Does the left atrium have thinner walls than the right atrium?

The left side of your heart is much stronger than the right because the blood that's in the left side of the heart needs to be pumped all over the body. Note also that blood that is pumped from the other side only needs to go to the lungs and there is not a huge distance from the heart to the lungs.

The heart is a organ that pumps blood all over the body. The right side receives blood (low in oxygen) from all over the body. The left side receives (oxygen rich) blood from the lungs. When blood is pumped all over the body, it loses its oxygen. After being pumped to all parts of the body, it returns to the heart and enters the right atrium and right ventricle. The blood is then pumped to the lungs and returns to the heart (this time on the left side). This blood (now rich in oxygen) flows all over the body. It looses its oxygen again and enters the heart on the right side and is then pumped to the lungs. Then the cycle starts again and again.