Question

WHY VENTRICLES HAVE THICKER WALLS THAN THE ATRIA ?

Answer 1

The ventricles which are lower chambers of heart, have to pump the blood throughout the body and to lungs for oxygenation i.e for purifying blood i.e Oxygen is given to impure blood in which Oxygen is absent in a great force and by applying force on blood for these activities (pump the purified blood throughout the body and for oxygenation) a great pressure creates which is required. And for this pressure and application of force the ventricles of heart must be thicker walls and because of this it helps to apply great force. If ventricles would have thinner walls like atria then these activities would not been taken place easily.

And atria of the heart have to just take blood from body for circulating the blood in heart. And for this activity atria must not be thicker in size.
That's why, ventricles have thicker walls than atria.