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tiny particle, nuclear reactions, atomic number, chemical element, number of protons

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Hydrogen, chemical element that exists as a gas at room temperature. Hydrogen gas is odorless, tasteless, colorless, and highly flammable. When hydrogen gas burns in air, it forms water. French chemist Antoine Lavoisier named hydrogen from the Greek words for “water former.”

Hydrogen has the smallest atoms of any element. (Atoms are the smallest particles that have the characteristics of a chemical element.) A hydrogen atom contains one proton, a tiny particle with a positive electrical charge, and only one electron, an even smaller, negatively charged particle. The proton is the center, or nucleus, of the hydrogen atom, and the electron travels around the nucleus. Pure hydrogen exists as hydrogen gas, in which pairs of hydrogen atoms bond together to make larger particles called molecules.

Hydrogen atoms were among the first atoms to form in the early universe. Hydrogen nuclei—that is, protons—formed within three minutes after the big bang, the explosion that scientists believe created the universe as we know it. The protons began to combine with electrons to form hydrogen atoms when the universe was about 300,000 years old. This process of combination continued until the universe was about one million years old. In stars, hydrogen nuclei combine with each other in nuclear reactions to build helium atoms. These high-energy reactions create the light and heat of the Sun and most other stars.

Hydrogen is the first element in the periodic table of the elements and is represented by the symbol H. The periodic table lists elements by their atomic number, which is the same as the number of protons in one atom of the element. Hydrogen, with only one proton, is the simplest element. It is usually placed in Period 1 (the first row) and Group 1 (the first column) of the periodic table. Hydrogen can combine chemically with almost every other element and forms more compounds (materials made of two or more different elements) than does any other element. These compounds include water, minerals, and hydrocarbons—compounds made of hydrogen and carbon—such as petroleum and natural gas.


Andersson, Laura A., B.S., Ph.D. Biochemistry

Visiting Assistant Professor, Department of Chemistry, Vassar College.

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