Which is the lightest element in the universe?
(A) Helium
(B) Hydrogen
(C) Nitrogen
(D) Silicon

Hint: It is the first element of the periodic table. It was discovered by scientist Henry Cavendish from a metal-acid reaction “inflammable air”. It is the most abundant element in the universe. It is highly flammable gas and it lacks colour, taste and odour.

Complete step by step answer:
- Hydrogen ($H_2$) is the lightest element in the universe. It is a gas.
- Its atomic number is 1 and atomic mass is 1.00794 amu.
- Not only on earth, it’s lightweight and has helped scientists in space also. It allows the gas to float in space, fuels the sun and so on.
- Two hydrogen atoms when attached to oxygen makes the wonder liquid which makes our planet habitable and that is water.
So, the correct option is B.

Additional information: Helium is the second most abundant element in this universe. Its very low density makes balloons float and buoyed up by dense air.
- Nitrogen is the most abundant gas in the atmosphere. Though nitrogen gas is non-toxic, liquid nitrogen can be dangerous if not handled properly. It can cause frostbite and if exposed to the whole body, it may kill us too
- Silicon is the second most abundant gas in the atmosphere. It has abundant use as a semiconductor.
Note: Though it has many good sides, its high concentration in the environment can cause an oxygen-deficient environment which results in headaches, nausea, vomiting and so on. Liquid hydrogen has the lowest density of all the liquids. Vehicles run by hydrogen fuel cells emit water vapour and warm air, so no harmful emissions.