. This means the composition of a compound is fixed. The atoms are present in a molecule of a compound in whole numbers. For example, one atom of hydrogen and one atom of chlorine makes one molecule of hydrogen chloride. So, a molecule of hydrogen chloride can be represented as \[{{H}_{1}}C{{l}_{1}}\] or HCl. The number of atoms present in a molecule is represented as a subscript written after the symbol of an element.

Note: Usually one (1) is not written subscript, since it is obvious. For example, two atoms of hydrogen and one atom of oxygen make one molecule of water. Thus, a molecule of water can be represented as \[{{H}_{2}}{{O}_{1}}\,or\,{{H}_{2}}O.\]

• To represent a certain number of atoms or molecules of a given element or a compound, the appropriate way is

\[2\,\,HCL\to \] Denotes two molecules of hydrogen chloride

\[2\,{{O}_{2}}\to \] Denotes two molecules of oxygen

• When iron (Fe) and sulphur (S) are heated, they combine to form a new substance called iron sulphide (FeS). It is an example of a chemical reaction and can be represented by chemical equations:

\[\underset{(Fe)}{\mathop{Iron}}\,\,+\underset{(S)}{\mathop{\,Sulphur}}\,\,\to \,\underset{(Fes)}{\mathop{Iron\,Sulphide}}\,\]

Note: Substances taking part in a chemical reaction are called reactants. In the above reaction, iron and sulphur are reactants. The reactants are placed on the left side of a chemical equation. The chemicals formed in a reaction are known as products, i.e., iron sulphide is the product in the above reaction. Products are placed on the right side of a chemical equation.

• A chemical equation is balanced so that total number of atoms of each element is identical on both sides. The balancing of an equation represents the fact that atoms are neither created nor destroyed during a chemical reaction. They are only rearranged.