If an atom were to hypothetically move at the speed of light and collide with a human being, it would have significant and catastrophic consequences. However, it's important to note that according to our current understanding of physics, it is impossible for massive particles like atoms to attain or exceed the speed of light.
Assuming we suspend the laws of physics for the sake of the scenario, the collision of an atom traveling at the speed of light would release an enormous amount of energy due to the high kinetic energy involved. This energy release would result in a violent and destructive impact.
The effects of such a collision would be similar to those caused by high-energy particles, such as those in particle accelerators or cosmic rays. The high-speed atom would release an intense burst of energy upon impact, leading to a release of radiation and the disruption of atomic and molecular structures in the human body.
This radiation and disruption of atomic structures would likely cause severe damage to the human's cells, tissues, and organs. The impact would be akin to a high-energy particle beam or a powerful explosion, resulting in widespread destruction, tissue damage, burns, and potentially fatal injuries.
In reality, due to the principles of special relativity, particles with mass cannot attain or exceed the speed of light. As particles with mass approach the speed of light, their energy and momentum increase significantly, requiring an infinite amount of energy to reach the speed of light itself. Therefore, the scenario of an atom moving at the speed of light colliding with a human being remains purely hypothetical and impossible within the framework of our current understanding of physics.