Question

Approximately what percentage of the universe is believed to consist of hydrogen?
A. $25\% $
B. $50\% $
C. $75\% $
D. $90\% $

Answer 1

Hint: Hydrogen is one of the gases that comprises our universe. It is the simplest gas that is formed and is also the most abundant gas present. It is the starting point for almost all the entities present in space.

Complete solution:
Hydrogen is a gas that contains one neutron and one proton. It also comprises two other isotopes as well. They are: deuterium and tritium. The three isotopes differ in the number of neutrons that are present in their atoms. That is, they differ in their atomic number and not in their mass number. Out of all these isotopes, hydrogen is the most abundant.
In the universe, every star, galaxy or other celestial body was once a swirling hot ball of gas. This gas was mostly hydrogen in the beginning stages. Even earth was a ball of hydrogen. Upon forming this cloud, the gas molecules start to accumulate together and become more defined in its shape. With time and a decrease in temperature and increase in pressure, the hydrogen molecules combine to form helium and these helium molecules combine to form larger molecules until the center of the star or planet becomes the heaviest possibility that is iron.
Hydrogen in general is abundant in the atmosphere. Thus, the percentage of hydrogen that is seen in the universe is usually, $75\% $ of the total gas present in the universe.

Therefore, the answer to this question will be option C.

Note: The universe consists of many gases that can combine to form other gases.
Hydrogen being the most common gas found comprises a near total of $75\% $ of the gas present in the universe.
Hydrogen was the first gas that is formed in the universe and can combine to form the other gases.