The substance of the universe—from a grain of sand to a star—is called matter. Scientists define matter as anything that occupies space and ...

Elements are the smallest units of matter that cannot be broken down further by chemical processes. There are 118 known elements, but not all of these are ...

The Greek philosopher Democritus proposed that everything on Earth is made up of tiny particles called atomos, Greek for "indivisible," from which ...

A chemical symbol is an abbreviation that is used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. We use the ...

The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element. Its value determines the identity of ...

Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists arbitrarily define this amount of mass as one atomic mass ...

In the early 1900s, English chemist Frederick Soddy realized that an element could have atoms with different masses that were chemically ...

Electrons are negatively charged subatomic particles attracted to and orbit around the positively-charged nucleus of an atom. They reside in spaces ...

Chemical Bonds The electrons of the outermost energy level determine the energetic stability of the atom and its tendency to form chemical bonds with ...

Overview When atoms gain or lose electrons to achieve a more stable electron configuration they form ions. Ionic bonds are electrostatic attractions ...

Overview When two atoms share electrons to complete their valence shells, they create a covalent bond. An atom's electronegativity—the force ...

A hydrogen bond is formed when a weakly positive hydrogen atom already bonded to one electronegative atom (for example, the oxygen in the water molecule) ...

Ions - When an atom participates in a chemical reaction that results in the donation or acceptance of one or more electrons, the atom becomes positively ...

Solids, liquids, and gases are the three states of matter commonly found on Earth. A solid is rigid and possesses a definite shape. A liquid flows and ...

Children at play often make suspensions such as mixtures of mud and water, flour and water, or a suspension of solid pigments in water known as tempera ...

All chemical reactions begin with a reactant, the general term for one or more substances entering the reaction. Sodium and chloride ions, for example, ...

The first law of thermodynamics holds that energy can neither be created nor destroyed—it can only change form. An organism's essential function ...

An exchange reaction is a chemical reaction in which both synthesis and decomposition occur, chemical bonds are both formed and broken, and chemical ...

Chemical reactions require sufficient energy to cause the matter to collide with enough precision and force that old chemical bonds can be broken and new ...

A variety of factors influence the rate of chemical reactions. For a chemical reaction to happen, atoms must collide with enough energy to overcome the ...

Acids and bases play several important roles in biology. The pH of a biological system can significantly impact the function of biological molecules, ...