STRUCTURE OF MATTER (GRADE 7)

An atom is the smallest particle of an element that can take part in a chemical reaction. It may or may not exist independently.

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

A substance that cannot be decomposed into simpler substances by chemical means and is made up of only one kind of atoms is called an element. The smallest particle of an element is an atom.

A substance formed by the chemical combination of two or more elements in fixed proportions is called a compound. The smallest particle of a compound is a molecule.

When two or more elements combine together not chemically but physically we get a mixture.

 Q. Why are the properties of different compounds so different from each other?

A. The properties of different compounds are different from each other because the molecules of the compounds are different from each other.

Q. Why are most elements not found in the free state in nature?

A. Many elements have a great tendency to combine with each other to form compounds. These elements are, therefore, not found in the free state in nature.

 

Properties of compounds:

1) A compound can be broken down into its constituent elements by chemical methods. 

2) A compound always contains the same elements combined together chemically in a fixed ratio.

3) The properties of a compound are different from those of its constituent elements.

 

Difference between compounds and mixtures:

1) In compounds, constituents are combined together chemically in a fixed ratio whereas in mixtures, constituents are combined physically in any ratio.

2) In compounds, constituents can be separated by chemical methods whereas in mixtures, constituents can be separated by physical methods. 

3) Energy is absorbed or released during the formation of compounds whereas no energy is absorbed or released when a mixture is formed.

 

Writing the full names of elements and compounds, and describing their reactions is very inconvenient. It is more convenient to use abbreviations or symbols. We use symbols to represent to elements. A compound is represented by a formula which contains symbols of all the elements present in a molecule of that compound. 

Some common elements and their symbols:

Element                                      Symbol

Aluminum                                   Al 

Calcium                                       Ca 

Carbon                                         C 

Chlorine                                       Cl 

Copper                                         Cu 

Gold                                              Au 

Helium                                          He 

Hydrogen                                      H 

Iron                                                Fe 

Nitrogen                                        N 

Oxygen                                          O 

Potassium                                    K 

Sodium                                         Na 

Sulphur                                         S 

Tungsten                                     W 

Zinc                                             Zn 

 

Atoms of some elements cannot exist independently in nature. They form molecules containing two or more atoms.

A group of atoms carrying a positive or negative charge is known as radical. Examples are ammonium and sulphate.

 

The number of atoms present in a molecule of an element is known as its atomicity. If a molecule consists one atom it is known as monoatomic element. If a molecule consists two atoms it is known as diatomic element.

 

 What information does a formula give?

1) It tells the type of elements present in the compound.

2) It tells how many atoms of each element are present in the compound.

3) The formula represents the molecule of the compound.

Chemical formula of ammonia is 

NH3

There are two elements present in ammonia- Nitrogen and Hydrogen. A molecule of ammonia contains one atom of nitrogen and three atoms of hydrogen. It represents one molecule of ammonia.

 

The valency of an element denotes its combining capacity. We take the valency of hydrogen as 1. The valencies of other elements is the number of hydrogen atoms which can combine with one atom of that element. Example In ammonia one atom of nitrogen combines with three atoms of hydrogen, the valency of Nitrogen will be 3.

Writing a formula

1) Write the symbols of the elements side by side. Write their valencies below the symbols.

2) Divide by any common factor in their valencies.

3) Interchange the numbers obtained and write them at the base to get the formula.

 

The elements that take part in a chemical reaction are known as Reactants. The new compounds formed in the chemical reaction are called Products. 

 A chemical equation shows the result of a chemical reaction in which the reactants and the products are represented by symbols or formulae.

A balanced chemical equation is the one in which the number of atoms of each element on the left hand side are same as the number of atoms on the right-hand side.

This is an example of a balanced chemical equation. In the above reaction, nitrogen and hydrogen are the reactants (written on the left hand side) and ammonia is the product formed (written on the right hand side).