Hydrogen atom

A hydrogen atom is an atom of the chemical element hydrogen. The electrically neutral hydrogen atom has one positively charged proton in the middle, and one negatively charged electron around it.

Atomic hydrogen makes up about 75% of the baryonic mass of the universe.

On Earth, single hydrogen atoms are very rare. Usually, hydrogen atoms join together to form diatomic hydrogen gas, H₂. A water molecule has two hydrogen atoms, but it does not have atomic hydrogen, which means single hydrogen atoms.

Atomic spectroscopy shows that atoms can exist in many different states. Learning about the hydrogen atom helped scientists understand quantum mechanics, because it is the simplest atom.

Isotopes

Main article: Isotopes of hydrogen

Hydrogen has three main types, called isotopes. The most common one, called protium, has just a proton and an electron and no neutrons. It makes up almost all natural hydrogen.

Another type, called deuterium, has one neutron along with a proton and an electron. It is used in some special machines. A third type, called tritium, has two neutrons but it doesn’t stay around for long—it changes over time.

Heavier types of hydrogen can only be made in special science tools and they disappear very quickly.

Hydrogen ion

Main articles: hydrogen cation and hydrogen anion

Hydrogen atoms are usually found connected to other atoms. They can also change forms. If a hydrogen atom loses its electron, it becomes a cation, written as "H⁺". Free protons are found in space and in solar wind. In water with acids like hydrochloric acid, what forms is hydronium, H₃O⁺, when the acid gives a hydrogen to a water molecule.

If a hydrogen atom gains an extra electron, it becomes an anion, written as "H^–" and called hydride.

Theoretical analysis

The hydrogen atom is important in quantum mechanics and quantum field theory because it is a simple system with two parts and has many clear answers.

Failed classical description

Experiments in 1909 showed that atoms have a small, positive center with a thin, negative area around it. This made scientists wonder how such a system could stay stable. Classical physics suggested that if an electron moved in a circle and gave off energy, it would quickly fall into the center. However, atoms appear stable. Also, atoms were seen to give off only specific colors of light, not a mix of colors. The answer would come with the development of quantum mechanics.

Bohr–Sommerfeld model

In 1913, Niels Bohr made a simple model to explain the hydrogen atom. His model had a few key ideas:

1. Electrons can only be in certain circles or paths, called stationary states.
2. Electrons do not give off light while in these stationary states.
3. Electrons can jump from one path to another, gaining or losing energy.

Bohr’s ideas matched experiments better than the old models.

Schrödinger equation

The Schrödinger equation is a main idea in quantum mechanics. It helps us find where the electron is likely to be. The simplest state of the hydrogen atom is called the ground state. This state has the lowest energy and is where the electron is usually found.

The Schrödinger equation works for simple systems like the hydrogen atom but needs more complex tools for bigger atoms or molecules.

Visualizing the hydrogen electron orbitals

The picture shows the first few shapes of the hydrogen atom. These shapes show where the electron is likely to be found. The simplest shape, called the 1s state, is the one with the lowest energy.

Features going beyond the Schrödinger solution

There are some small effects that the Schrödinger equation does not fully explain. These include the speed of the electron, the interaction between the electron’s spin and its motion, and small shifts in energy caused by quantum effects. These effects are explained by more advanced theories.

Alternatives to the Schrödinger theory

Scientists have found different ways to understand the hydrogen atom. One way was created using special math rules by Wolfgang Pauli. Another method was shown in 1979 by Duru and Kleinert. There are also other models, like Bohm mechanics and a special math method.