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Absolute Density The absolute density is a measure of the mass of one milliliter of gas at standard temperature and pressure.

More Information: Gases
Acicular Habit An acicular habit describes the shape of a large crystal that looks like spikes coming out from one point. Think about those koosh balls for this example.

More Information: Solids
Actinide Series The actinide series is one of two series of inner transition elements. Elements 89 through 103 are a part of this series. The elements include uranium, berkelium, and nobelium.

More Information: Actinide Series
Actinium Symbol:"Ac" Atomic Number:"89" Atomic Mass: 227.03amu. It is one of the elements in the actinide series of inner transition elements. It may also be classified as a rare earth element. Actinium is the first element of the actinide series. It is used as a source of neutrons in experiments that involve radioactivity. You will not find the element in regular use anywhere in the natural world.

More Information: Actinide Series
Activation Energy When reactions proceed, a certain amount of energy is needed for the whole process to begin. The energy needed to get the reaction started (get it over the hump) is called the activation energy. The energy required to start a chemical reaction. If a reaction is not spontaneous, it requires a specific amount of energy to proceed. That required energy is the activation energy. Enzymes and catalysts can decrease the activation energy of a reaction.

More Information: Catalysts and Inhibitors
Adhesion Adhesion is one type of attraction force between the molecules of a substance and the container or another object. You stay wet when you get out of the bathtub because of adhesive forces. Some of the water molecules want to stick to you.

More Information: Liquids
Alchemy Alchemy is an ancient, non-scientific form of chemistry. In the middle ages, alchemists were people who tried to turn one element into another (usually lead (Pb) into gold (Au)).

More Information: Chemical and Physical Changes in Matter
Aluminum Symbol:"Al" Atomic Number:"13" Atomic Mass: 26.98amu. Aluminum is a light element and classified as a basic metal. There is more aluminum than any other metal in the Earth's crust. You will also find aluminum in utensils, foil wrap, power lines, soda cans, and airplane structures.

More Information: Aluminum
Americium Symbol:"Am" Atomic Number:"95" Atomic Mass: (243)amu. Americium is one of the elements in the actinide series of inner transition elements. It may also be classified as a rare earth element. This element is radioactive and can be safely used in small amounts. You might find it in nuclear reactors and some smoke detectors.

More Information: Actinide Series
Amino Group The amino group is a functional group with one nitrogen and two hydrogen atoms. You will find them on all of the amino acids. It can be synthesized from ammonia.

More Information: Amino Acid Structure
Amorphous Solid An amorphous solid has no specific organization of molecules. The other end of the spectrum would be a crystal with a highly organized set of molecules. Plastic is an example of a vitreous solid. Because of their combination of atoms, amorphous solids do not have a specific melting point. They become a liquid over a wide temperature range.

More Information: Solids
Amphipathic Amphipathic molecules have one hydrophobic end and one hydrophilic end. You can find these molecules in the cell membrane.

More Information: Compounds
Antimony Symbol:"Sb" Atomic Number:"51" Atomic Mass: 121.75amu. Antimony has been used for thousands of years. The pure metal is quite brittle. You will find the element used in batteries, alloys, and in the creation of paints and enamels.

More Information: Families of Elements
Argon Symbol:"Ar" Atomic Number:"18" Atomic Mass: 39.95amu. Argon has no color, no smell, and is one of the noble or inert gases. It can be found in welding, making silicon chips, light bulbs, and radio tubes.

More Information: Argon
Arsenate Mineral A mineral that is made up of compounds with an arsenic atom or arsenic oxide group bonded to a metal. Erythrite is a good example of a arsenate mineral.

More Information: Arsenic
Arsenic Symbol:"As" Atomic Number:"33" Atomic Mass: 74.92amu. Classified as a semi-metallic element, Arsenic is probably best known as a poison. While highly toxic, this grey, semi-metallic element is used in lasers, insecticides, and in some fireworks.

More Information: Arsenic
Astatine Symbol:"At" Atomic Number:"85" Atomic Mass: (210)amu. Astatine is a member of the halogen group. This element is often found during reactions with uranium in nuclear reactors. It is not found in nature and has no uses.

More Information: Halogens
Atomic Heat This is a measure of the atomic weight of a substance multiplied by its specific heat.

More Information: Atoms
Atomic Mass The atomic mass is the total mass of one atom of an element. It is the mass of the protons, electrons, and neutrons combined. The mass of all atoms is based on the mass of carbon. Carbon's mass is twelve AMU. Atomic mass is not measured in pounds or grams, scientists used something called Daltons. One Dalton is equal to one-twelfth the mass of a carbon atom. Scientists also call a Dalton an Atomic Mass Unit (amu).

More Information: Atoms
Atomic Number The atomic number is the number of an element on the periodic table. It is also equal to the number of protons inside of an atom. A neutral atom has equal numbers of electrons and protons. The atomic number for oxygen is 8.

More Information: Periodic Table
Attraction Forces Attraction forces hold molecules next to each other. When something is in a liquid, the attraction forces hold the liquid together. If you raise the temperature of the liquid, the molecules are given more energy and break free of the attraction forces to become a gas.

More Information: Liquids
Average Energy This is a measure of the overall energy of a sample. Some areas might be hotter and some might be colder, but the average energy is how scientists measure the temperature of a system.

More Information: Thermodynamics
Avogadro's Hypothesis Amadeo Avogadro came up with this idea. He said that if you have two volumes of gas, they would have the same number of molecules inside if the pressure and temperatures inside were equal. One liter of oxygen and one liter of carbon dioxide (at the same temperature and pressure) would have an equal number of molecules.

More Information: Gases
Avogadro's Number This is one of the constants of Chemistry. Amadeo Avogadro determined that there were 6.02 x 10 23 atoms in one mole of any substance. So if you have one gram of hydrogen, you have that many 6.02 x 10 23 atoms. 6.02 x 10 23 OR 60,220,000, 000,000 ,000,000,000. The value represents the number of atoms or molecules in one mole of a substance.

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