Which gas is most effective in absorbing incoming harmful ultraviolet radiation in the earth's stratosphere before that radiation reaches earth's surface?

Ozone (O₃) gas

Explanation:

In the Earth's stratosphere, oxygen gas (O₂) is combined with atomic oxygen (O) to form ozone gas (O₃). This is very important because ozone protects the life on Earth from ultraviolet (UV) radiation from the Sun. UV radiation is high-energy radiation, and it is very dangerous for living organisms. Ozone molecule (O₃) absorbs UV radiation and it breaks down by forming O₂ and atomic oxygen (O) again. In this way, it protects the Earth from nocive radiation.

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the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.