Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


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 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.


The chemical industry uses hydrogen gas in many industrial chemical processes. The most important of these processes uses hydrogen to make ammonia; it is called the Haber process after German chemist Fritz Haber, who developed it in 1908. The industry can then use ammonia to make other important products, such as explosives and fertilizers. Industrial chemists also use hydrogen in large amounts to make compounds such as the fuel methane and the alcohol methanol, which is used as antifreeze and to make other chemicals. The food industry hydrogenates (adds hydrogen to) liquid oils (see Hydrogenation). When hydrogen atoms are added to the molecules of liquid oils, the oils become solid fats, such as margarine or vegetable shortening (for example, Crisco). Metallurgists use hydrogen to separate pure metals from their oxides. For example, hydrogen bonds with and removes oxygen from copper oxide, leaving pure copper.

Physicists use liquid hydrogen, which is extremely cold, to study elementary particles and low-temperature effects. Elementary particles, the smallest building blocks of matter, form in nuclear reactions, but they are too small and move too quickly to be visible to scientists. Scientists can view them indirectly by looking at the evidence the particles leave behind. In a device called a bubble chamber, this evidence is a little ripple pattern, or a track, in liquid hydrogen. Laboratory scientists also use liquid hydrogen to cool objects to extremely low temperatures to study effects such as superconductivity, which is the ability of a material to conduct electricity with no resistance (no loss of energy). Substances usually only become superconducting at very low temperatures.

People may someday use hydrogen as fuel for automobiles, refrigerators, and airplanes, if it becomes easier to distribute, store, and use. Automobile manufacturers are developing vehicles that are powered by hydrogen fuel cells, devices that use hydrogen to produce electricity. The aerospace industry, the industry that designs and builds airplanes and spacecraft, already uses liquid hydrogen as a fuel for rockets. Aerospace engineers are interested in using hydrogen fuel for airplanes because of its low density. Conventional hydrocarbon fuels add much weight to an aircraft. Low-weight, high-energy hydrogen would decrease the amount of fuel needed to lift the airplane at takeoff and increase the distance the airplane could fly without stopping. Hydrogen fuel could also cut pollution, since it mostly produces water when it burns. Spacecraft use hydrogen as a primary rocket fuel that reacts with fluorine or oxygen to produce energy.