Elements and Compounds

There are many substances that consist of the same type of atoms, such as oxygen, hydrogen, gold, copper, silver, iron, nitrogen, chlorine, sulfur, magnesium, aluminum. Such substances which consist of the same type of atoms are called elements.

So far, 109 elements have been detected out of which 92 elements are found naturally.

Since the names of so many elements are difficult to remember, that is why each element is written by a special symbol. This sign is represented by the first one or two letters of the Greek or Latin name of that element.


Compounds are formed by the chemical combination of two or more atoms. The constituent atoms of a compound cannot be separated by physical methods. A compound is a new substance. Its properties differ from the properties of the elements or substances present. Most chemical substances are compounds. Examples of compounds are salt, molasses, blue stone, alum, saline, sugar and sulfuric agglomeration.

Chemical Formulas

Just as the atoms of elements are written by signs, the molecules of compounds are written by chemical formula. From the chemical formula of a substance, we get to know the following things about that substance: 

  • The substance is made up of how many chemical elements?
  • How many atoms of each element are there in a molecule?
  • What is the molecular weight of its molecule?

For example, we know that a molecule of water consists of two atoms of hydrogen and one atom of oxygen, so if we were to write its chemical formula, it would be H2O.

Similarly, one molecule of glucose is composed of 6 atoms of carbon, 12 atoms of hydrogen and 6 atoms of oxygen. Therefore, the chemical formula of glucose is C6H12O6 which is also known as its molecule.

How to Find Molecular Weight

To find the molecular weight of glucose, we need to know the atomic weight of carbon, hydrogen and oxygen, which is as follows - Atomic weight of carbon-12, Atomic weight of hydrogen-1, Atomic weight of oxygen-16.

Since glucose has a molecule - C6H12O6, its molecular weight will be: 

Atomic mass = Atomic weight of 6 x C + Atomic weight of 12 x H + Atomic weight of 6 x O

= (6 x 12) + (12 x 1) + (6 x 16) 

= 72 + 12 + 96

= 180


The atoms of different elements form molecules of compounds, such as one molecule of water consisting of two hydrogen atoms and one atom of oxygen. Similarly, different elements combine to form different types of compounds. The combining ability of a chemical element is called its valency. For example,

Valency of Cl in HCl = 1

Valency of O H2O = 2

Valency of N in NH3 = 3

Valency of C in CH4 = 4

These compounds combine 1, 2, 3 and 4 atoms of hydrogen to one atom of chlorine, oxygen, nitrogen and carbon respectively to form HCl (hydrogen chloride acid), H2O (water), NH3 (ammonia), CH4 (methane). Hence, the valency of these elements will be 1, 2, 3 and 4 respectively.

Chemical Equation

Two or more substances are required for any chemical reaction to occur. These substances, through which the reaction takes place, are called reactants. The substances that are formed as a result of the process are called products. To write a chemical equation, an arrow is placed between these two types of substances.

For example, if we burn magnesium in oxygen, then magnesium oxide is formed and heat is released. This will be written in words like this: 

Magnesium + Oxygen → Magnesium Oxide + Heat

This reaction consists of magnesium and oxygen reactants and the product produced (i.e. obtained at the end of the action) is magnesium oxide.

If we write chemical formulas of the above substances instead of words, we can write the above reaction by the following equation:

2Mg + O2 → 2MgO + Heat 

This is called a chemical equation.