Class 7 Science The Language of Chemistry Exam Notes

Class 7 Science The Language of Chemistry Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Covers the following topics:

1.Symbols

2.Formulae

3.Formulae of Elements

4.Variable Valency

5.What are compound radicals

6.Chemical equation

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 (i.e., a reaction) by a chemical equation.    

Symbol

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)

◊  What does a symbol represent ?

The symbol of an element represents the following.

1. An element in particular For example, you know that 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. This is explained in the next section.

→ FURMULAE 

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 numbers(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(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. The formula of hydrogen is H₂, 2 being 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, viz., 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.

It will be evident from the following that the combining capacities of all elements are not the same.

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 thrice that of Cl.

H is assigned a valency of 1. So the valencies of Cl, O and N are 1, 2 and 3 respectively. However, 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, i.e., 1.

◊ The valency of an element is given by the number of H or Cl atoms that an atom o 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, etc., are said to be monovalent, divalent (or bivalent), trivalent, and so on. The valencies of some common elements are indicated in Table

Table: The valencies of some common elements.

◊ Obtaining the formulae of compounds

The formula of a binary compound, i.e., 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 AxBy. The subscripts should be divided by a common factor, if any.

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

The formulae of some common compounds are given in table Table: Formulae of some common compounds.

→ VARIABLE VALENCY

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

Table: Some elements with variable valencies

→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)

Table: Formulae of some compounds containing compound radicals.

→ CHEMICAL EQUATIONS

You know that 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.

If the supply of oxygen is insufficient, carbon monoxide (CO) is formed. Let us express the reaction thus:
C + O₂ → CO
The substances that react among themselves are called reactants and those that are formed are called products. Remember that 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. For example, the equation showing the formation of carbon dioxide is a balanced chemical equation. But the one showing the formation of carbon monoxide is not. Because there are two O atoms on the reactant side, the only one on the product side. To balance this equation, we place the numeral 2 before C on the reactant side and CO on the product side.

2C + O₂ → 2CO

Only balanced chemical equations are acceptable. Here are a few examples of such equations.

H₂ + Cl₂ → 2HCl

2H₂ + O₂ → 2H₂O

2Mg + O₂ →2MgO

Sometimes, an equation is made more informative by mentioning the conditions o the reaction above the arrow.

C + O₂ —→ ^heat CO₂

POINTS TO REMEMBER

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

◊ A symbol represents an element and an atom of the element.

◊ 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.

◊ 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.

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

◊ The formula of a compound containing two elements is obtained by transposing their valencies.

◊ Some elements like iron (Fe), copper (Cu), lead (Pb), phosphorus (P) and sulphur(S) have variable valency.

◊ 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.

◊ Chemical changes are represented by chemical equations.

◊ The number of atoms of each elements 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.

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