Atomic Number One on the Periodic Table

Hydrogen is the element that is atomic number 1 on the periodic table. The element number or atomic number is the number of protons present in the atom. Each hydrogen atom has one proton, which means it has a +1 effective nuclear charge.

Basic Atomic Number 1 Facts

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Hydrogen Facts - H or Atomic Number 1

By Anne Marie Helmenstine, Ph.D.

• At room temperature and pressure, hydrogen is a colorless, odorless gas.
• While ordinarily classified as a nonmetal, the solid form of hydrogen acts like other alkali metals in the same column of the periodic table. Hydrogen metal forms under intense pressure, so it is not seen on Earth, but it does exist elsewhere in the solar system.
• The pure element bonds to itself to form diatomic hydrogen gas. This is the lightest gas, although it is not significantly lighter than helium gas, which exists as a monatomic element.
• Element atomic number 1 is the most abundant element in the universe. In terms of a sheer number of atoms, about 90% of atoms in the universe are hydrogen. Because the element is so light, this translates into around 74% of the universe by mass.
• Hydrogen is extremely flammable, but it doesn't burn without the presence of oxygen. If you were to place a lit match into a container of pure hydrogen, the match would simply go out, not cause an explosion. Now, if it was a mixture of hydrogen and air, the gas would ignite!
• Many elements can exhibit a variety of oxidation states. While atomic number 1 usually displays a +1 oxidation state, it can also pick up a second electron and exhibit a -1 oxidation state. Because two electrons fill the s subshell, this is a stable configuration.

Atomic Number 1 Isotopes

There are three isotopes that all have atomic number 1. While an atom of each isotope has 1 proton, they have different numbers of neutrons. The three isotopes are proton, deuterium, and tritium.

Protium is the most common form of hydrogen in the universe and in our bodies. Each protium atom has one proton and no neutrons. Ordinarily, this form of element number 1 has one electron per atom, but it readily loses it to form the H⁺ ion. When people talk about "hydrogen", this is the isotope of the element usually being discussed.

Deuterium is a naturally occurring isotope of element atomic number 1 that has one proton and also one neutron. Since the number of protons and neutrons is the same, you might think this would be the most abundant form of the element, but it's relatively rare. Only around 1 in 6400 hydrogen atoms on Earth are deuterium. Although it's a heavier isotope of the element, deuterium is not radioactive.

Tritium also occurs naturally, most often as a decay product from heavier elements. The isotope of atomic number 1 is also made in nuclear reactors. Each tritium atom has 1 proton and 2 neutrons, which is not stable, so this form of hydrogen is radioactive. Tritium has a half-life of 12.32 years.