Hydrogen

I

Introduction

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. See also Atom.

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 (see Big Bang Theory). 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 (see Periodic Law). 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.

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II

Occurrence

Hydrogen is the tenth most common element on Earth. Because it is so light, though, hydrogen accounts for less than 1 percent of Earth's total mass. It is usually found in compounds. Pure hydrogen gas rarely occurs in nature, although volcanoes and some oil wells release small amounts of hydrogen gas. Many minerals and all living organisms contain hydrogen compounds. Hydrogen is in nearly every compound in the human body. For example, it is in keratin, the main protein that forms our hair and skin, and in the enzymes that digest food in our intestines. Hydrogen is in DNA, the molecules that code our genetic information and make each species of plant or animal unique (and every person unique). Hydrogen is in the molecules in food that provide energy: fats, proteins, and carbohydrates.

Hydrogen is in nearly all organic compounds, or compounds that contain carbon (see Organic Chemistry). Fats, proteins, and carbohydrates are all organic compounds. Other organic compounds that contain hydrogen include the hydrocarbon fuels methane (CH₄), ethane (C₂H₆), propane (C₃H₈), and butane (C₄H₁₀). Alcohols, such as methanol (CH₃OH) and ethanol (C₂H₅OH), are organic compounds that contain hydrogen, carbon, and oxygen. Hydrogen also forms inorganic compounds, or compounds that do not include carbon, such as water (H₂O), ammonia (NH₃), hydrochloric acid (HCl), and sodium hydroxide (NaOH) (see Inorganic Chemistry).

Hydrogen accounts for about 73 percent of the observed mass of the universe and is the most common element in the universe. Spectroscopes, instruments that measure properties of light to detect the element producing it, reveal that hydrogen exists in the Sun and in most, if not all, other stars. Most scientists believe that hydrogen atoms were the first atoms to form in the early universe and that the atoms of the other elements formed later from the hydrogen atoms. Scientific experiments show that about 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent.

Heavier elements form from hydrogen through a process called fusion, the joining of two atoms or parts of atoms to produce a new, larger atom. Fusion is not the same as chemical bonding. In chemical bonding, atoms share electrons to join together as a molecule that can be broken apart to yield the atoms again. In fusion, an atom permanently changes into an atom of another element. Fusion only occurs in nature in stars that reach a temperature of about 200 million degrees C (400 million degrees F). At this temperature, atoms collide with each other at great speeds, enabling them to fuse together. Hydrogen atoms in stars fuse together to form helium atoms, and the fusion reaction releases energy. Once this fusion starts, it heats the star such that heavier elements can form: elements through atomic number 22 (titanium) can form at about 1 billion degrees C (roughly 2 billion degrees F). At higher temperatures, all the natural elements can form.

Hydrogen exists in interstellar space (between stars) as atoms of the gas and as hydrogen molecules, spread out at about one atom or molecule per cubic centimeter. However, a surprisingly large amount of ionized hydrogen (H⁺), hydrogen atoms missing their electron, also exists in the galaxy. Scientists do not understand where it comes from, but they are reexamining theories of astronomy and cosmology to explain its presence.

III

Physical Properties

Pure hydrogen is a gas under normal conditions—that is, at room temperature and normal atmospheric pressure. Like most gaseous elements, hydrogen is diatomic, meaning its molecules contain two atoms. Molecular hydrogen is represented symbolically as H₂. Hydrogen gas is much lighter than air. At 0°C (32°F) and regular atmospheric pressure, hydrogen has a relative density of 0.090 grams/liter (g/L), whereas the relative density of ordinary air is 1.0 g/L. Hydrogen has such a small mass that it can escape Earth’s gravitational pull and fly off into space. As a result, it is not found in large amounts in the atmosphere. Hydrogen has a lower boiling point and freezing point than does any other substance except helium. Hydrogen boils at –252.8°C (-423.0°F) and freezes at –259.14°C (-434.45°F). Liquid hydrogen, first obtained by British chemist Sir James Dewar in 1898 (see Cryogenics), is colorless in small amounts but light blue in thick samples. Solid hydrogen is colorless.

Hydrogen exists in nature as three different isotopes. Isotopes are atoms of the same element that contain different numbers of neutrons, uncharged elementary particles, in their nuclei. The majority of hydrogen atoms have no (zero) neutrons in their nuclei. Scientists represent these hydrogen atoms with the symbol ¹H. Atoms of ¹H have just one proton in their nucleus and have an atomic mass of 1. This isotope, which accounts for 99.98 percent of hydrogen atoms, is sometimes called protium. About 0.02 percent of hydrogen atoms have one neutron and one proton in their nucleus. This isotope is called deuterium. Deuterium was the first isotope of any element that scientists discovered and isolated from a sample. It is used in a variety of scientific experiments. Deuterium is represented by the symbol ²H, or by the symbol D, and has an atomic mass of 2. The third isotope of hydrogen is called tritium (³H). This isotope has two neutrons and one proton in each atom’s nucleus, and it has an atomic mass of 3. Tritium accounts for fewer than one in 10,000 atoms of hydrogen. It is radioactive, meaning its nucleus can decay, or spontaneously change, into other particles (see Radioactivity). The half-life of a radioactive substance, such as tritium, is the length of time necessary for half of a sample of the substance to decay into other particles. Tritium has a half-life of 12.4 years. Scientists can make tritium in the laboratory in nuclear reactions. The names protium, deuterium, and tritium come from the Greek words for first, second, and third, respectively.

IV

Chemical Properties

Hydrogen gas does not usually react with other chemicals at room temperature. That is, it does not split into two hydrogen atoms to combine with other chemicals. The bond between the hydrogen atoms is very strong and can only be broken with a large amount of energy. However, when heated with a flame or a spark, hydrogen gas will react violently with oxygen in the air to produce water in the following reaction:

2H₂ + O₂ → 2H₂O

This chemical equation shows that two hydrogen molecules (H₂) and one oxygen molecule (O₂), combine to form two molecules of water, or H₂O. This reaction releases energy.

Hydrogen atoms form covalent bonds with each other and with other atoms. Two atoms form a covalent bond when they share some or all of their electrons. Two hydrogen atoms bond covalently to form the hydrogen molecule (H₂), the smallest and lightest molecule that exists. In an H₂ molecule, each hydrogen nucleus shares two electrons. Hydrogen can also bond covalently with other elements, for example, with carbon in hydrocarbons, such as methane (CH₄), and with oxygen in water (H₂O).

In some molecules containing hydrogen, the covalent bond between one of the hydrogen atoms and another atom is weak and breaks easily. Chemists call compounds made of these molecules acids (see Acids and Bases). Acids tend to be corrosive, that is, they destroy metals. Weak acids, such as acetic acid (CH₃CO₂H), which is found in vinegar, and citric acid (HOC[CH₂CO₂H]₂CO₂H), which is found in citrus fruits, give foods a tart taste. When an acid mixes with water, it dissolves and the acid’s weakly-bound hydrogen atom breaks off, leaving its electron behind. (Some acids, such as citric acid, have more than one weakly-bound hydrogen atom.) The hydrogen atom becomes a positively charged particle called a hydrogen ion, or H⁺. This ion is the hydrogen’s nucleus, a proton. The negatively charged remnant of the molecule (for example, CH₃CO₂^-, the remnant from acetic acid) is called an anion.

Hydrogen also forms ionic bonds with some metals, creating a compound called a hydride. Two atoms form an ionic bond when one atom donates an electron to the other atom. The resulting difference in electric charge between the two atoms makes them attract each other and bond together. In the ionic bonds of hydrides, the metal atom gives hydrogen an electron, making hydrogen a negatively charged ion (H^-) and the metal a positively charged ion, for example a sodium ion (Na⁺). The two oppositely charged ions then attract each other and bond to form a salt, such as sodium hydride (NaH).

Hydrogen can also form a unique bond known as a hydrogen bond. Hydrogen bonds only form between hydrogen and the elements oxygen (O), nitrogen (N), or (less commonly) fluorine (F). Hydrogen bonds actually form between a hydrogen atom in one molecule, and the oxygen, nitrogen, or fluorine atom in another molecule. These elements (O, N, and F) are extremely electronegative, that is, when they form a covalent bond with hydrogen, they pull hydrogen’s single electron more tightly toward themselves and away from the hydrogen nucleus. This creates a slight positive charge on the hydrogen atom and a slight negative charge on the oxygen, nitrogen, or fluorine atom. The hydrogen’s proton and its positive charge are exposed. When the slightly positively charged hydrogen attracts a slightly negatively charged oxygen, nitrogen, or fluorine atom in another molecule, the two atoms form a hydrogen bond.

Water (H₂O) is a good example of hydrogen bonding. The oxygen atom pulls the electrons more tightly toward itself and away from the hydrogen atoms. The oxygen gains a slight negative charge, while the two hydrogen atoms each become slightly positive. These small charges on the atoms allow them to attract atoms of neighboring water molecules. Each hydrogen atom of water molecule A forms a hydrogen bond to the oxygen atom of another water molecule, such as molecule B or molecule C, and so forth.

These special hydrogen bonds are so important in living systems that some scientists consider the hydrogen bond to be the most important chemical bond of all. Hydrogen bonds keep water molecules together in the liquid state, preventing the molecules from separating and evaporating at a lower temperature. Without hydrogen bonds, water would boil near –80º C (near -110º F) instead of at 100º C (212º F). Liquid water would not exist in most places on Earth. Hydrogen bonds also hold together the paired strands of compounds that make up deoxyribonucleic acid (DNA), the genetic material essential in living organisms.

Hydrogen is usually listed in the periodic table in the first column, with the elements called alkali metals (the elements lithium, sodium, potassium, rubidium, cesium, and francium). Chemists list elements in the periodic table according to the number of protons in the nucleus of each of the element’s atoms. The number of protons in an atom’s nucleus is equal to the number of electrons that atom contains. The number of electrons in an element’s atoms, specifically the number of outermost electrons, determines the chemical behavior of the element. Elements in each column have the same number of outermost electrons, so they behave in a similar manner. Since hydrogen has one electron, it is placed in the first column with the alkali metals, which also have one outermost electron. However, hydrogen is not considered an alkali metal, because under ordinary conditions it does not behave like a metal.

Under extreme pressures, hydrogen can actually act like a metal by, for example, conducting electricity and reflecting light. Some planetary scientists believe that Jupiter's immense magnetic field is created by metallic hydrogen in its core. The immense pressure at the center of Jupiter might prevent each hydrogen atom’s electron from binding to a single nucleus. Instead, the electrons might be shared by all the nuclei, as are electrons in a metal. This would make hydrogen conduct electricity like other magnetic metals. Scientists have used extremely high temperatures (approximately 5000° C or 9000° F) and high pressures (1.8 million times the normal pressure of Earth’s atmosphere at sea level) to temporarily transform hydrogen into a metal.

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