Class 7 Science The Language of Chemistry Exam Notes

Covers the following topics:

1.Symbols

2.Formulae

This Class 7 Science exam notes page on The Language of Chemistry explains how elements are written as symbols, how molecules and compounds are written as formulae, how valency is used to write formulae, what compound radicals are, and how chemical reactions are written as balanced chemical equations.

INTRODUCTION

You would need many words to describe a chemical change in ordinary language. But you can do so with only a few letters and numbers in the language of chemistry. J.J. Berzelius laid the foundation of this language in the early nineteenth century. It gradually developed into its present form. In this language, an atom is represented by a symbol, a molecule by a formula, and a chemical change (that is, a reaction) by a chemical equation.

SYMBOLS

A symbol is an abbreviation of the name of an element. The symbols of elements have been derived in three different 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 (this becomes necessary when the names of two or more elements begin with the same letter):

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

What does a symbol represent?
The symbol of an element represents the following:
1. An element in particular. For example, the symbol of sodium is Na and that of chlorine is Cl. So, instead of saying that the compound common salt is made up of the elements sodium and chlorine, you can say that it is made up of Na and Cl. You can also say that Cu is red-brown whereas Au is yellow, and that Ca is a metal whereas Cl is a nonmetal.
2. An atom of an element. In formulae and equations, a symbol represents an atom of an element. More than one atom in a molecule is shown by a numeral subscript, as explained in the next section.

FORMULAE

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. It is represented by a formula.

The formula of a molecule gives the number(s) of atoms of the same or different elements present in the molecule.

FORMULAE OF ELEMENTS

When an atom of an element combines with another atom or atoms of the same element, a molecule of the element is formed. For example, two atoms of hydrogen combine to form a molecule of hydrogen. The formula of hydrogen is H₂, with 2 as the subscript showing the number of H atoms in the molecule.

Similarly, molecules of nitrogen, oxygen, fluorine, chlorine, bromine and iodine contain two atoms of the element. So they are represented as N₂, O₂, F₂, Cl₂, Br₂ and I₂ respectively. As these molecules have two atoms of the element, they are said to be diatomic. A common example of a triatomic gas is ozone (O₃).

There are a few highly inactive gases present in very small amounts in the air. These gases, namely helium (He), neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe), are called noble gases. A molecule of a noble gas contains only one atom of the element. In other words, noble gases are monoatomic. So the formula of a noble gas is the same as its symbol.

VALENCY - THE COMBINING CAPACITY OF AN ELEMENT

When atoms of two or more elements combine, a molecule of a compound is formed. The capacities of these elements to combine with each other determine the formula of the compound formed.

The capacity of an element to combine with other elements is known as its valency.

The combining capacities of all elements are not the same, as the following examples show:
1. One atom of Cl combines with one atom of H to form a molecule of hydrogen chloride.
2. One atom of O combines with two atoms of H to form a molecule of water.
3. One atom of N combines with three atoms of H to form a molecule of ammonia.

Thus, the combining capacity of O is twice that of Cl, and that of N is three times that of Cl. H is assigned a valency of 1. So the valencies of Cl, O and N are 1, 2 and 3 respectively. Many atoms do not combine with H. Their valencies are calculated by the number of Cl atoms they combine with, since Cl and H have the same valency, that is, 1.

The valency of an element is given by the number of H or Cl atoms that an atom of the element combines with. (You will learn in higher classes that the valency of an element can be expressed in many ways.)

Elements with valencies 1, 2, 3, and so on, are said to be monovalent, divalent (or bivalent), trivalent, and so on. The valencies of some common elements are given in the table below.

Table: The valencies of some common elements

OBTAINING THE FORMULAE OF COMPOUNDS

The formula of a binary compound, that is, a compound formed by two elements only, 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 usually has the formula A_xB_y. The subscripts should be divided by a common factor, if any.

Example: If A has valency 4 and B has valency 2, the formula is A₂B₄. Dividing the subscripts by the common factor 2 gives AB₂.

There are some exceptions, such as H₂O₂ (hydrogen peroxide), in which the numeral subscripts are not divided by the common factor.

Table: Formulae of some common compounds

VARIABLE VALENCY

Some elements have variable valency. For example, iron has valencies of 2 (as in FeCl₂) and 3 (as in FeCl₃). FeCl₂ is named iron(II) chloride, and FeCl₃ is named iron(III) chloride. This method of naming a compound is adopted when it contains an element with variable valency.

Table: Some elements with variable valencies

Some examples of compounds containing elements of variable valency:
Copper(I) chloride: copper with valency 1 and chlorine with valency 1 give CuCl
Copper(II) chloride: copper with valency 2 and chlorine with valency 1 give CuCl₂
Iron(II) oxide: iron with valency 2 and oxygen with valency 2 give FeO
Iron(III) oxide: iron with valency 3 and oxygen with valency 2 give Fe₂O₃

Can you guess the valencies of phosphorus in PCl₃, and of sulphur in H₂S, SO₂ and SO₃?
Phosphorus has valency 3 in PCl₃. Sulphur has valency 2 in H₂S, 4 in SO₂ and 6 in SO₃.

WHAT ARE COMPOUND RADICALS

Certain groups of atoms of different elements remain intact in many chemical reactions. In fact, they behave like single atoms and have a valency. They are called compound radicals. They do not exist independently, but only as parts of compounds.

Common examples of monovalent radicals are hydroxide (OH) and nitrate (NO₃). Carbonate (CO₃) and sulphate (SO₄) are examples of divalent radicals. The phosphate (PO₄) radical is trivalent.

The formulae of compounds containing compound radicals are also obtained by transposing valencies.

Table: Formulae of some compounds containing compound radicals

CHEMICAL EQUATIONS

An element is represented by a symbol and a compound by a formula. A chemical change is represented by an equation called a chemical equation. For example, the burning of carbon in a sufficient supply of oxygen to form carbon dioxide is represented by the following equation:

C + O₂ → CO₂
(reactant + reactant → product)

If the supply of oxygen is insufficient, carbon monoxide (CO) is formed. This reaction is expressed as:

C + O₂ → CO

The substances that react among themselves are called reactants, and those that are formed are called products. No atoms are lost or gained in a chemical reaction. So the number of atoms of each element on the reactant side must be the same as that on the product side.

An equation satisfying this rule is called a balanced chemical equation. The equation showing the formation of carbon dioxide is balanced. The equation showing the formation of carbon monoxide is not balanced, because there are two O atoms on the reactant side and only one on the product side. To balance it, the numeral 2 is placed before C on the reactant side and before CO on the product side:

2C + O₂ → 2CO

Only balanced chemical equations are acceptable. Here are a few examples:
H₂ + Cl₂ → 2HCl
2H₂ + O₂ → 2H₂O
2Mg + O₂ → 2MgO

Sometimes an equation is made more informative by mentioning the conditions of the reaction above the arrow:

C + O₂ → CO₂ (in the presence of heat)

POINTS TO REMEMBER

1. A symbol is an abbreviation of the name of an element.
2. A symbol represents an element and an atom of the element.
3. The formula of an element or a compound represents a molecule of the element or compound. It gives the number(s) of atoms of the same or different elements present in the molecule.
4. Noble gases (helium, neon, argon, krypton and xenon) are monoatomic, whereas hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine are diatomic. Ozone (O₃) is a triatomic gas.
5. The capacity of an element to combine with other elements is known as its valency.
6. The formula of a compound containing two elements is obtained by transposing their valencies.
7. Some elements like iron (Fe), copper (Cu), lead (Pb), phosphorus (P) and sulphur (S) have variable valency.
8. A group of atoms of two or more elements, which behaves like a single atom and has a valency, is known as a compound radical.
9. Chemical changes are represented by chemical equations.
10. The number of atoms of each element on the reactant side must be the same as that on the product side. An equation that satisfies this rule is called a balanced chemical equation.

6.Chemical equation