At the beginning of the universe, the first substances to appear were hydrogen and a small amount of helium, because these two elements are the smallest atomic number, the lightest mass, and therefore the most easily formed.
The hydrogen atom has only one proton, and the helium atom has only 2 protons and 2 neutrons, which naturally became the first form of matter to come to this universe, and even now, hydrogen and helium account for 98% of the total amount of matter in the universe, while the rest of the elements share the remaining 2%. Although the universe is a world of hydrogen and helium, it is not the case on Earth. Who is the protagonist in the Earth's atmosphere? Oxygen? No, it's nitrogen. The composition of the Earth's atmosphere is roughly this: 78% nitrogen, 21% oxygen, 0.94% noble gases, 0.03% carbon dioxide, and small amounts of water vapor and other gases.
Why is there such a large gap between the composition of the Earth's atmosphere and the composition of cosmic matter? It all starts with the birth of the solar system.
After the birth of the universe, hydrogen and helium were formed, after which matter gradually converged under the mutual gravitational force to form a nebula, which gradually condensed to form a star, and the huge pressure inside the star triggered a fusion reaction, and through nuclear fusion, heavier elements appeared one after another. Although the variety of matter in the universe has gradually become abundant, hydrogen and helium are still the protagonists of the universe. The time came about 5 billion years ago, when a nebula of many materials condensed in the position of the present solar system, which we call the "solar nebula". The solar nebula is no longer a primitive nebula with a single elemental species, in addition to hydrogen and helium, there are other elements such as oxygen, carbon, neon, iron, nitrogen, etc. Of course, the proportion of these elements is very small, and the protagonist is hydrogen and helium.
Later, gravitational exploration occurred in the solar nebula, the size of the nebula became smaller and smaller, the density became larger and larger, and finally, a new star was born, it was the sun.
After the formation of the Sun, the remaining nebula material continued to orbit the Sun under the gravitational action of the Sun and gradually condensed into a variety of Solar System objects. Initially, the number of celestial bodies in the solar system was very large, and then with the continuous collision and merger, it eventually formed what it is today. Since the most important material in the nebula is hydrogen and helium, and the earth is condensed from the nebula material, why is there no hydrogen and helium in the earth's atmosphere? Hydrogen and helium are the most abundant elements in the universe because they are light, and the Earth's atmosphere is not, also because of lightness.
Earth is not a very large planet, its mass is about 5.965X10∧24 kg, such a scale of planet, its gravity is not enough to bind such a hydrogen gas, so the hydrogen flies higher and higher, drifting into the universe.
Fortunately, hydrogen helium is a relatively active element, so many hydrogen elements combine with oxygen to form water, which is retained on the earth in this special form. In addition to hydrogen, helium is the lightest element, but unlike hydrogen, helium is an extremely stable element that reacts hardly with any matter, so after the formation of the earth, helium is completely dispersed into the universe, and there is nothing left. So where does the existing helium on Earth come from? There are some radioactive elements in the Earth's interior, which are the source of helium.
Uranium-238 and plutonium-232 release an alpha particle that contains two protons and two neutrons, essentially a helium nucleus, which is released and acquires two electrons, resulting in a helium atom.
Helium is produced and mixed with natural gas underground, which is obtained by separation when the natural gas is extracted. However, the decay cycle of these heavy elements is often billions of years, so helium is definitely a rare resource on Earth. There is no hydrogen and helium in the Earth's atmosphere because they are light, so why is there so much nitrogen? Because nitrogen is very chemically active, they react with hydrogen as early as in the nebula to form NH3, or ammonia. Ammonia is a very heavy gas, so it can't get rid of the earth's gravity, so it stays on the earth.
In the earth's primordial atmosphere, the most is not nitrogen, but ammonia, so how does ammonia become nitrogen? This is the credit of the sun's rays.
When sunlight hits the earth, short-wave radiation will decompose ammonia, and the decomposed ammonia will become nitrogen and hydrogen. Without suspense, the decomposition of the hydrogen will be busy escaping into the universe, while the nitrogen is firmly bound by the gravitational pull of the earth, because nitrogen is a gas heavier than ammonia, its molecular weight is about 28, while the ammonia is only 17. Over time, ammonia gradually decreased, nitrogen gradually increased, so the earth's atmosphere became the world of nitrogen, until later, with the appearance of plants on the earth, a large amount of oxygen was generated through photosynthesis, so that oxygen occupied a place in the earth's atmosphere.