ICSE Class 7 Chemistry Chapter 01 Elements and Compounds

Elements and Compounds

Things around us, like air, water and gold, are all made of matter. Yet they differ from each other. Matter exists in different forms. To make the study of matter easier, scientists have classified it into three groups - elements, compounds and mixtures.

Elements, Compounds and Mixtures

An element is a substance that cannot be split into simpler substances by a chemical means.

A large number of elements - about 114 - are known at present. Carbon, nitrogen, oxygen, chlorine, sodium, iron, gold, copper, zinc, etc., are all elements. They cannot be split into simpler substances by a chemical means.

A compound is a substance that can be split into two or more simpler substances by a chemical means.

For example, water can be split into two simpler substances - hydrogen and oxygen - which are elements. Similarly, sand is a compound, called silica, which can be split into the elements silicon and oxygen. And the common salt we eat is a compound, called sodium chloride, which can be split into the elements sodium and chlorine.

The amounts of different elements in a given amount of a compound are always fixed. For example, 9 g of pure water will always contain 1 g of hydrogen and 8 g of oxygen - no matter from which source the water has been obtained.

Elements combine among themselves in definite proportions to form compounds.

Remember that compounds differ in properties and appearance from the elements that make them up. (See Figure 1.1.) The metallic red copper turnings (element) are very different from the blue, crystalline copper sulphate (a compound formed by copper). Also, the greyish iron (element) pins are very different from the green crystals of ferrous sulphate (a compound formed by iron).

A mixture is a substance containing more than one element or compound in any proportion. The components of a mixture can be separated by physical means like filtration, sublimation, distillation, etc.

Air is a mixture that contains elements (like oxygen and nitrogen) as well as compounds (like carbon dioxide and water vapour). Unlike compounds, the composition of a mixture may vary. For example, the air in cities has more carbon dioxide than does the air in villages. Also, mixtures of sand and water may contain the two components in any proportion.

Atoms and Molecules

Matter is not continuous. It consists of extremely small parts called atoms and molecules.

Atoms

If we keep dividing a given mass of an element, we will finally get an indivisible part that has all the properties of the element.

Such an indivisible part is called an atom (derived from the Greek word atomos, meaning indivisible). It is the atoms that take part in a chemical reaction.

An atom is the smallest part of an element that takes part in a chemical reaction.

Characteristics of atoms

Atoms have the following characteristics.

1. The atoms of an element are all alike but different from those of others. For example, the atoms of hydrogen are alike and so are those of oxygen. But the atoms of hydrogen are different from those of oxygen.

2. The atoms of an element show all the properties of the element. Each atom of hydrogen or oxygen will retain all the properties of the element.

3. One kind of atom cannot be changed into another. And an atom can neither be created nor destroyed by a chemical means. An atom of hydrogen cannot be changed into that of oxygen or vice versa. And neither can it be destroyed.

Atoms are Represented by Symbols

A symbol is an abbreviation for the name of an element.

It represents an atom of an element. The symbols of different elements have been derived in three ways.

1. The first letter (in capital) of the English name of an element

2. The first letter along with one more letter of the English name of an element

3. One or two letters of the Latin name of an element

Teacher's Note

When you look at a piece of gold jewellery and a piece of iron, you are observing two different elements with distinct properties that cannot be changed by physical means - only through chemical reactions.

Molecules

Atoms usually do not exist independently. They generally combine among themselves to form molecules.

A molecule is the smallest part of an element or a compound that can exist independently.

When an atom of an element combines with another atom(s) of the same element, a molecule of the element is formed.

For example, two atoms of hydrogen combine to form a molecule of hydrogen. Similarly, two atoms of nitrogen combine to form a molecule of nitrogen.

Not all molecules contain more than one atom. A molecule of a noble gas, i.e., helium, neon, argon, krypton or xenon, contains only one atom of the element. When a noble-gas atom exists independently, for example, in a sample of the gas, it is called a molecule.

When atoms of two or more elements combine among themselves, a molecule of a compound results. For example, two atoms of hydrogen combine with one atom of oxygen to form a molecule of water. A molecule of water can exist independently.

Elements and compounds can now be redefined as follows.

An element is a substance a molecule of which contains atom(s) of the same kind.

A compound is a substance a molecule of which contains atoms of two or more different kinds.

Molecules are represented by formulae

A molecule of an element or a compound is represented by a formula. A formula directly gives the number of atoms of the same or different elements present in the molecule.

Formulae of elements: A molecule of a noble gas, e.g., helium (He), neon (Ne) or argon (Ar), contains only one atom of the element. Hence the molecules of these elements are represented by their symbols. They are said to be monoatomic. However, a molecule of hydrogen, nitrogen, oxygen or chlorine contains two atoms of the element. They are diatomic and are represented as \(H_2\), \(N_2\), \(O_2\), and \(Cl_2\) respectively. A common example of a triatomic gas is ozone (\(O_3\)).

The number of atoms contained in a molecule of an element is called its atomicity.

The atomicity of phosphorus is 4 (\(P_4\)) and that of sulphur is 8 (\(S_8\)).

Formulae of compounds: The formula of a compound formed by two elements is obtained by transposing their valencies. Suppose an element A has a valency y and element B has a valency x. Then the compound formed between A and B will have the formula \(A_x B_y\). The subscripts should be divided by a common factor, if any.

There are some exceptions like \(H_2 O_2\) (hydrogen peroxide) in which the numeral subscripts are not divided by the common factor. You will understand the reasons for this in higher classes.

The formulae of some common compounds are given in Table 1.3.

Teacher's Note

Water molecules are made of hydrogen and oxygen atoms bonded together - the same elements that make up air, yet water has completely different properties because of how those atoms are arranged.

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