Prabin Kalikote

The elements or compounds that take part in a chemical reaction are reactants and the elements or compounds that are formed as a result of a chemical reaction are called products.

for example

H₂SO₄ + Zn ----------> ZnSO₄ + H₂

Reactants                    Products

A chemical equation is an equation that represents the chemical reaction with the symbols and molecular formulae of the reactants and products.

For example:    2H₂ + 0₂ --------> 2H₂O

A chemical reaction expressed by writing the full names of reactants and products is called a word equation. For example:

Hydrogen + oxygen -----------> Water

The combination or displacement that occurs in the molecules of matter during a chemical change called a chemical reaction. Any two examples are:

1. 2H₂ + 0₂ -------> 2H₂0
   
2. H₂SO₄ + Zn ---------> ZnSO₄ + H₂

We know, lanthanides are rare earth metals, and actinides are radioactive metals. The properties of lanthanides resemble each other very closely but are different from the rest elements due to the preferential filling of f-orbitals. So, they are placed separately to avoid unnecessary sidewise expansion of the periodic table.

When we move from top to bottom in a group of non-metal, the atomic size increases gradually. Hence, nuclear attraction for incoming electrons decreases. As a result, the tendency of non-metallic atoms to gain electrons decreases. Therefore, chemical reactivity of non-metals decreases on moving down in a group.

When we move from top to doon of periodic groups of metals, the atomic size increases gradually. Due to this, nuclear attraction to the valence electron decreases. As a result, the tendency of metal atoms to lose electrons increases. Hence chemical reactivity of metals increases when moving from top to bottom in a group.