Period 1 element

A **period 1 element** is an element in the first period (row) of the periodic table. The periodic table is arranged in rows to show repeating properties of the elements. When the atomic number increases, the element have different properties. A new row begins when chemical properties repeat. It means that elements in the same group have similar properties. The first period has less elements than any other periods in the periodic table. There are only two elements in the first period: hydrogen and helium. We can explain why *there are less elements in the first row* in modern theories of atomic structure. This is because in quantum physics, this period fills up the 1s orbital. Period 1 elements follows the duet rule, they only need two electrons to complete their valence shell. These elements can only hold two electrons, both in the 1s orbital. Therefore, period 1 can have only two elements.

## Periodic trends
As period 1 only has two elements, there are no remarkable periodic trends.
## Position of period 1 elements in the periodic table
Although both hydrogen and helium are in the s-block, they do not behave similarly to other s-block elements. There is argument over where these two elements should be placed in the periodic table.

### Hydrogen
The position of hydrogen is sometimes above lithium, sometimes above fluorine, sometimes above both lithium and fluorine (appearing two times), in the periodic table.

### Helium
The position of helium is almost always above neon (which is in the p-block) in the periodic table because it is a noble gas. However, sometimes the position of it is above beryllium because they have similar electron configuration.

## Elements in period 1
### Hydrogen
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Hydrogen (symbol:**H**) is a chemical element. Its atomic number is 1. At standard temperature and pressure, hydrogen has no color, no smell and no taste. It is a nonmetal, and it is highly flammable. It is a diatomic gas with the molecular formula H2. Its atomic mass is 1.00794&nbsp;amu Hydrogen is the lightest element.

Hydrogen is the most abundant of the chemical elements. The abundance of hydrogen is roughly 75%. Stars in the main sequence are mainly composed of hydrogen in its plasma state. However, there is less hydrogen on Earth. Therefore, hydrogen is industrially produced from hydrocarbons (e.g. methane). We use elemental hydrogen locally at the production site. The largest markets are almost equally divided between fossil fuel upgrading, such as hydrocracking, and ammonia production, mostly for the fertilizer market. Hydrogen may be produced from water using the process of electrolysis, but this process is significantly more expensive commercially than hydrogen production from natural gas.

The most common naturally occurring isotope of hydrogen, protium, has a single proton and no neutrons. In ionic compounds, it can take on either a positive charge, becoming a cation composed of a bare proton, or a negative charge, becoming an anion known as a hydride. Hydrogen can form compounds with most elements and is present in water and most organic compounds. It plays a particularly important role in acid-base chemistry, in which many reactions involve the exchange of protons between soluble molecules. As the only neutral atom for which the Schrödinger equation can be solved analytically, study of the energetics and spectrum of the hydrogen atom has played a key role in the development of quantum mechanics.

The interactions of hydrogen with various metals are very important in metallurgy, as many metals can suffer hydrogen embrittlement, and in developing safe ways to store it for use as a fuel. Hydrogen is highly soluble in many compounds composed of rare earth metals and transition metals and can be dissolved in both crystalline and amorphous metals. Hydrogen solubility in metals is influenced by local distortions or impurities in the metal crystal lattice.

### Helium
Helium (He) is a colorless, odorless, tasteless, non-toxic, inert monatomic chemical element that heads the noble gas series in the periodic table and whose atomic number is 2. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions.

Helium was discovered in 1868 by French astronomer Pierre Janssen, who first detected the substance as an unknown yellow spectral line signature in light from a solar eclipse. In 1903, large reserves of helium were found in the natural gas fields of the United States, which is by far the largest supplier of the gas. The substance is used in cryogenics, in deep-sea breathing systems, to cool superconducting magnets, in helium dating, for inflating balloons, for providing lift in airships, and as a protective gas for industrial uses such as arc welding and growing silicon wafers. Inhaling a small volume of the gas temporarily changes the timbre and quality of the human voice. The behavior of liquid helium-4's two fluid phases, helium I and helium II, is important to researchers studying quantum mechanics and the phenomenon of superfluidity in particular, and to those looking at the effects that temperatures near absolute zero have on matter, such as with superconductivity.

Helium is the second lightest element and is the second most abundant in the observable universe. Most helium was formed during the Big Bang, but new helium is being created as a result of the nuclear fusion of hydrogen in stars. On Earth, helium is relatively rare and is created by the natural decay of some radioactive elements because the alpha particles that are emitted consist of helium nuclei. This radiogenic helium is trapped with natural gas in concentrations of up to 7% by volume, from which it is extracted commercially by a low-temperature separation process called fractional distillation.

## References
## Further reading
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