Final answer:
Hydrogen is the universe's most abundant element, primarily found in stars and on Earth as water. Diatomic hydrogen (H₂) is relatively rare on Earth due to its low density and high reactivity. As an element, hydrogen is incredibly versatile, constituting parts of many essential substances and existing in multiple isotopes.
Step-by-step explanation:
Hydrogen is the most abundant element in the universe and is principally found in stars such as the sun. On Earth, hydrogen mainly exists in combination with oxygen as water (H₂O), covering about 70% of the planet's surface. While diatomic hydrogen (H₂) is a stable molecule and can exist in a free state, it is rare on Earth due to its low density and high reactivity. In fact, on our planet, hydrogen gas is extremely flammable and only present at about 0.5 parts per million in the atmosphere, because it can easily escape into space due to its light weight.
Though hydrogen in its molecular form (H₂) is less common on Earth, hydrogen as an element is extremely versatile. It is a vital component of countless substances, including petroleum, minerals, sugars, fats, alcohols, and acids, making it essential for life and various industrial applications. Furthermore, hydrogen exists in various isotopes, with protium (one proton), deuterium (one proton and one neutron), and tritium (one proton and two neutrons) being the most stable.