Final answer:
The greatest amount of radon entry in homes is typically caused by radon seeping through cracks in foundations and openings for pipes. Radon is a byproduct of uranium-238 decay and is more dense than air, which causes it to accumulate in basements and lower floors.
Step-by-step explanation:
The radon entry mechanism that typically causes the greatest amount of radon entry in homes is through cracks in concrete foundations and basement floors, stone or porous cinderblock foundations, and openings for water and gas pipes. Radon-222 seeps into houses and other buildings from rocks that contain uranium-238, a radon emitter. It slowly escapes from the ground and accumulates in basements and lower floors, diffusing throughout the building due to its greater density compared to air.