Final answer:
Hydrogen sulfide (H₂S) is a colorless gas with a characteristic odor of rotten eggs, is heavier than air, and extremely toxic, potentially paralyzing the olfactory nerves.
Step-by-step explanation:
Hydrogen sulfide, often denoted as H₂S, is a colorless gas recognized for its offensive odor reminiscent of rotten eggs. It is known to be heavier than air and can pose significant health risks, as it is as toxic as hydrogen cyanide. Precautions must be taken when handling H₂S because it can paralyze the olfactory nerves, rendering a person's sense of smell inactive after short-term exposure. Due to its properties and potential for toxicity, H₂S is also treated as a pollutant when found in natural gas.