Assertion: Earth has an atmosphere around it but the moon does not.Reason: Moon is very small as compared to the earth

The assertion and reason given are partially correct. It's true that Earth has an atmosphere and the Moon does not. However, the size of the Moon compared to Earth is not the primary reason for this.

The Earth's atmosphere is held in place by gravity. The Moon also has gravity, but it's much weaker than Earth's due to its smaller size and mass. This means the Moon's gravity is not strong enough to hold a significant atmosphere in place.

However, the main reason the Moon lacks an atmosphere is because it doesn't have a magnetic field to protect it from solar winds. Solar winds are streams of charged particles emitted by the sun. On Earth, our magnetic field deflects these particles, protecting our atmosphere. The Moon, lacking such a magnetic field, has its atmosphere stripped away by solar winds.

So, while the Moon's smaller size and weaker gravity do play a role, the lack of a protective magnetic field is the primary reason it doesn't have a significant atmosphere.