Period 1 element

A period 1 element is any chemical element located in the very first row of the periodic table. This row is unique because it contains only two elements: hydrogen and helium. The periodic table is organized into rows called periods to show recurring trends in chemical behavior. As the atomic number of elements increases, a new row begins when chemical properties start to repeat. Because the first period is so short, it does not show the same horizontal trends found in larger periods. These two elements are also the oldest and most abundant substances in the entire universe.

The structure of these elements is explained by quantum mechanics. In an atom, electrons occupy specific regions called orbitals. Period 1 elements correspond to the filling of the 1s orbital. This is the first electron shell, and it can only hold a maximum of two electrons. This capacity is known as the duet rule. Because the first shell lacks "p" orbitals or any other types of orbitals, the period is limited to exactly two elements. Both hydrogen and helium belong to the s-block, which is a specific section of the periodic table.

Hydrogen is the lightest element, with an atomic mass of 1.00794 amu. At standard temperature and pressure, it is a colorless, odorless, and highly flammable diatomic gas. It makes up roughly 75% of the elemental mass in the universe. In stars, hydrogen exists in a plasma state within the main sequence. On Earth, hydrogen is often produced from hydrocarbons like methane. It can also be made from water through electrolysis, though this is more expensive. Hydrogen is vital in acid-base chemistry because it involves the exchange of protons between molecules.

Helium is the second lightest and second most abundant element in the observable universe. It is a colorless, odorless, and non-toxic monatomic gas. Helium is categorized as a noble gas because it is chemically inert, meaning it does not react easily. Most helium was created during the Big Bang, but stars create more through nuclear fusion. On Earth, helium is relatively rare. It is produced by the natural decay of radioactive elements. This radiogenic helium is often trapped in natural gas fields.

Placement on the periodic table is a subject of scientific debate for these two elements. Hydrogen is often placed in group 1 because it has one electron in its outermost shell. Like alkali metals, it can lose this electron in chemical reactions. However, it is a nonmetallic gas rather than a reactive solid metal. It also shares traits with halogens, such as forming hydrides by gaining an electron. Some scientists suggest hydrogen could belong in group 17, or even float separately from all groups.

Helium is almost always placed in group 18, the noble gases. This is because it has a full outer shell and is unreactive at standard conditions. However, helium only has two outer electrons, while other noble gases have eight. Helium is also an s-block element, while other noble gases are in the p-block. Some researchers, like Henry Bent, argue helium belongs in group 2. This is because its solid form crystallizes in a structure similar to beryllium and magnesium.

History shows how our understanding of these elements grew over time. French astronomer Pierre Janssen discovered helium in 1868. He detected it by observing a unique yellow spectral line during a solar eclipse. Later, in 1903, large reserves of helium were discovered in United States natural gas fields. Today, helium is used for many critical tasks. It is used in cryogenics, deep-sea breathing systems, and cooling superconducting magnets. It is also used to inflate balloons and provide lift in airships.