## What do you mean by Hydrogen Atom?

Hydrogen is one of the most fundamental elements on Earth which is colorless, odorless, and a flammable chemical substance. The representation of hydrogen in the periodic table is H. It is mostly found as a diatomic molecule as water H2O on earth. It is also known to be the lightest element and takes its place on Earth up to 0.14 %. There are three isotopes of hydrogen- protium, deuterium, and tritium.  There is a huge abundance of Hydrogen molecules on the earth's surface. The hydrogen isotope tritium has its half-life equal to 12.32 years, through beta decay. In physics, the study of Hydrogen is fundamental.

## Mass of Hydrogen Atom

Atomic mass gives us the mass of that particular atom in amu (atomic mass unit). The average atomic mass is the weighted average of all the naturally occurring isotopes of the element.

Formula mass is the sum of the masses of all the atoms present in that ionic compound and shown in the chemical formula.

Molecular mass is the total of all the atomic masses of atoms present in a molecule. Molar mass is the mass of 1 mole of atoms molecules in a gram. The atomic molar mass of the hydrogen is 1.0078 u. When the quantum number l=0 there is only one value for m, which would be zero. The first electron in helium has four quantum numbers of the first electron hydrogen.

## Bohr model of the Hydrogen Atom

The atoms are generally made up of protons, neutrons and electrons. The protons and neutrons are tightly packed and present at the center of the atom as nucleus. Due to protons, the nucleus is positively charged.

The electrons keep revolving around different energy levels or shells of an atom. Each level is characterized by a certain amount of energy.  The energy of the shell increases as we go away from nucleus. When a photon is absorbed by the electron, the energy is transferred to the electron and this electron moves from a lower level of energy to a high level of energy state by emitting photons, positive ions.

When the electrons jump from a higher energy level to a lower one, the energy absorbed is the difference of the energy in both levels. This energy change causes the absorption of radiation energy where it equals the Planks constant or number. This absorption or emission is analyzed by a spectrometer and this study is known as spectroscopy.

The Balmer series of hydrogen is visible at the low resolution of the spectrometer. The hydrogen spectrum is an important factor to understand the quantized structure of its atom. The quantized energy causes the spectrum to confine the value of the wavelength. The transition from the 1st level to other lower atomic energy levels are called Lyman series

The transition from the 2nd level to other lower atomic energy levels is called Balmer series.

The transition from the 3rd level to other lower atomic energy levels are called Paschen series

## Postulates

1) The electrons revolve around the nucleus with definite velocities in concentric circular orbits with energy equal to the binding energy at a certain distance from the nucleus. The energy of the electron remains constant till it stays in that energy level which is known as the main energy level.

2) Bohr postulated the angular momentum of an electron is quantized. Hence, the motion of the electron is constrained to those orbits where the angular momentum is a multiple of $\text{h/2π}$. The stationary states that are allowed energy levels are when n=1,2,3.. known as the Bohr’s quantum condition.

3) The energy of an electron changes when it moves from one energy level to another energy level. The energy released in the form of radiation of a certain frequency appears in the form of a line in the atomic spectrum. If the energy of the electron in the outer orbit is E2 and the inner orbit energy is E1, the frequency (n) of released radiation satisfies

$h\nu ={E}_{1}-{E}_{2}$

where h is the Planck’s constant.

4)The value of n would be only integer values and the quantum states are shells for the electron known as the principal quantum number.

5) As per Bohr's theory the radii of the various orbits and the energies associated with the electrons are quantized.

## States and Forms of Hydrogen

Hydrogen is present in all the states solid, liquid, gaseous, slush, and metallic hydrogen

Solid-state hydrogen is formed when the hydrogen is cooled down and has an electronic density of 0.086 g/cm3. At low temperature and pressure, Hydrogen exists as solids. The crystalline atomic structure of hydrogen is rigid and complex.

Liquid Hydrogen: It contains less energy per unit volume and is generally used as fuel for space engines due to its lightweight, ease of storage, and adjustable pressure. This has a higher density, boiling point value, and less explosive rate. To convert into liquid, a process called Liquefaction is done where it cools down the hydrogen by using expansions, heat exchangers, and compressors.

Gaseous Hydrogen: They are usually in this state, colorless, odourless, and highly flammable gas. Due to its non-corrosive nature, it is easily stored and transported. They are manufactured by steam reforming of natural gas. Steam reforming is a process when both hydrogen and carbon monoxide mixes to give syngas. These gaseous hydrogen are used in metallurgical industries.

Slush hydrogen: It is the combination of liquid hydrogen and solid hydrogen at the triple point at a lower temperature and higher electronic density than liquid hydrogen. It is also proposed to be used as a rocket fuel due to its low atomic mass and compressibility.

Metallic hydrogen: It is a state of hydrogen where it behaves like a conductor. It is known as the rocket propellant due to its high flammability, efficiency, and high volume.

## Uses of the Hydrogen Atom

It is used as a fuel in space engines, welding, metallurgical industry, methanol production, hydrosuplherization, commercial fixation of nitrogen from the air in the Haber ammonia process, production of hydrochloric acid, reduction of metallic ores and finally existing as water in various states of matter on earth

## Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• Bachelors in Science (Physics)
• Masters in Science (Physics)

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