At the beginning of the birth of the universe, the first substances to appear were hydrogen and a small amount of helium. Because these two elements have the smallest atomic number and the lightest mass, they are also the easiest to form. Hydrogen atom has only one proton, while helium atom has only two protons and two neutrons. Naturally, it has become the earliest material form in the universe. Even now, hydrogen and helium account for 98% of the total material in the universe, while the other elements share the remaining 2%. Although the universe is a world of hydrogen and helium, this 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 generally as follows: 78% nitrogen, 21% oxygen, 0.94% rare gas, 0.03% carbon dioxide and a small amount of water vapor and other gases. Why is there such a big gap between the composition of the earth's atmosphere and the composition of cosmic matter? Everything starts with the birth of the solar system. After the birth of the universe, hydrogen and helium were generated, and then the matter gradually approached under the action of mutual gravity to form a nebula. The nebula gradually condensed to form stars. The huge pressure inside the stars triggered the fusion reaction. Through nuclear fusion, heavier elements appeared one after another. Although the species of matter in the universe become more and more abundant, hydrogen and helium are still the protagonists in the universe. Time came about 5 billion years ago. At this time, a nebula composed of many substances condensed in the current solar system, which we call "solar nebula". The solar nebula is no longer a primitive nebula with a single type of elements. In addition to hydrogen and helium, there are other elements such as oxygen, carbon, neon, iron and nitrogen. Of course, the proportion of these elements is very small, and the protagonist is hydrogen and helium. Later, gravitational exploration took place in the solar nebula. The volume of the nebula became smaller and smaller, and the density became larger and larger. Finally, a new star was born, which is the sun. After the formation of the sun, the remaining Nebula materials continue to run around the sun under the gravitational action of the sun, and gradually condense into a variety of solar system objects. At first, the number of celestial bodies in the solar system was very large. Later, with the continuous collision and merger, it finally formed what it is today. Since the most substances in the nebula are hydrogen and helium, and the earth is formed by the condensation of nebula substances, why can't hydrogen and helium be seen in the earth's atmosphere? Hydrogen and helium are the largest elements in the universe because they are light, but not in the earth's atmosphere. The earth is not a big planet. Its mass is about 5.965x10 Λ 24 kg. The gravity of a planet of this size is not enough to bind the hydrogen gas, so the hydrogen will fly higher and higher and float into the universe. Fortunately, hydrogen and helium are relatively active elements, so many hydrogen elements combine with oxygen to form water and remain 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 hardly reacts with any substance. Therefore, after the formation of the earth, helium will all float into the universe, leaving nothing left. So where does the existing helium on earth come from? There are some radioactive elements in the earth's interior, which is the source of helium. In the process of decay, uranium 238 and plutonium 232 will release an alpha particle, which contains two protons and two neutrons. In essence, it is a helium nucleus. After the helium nucleus is released, it will obtain two electrons, so a helium atom is generated. Helium will be mixed in the underground natural gas after it is generated and obtained by separation during natural gas production. However, the decay period 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 the chemical properties of nitrogen are very active, as early as in the nebula, they reacted with hydrogen to become NH3, that is, 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 primitive atmosphere, the most is not nitrogen, but ammonia. How does ammonia change into nitrogen? This is the credit of the sunlight. When sunlight shines on the earth, short wave radiation will decompose ammonia, and the decomposed ammonia will become nitrogen and hydrogen. There is no suspense. The hydrogen generated by decomposition will escape to the universe, while nitrogen is firmly bound by the earth's gravity, because nitrogen is a heavier gas than ammonia, with a molecular weight of about 28, while ammonia is only 17. With the passage of time, ammonia gradually decreased and nitrogen gradually increased, so the earth's atmosphere became a nitrogen world. Until later, with the emergence of plants on the earth, a large amount of oxygen was generated through photosynthesis, so oxygen occupied a place in the earth's atmosphere. (Xinhua News Agency)