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In previous articles, we described how the first atoms were formed from the thermal plasma of the Big Bang: the first protons and neutrons condensed from basic quark particles, which led to mainly hydrogen and helium nuclei in the early universe. 380,000 years later, the hot universe became cold enough for electrons to bind to these nuclei, thus forming the first hydrogen atoms and some helium atoms.

In this way, we get a universe composed mainly of these two kinds of atoms. Larger atoms appeared later, and millions or even billions of years later, many heavier elements were formed through the fusion of star cores and supernova explosions.

But to make more complex structures, especially creatures like us, we need not only atoms, but also complex combinations of atoms: molecules. These molecules are not produced inside the star and must be produced in a completely different way. How does the universe go from simple atoms to molecules? How did the first molecule form?

Why atoms form molecules to answer these questions, we must first define the meaning of molecules. In short, when there are two or more atoms connected by chemical bonds, a molecule is formed. When molecules are formed, the formation of chemical bonds between two or more atoms is more conducive to energy formation than remaining independent. In other words, the total energy of a system made up of atoms can be lower than that of individual atoms. If we write the wave function of the hydrogen atom, we will see that the total energy of the system becomes smaller when they form a chemical bond.

So when the universe is only about 380000 years old, hydrogen will be one of the first molecules to be formed. These molecules still exist today, making them one of the oldest living molecules. But it is worth noting that these molecules will not be the first to form, and one molecule will form even earlier than hydrogen. It is helium hydride, and it will be formed when the universe is only 120,000 years old. The molecule is a positively charged ion consisting of a hydrogen atom and a helium atom, one of which is removed. There is a surprising but very good reason why it was formed in the first place.

Hydrogen molecules for the formation of helium hydride can only be made from hydrogen atoms, which require an electron and a proton to form hydrogen atoms. But electrons can attach themselves to hydrogen nuclei only if the universe is cool enough. This happened during the so-called cosmic restructuring period, 380,000 years after the Big Bang, where the cosmic microwave background was formed.

Hydrogen molecules are formed only after recombination, because hydrogen atoms are not available before, and it is not possible to make a molecule from two protons because they repel each other. But before the reorganization, when the universe was only 120000 years old, helium atoms were formed. Why can they form earlier? Because helium can absorb electrons at higher temperatures than hydrogen, it is a heavier element with higher ionization energy. It takes more energy to remove an electron from helium, so it tends to attract higher-level electrons.

So it was formed when the universe was hotter, when the universe was only 120000 years old. Hydrogen did not form until the universe was colder, when the universe was 380000 years old. But remember, in the early universe, the number of helium nuclei formed was very small, which is why we are not a universe made up mainly of helium atoms today.

Earlier helium can combine with protons to form positively charged helium hydride molecules, so these molecules will be the first to form. However, this molecule cannot form any useful substance. In fact, it is a highly reactive molecule that can only survive in isolation. These primitive molecules from the early universe are expected to survive for about 5,000 years after the formation of the first hydrogen molecule. It is impossible to confirm whether all the hydrogenated helium formed after the Big Bang has disappeared, but statistically speaking, it is almost impossible to leave anything behind today.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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