When atoms combine to form compounds, some of their electrons arrange around the nuclei in a way different from the arrangement in the atoms. When atoms of nonmetals combine to form compounds, molecules result. A molecule of a compound is composed of two or more atoms of at least two different elements. For example, water is a compound of hydrogen and oxygen. Both hydrogen and oxygen are nonmetals, and the units of water are water molecules. A molecule is the smallest unit of an element or compound that has the chemical properties of the element or compound. A molecule does not have a net electric charge—that is, a molecule is electrically neutral. In a water molecule, two atoms of hydrogen and one atom of oxygen are joined together. A water molecule is the smallest unit into which water can be divided without chemical change. Compounds that have molecules as units are called molecular compounds.
A molecule of an element consists of one or more atoms of the same element. For example, a single atom of helium is a molecule of helium. A molecule of hydrogen gas is made up of two atoms of hydrogen joined together. Molecules made up of two atoms are called diatomic molecules. Besides hydrogen, other common elements that have diatomic molecules as their units under ordinary conditions are nitrogen, oxygen, fluorine, chlorine, bromine, and iodine. Molecules of some compounds are also diatomic, but the two atoms are different. Hydrogen chloride gas is an example of a compound with diatomic molecules.
Formulas are used to represent molecules. Formulas consist of the symbols for each of the elements making up a compound, followed by a subscript showing how many of that kind of atom are in a molecule of the compound. If no subscript is written after a symbol, the number 1 is assumed. For example, the formula for a molecule of water is H2O, showing that one molecule of water is a combination of two atoms of hydrogen and one atom of oxygen.
The formula for a hydrogen molecule is H2, and the formula for a molecule of hydrogen chloride is HCl. In formulas, the element that is to the left in the periodic table is usually written on the left side of the formula. If two elements are in the same group, the one that is lower in the column in the periodic table is usually written first. The formula SO3 is an example.
A molecule is one of the basic units of matter. It is the smallest particle into which a substance can be divided and still have the chemical identity of the original substance. If the substance were divided further, only molecular fragments or atoms of chemical elements would remain. For example, a drop of water contains billions of water molecules. If one of those water molecules were separated from the rest, it would still behave as water. But if that water molecule were divided, only atoms of the elements hydrogen and oxygen would remain.
Molecules are made up of atoms held together in certain arrangements. Scientists use chemical formulas to show the composition of molecules. For example, a water molecule consists of two hydrogen atoms and one oxygen atom. A molecule's size and shape depends on the size and number of its atoms. A molecule that consists of two atoms, such as nitric oxide, is called a diatomic molecule. A molecule made up of three atoms, such as water, is called a triatomic molecule. A large molecule, such as DNA, can contain millions of atoms.
Atoms link together in molecules through strong attractive forces called bonds. The shape of a molecule depends upon two factors: (1) The atoms tend to take up positions relative to one another such that the bonds formed are the strongest of all the bonds that this particular group of atoms could form. (2) Atoms that are not bonded to each other tend to move far apart. For example, an ammonia molecule has the shape of a tetrahedron (a pyramidlike figure with four faces). It consists of three hydrogen atoms attached to a nitrogen atom. Normal butane molecules have 4 carbon atoms arranged in a zigzag chain with 10 hydrogen atoms attached. Large protein molecules can form long spiral chains.
The mass of a molecule is indicated by its molecular mass. Molecular mass can be found by adding the atomic masses of all the atoms in a molecule. The molecular mass of carbon dioxide can be found by adding the atomic mass of carbon, which is 12, and the masses of the 2 oxygen atoms, which are about 16 each. Carbon dioxide has a molecular mass of about 44. A molecule's mass can also be measured with an instrument called a mass spectrometer.
Each atom in a molecule consists of a positively-charged nucleus surrounded by a cloud of negatively-charged electrons. In a neutral molecule, the positive and negative charges are evenly balanced throughout the molecule. In polar molecules, the charges are not evenly balanced. In a polar molecule, more positive charge collects at one location in the molecule and more negative charge collects at a different location. Some molecules are magnetic because of the way the electrons are unevenly distributed within the molecule.
Almost all gases, most common liquids, and many solids are made up of neutral or polar molecules. But some substances are made up of units called ions (atoms or groups of atoms with a positive or a negative charge). These substances are called ionic substances.
Salts are examples of ionic substances. For example, sodium chloride, common table salt, consists of positive sodium ions and negative chloride ions. Electric forces among the ions hold them together in a regular framework. Metals are also different from molecular substances. In addition to positive ions, metals consist of a large number of electrons that move about freely throughout the metal.
Molecules are held together in a group by electrical forces called Van der Waals forces. These forces are usually weaker than those that hold a molecule itself together. The force between molecules depends on how far apart they are. When two molecules are widely separated, they attract each other. When they come very close together, they repel each other.
In a solid, the molecules are so arranged that the forces which attract and repel are balanced. The molecules vibrate about these positions of balance, but they do not have enough energy to move to different parts of the solid. As the temperature of a solid is raised, the molecules vibrate more strongly. When the Van der Waals forces can no longer hold the molecules in place, the solid melts.
In a liquid, the molecules move about easily, but they still have some attractive force on one another. These forces are strong enough to keep the liquid together. Certain organic compounds called liquid crystals have properties of both liquids and solids. Within a particular temperature range, such a compound flows like a liquid, but has a more ordered molecular arrangement. Its molecules line up side by side and form tiny groups or clusters that slide past one another in certain directions.
In a gas, the molecules move about so fast that the attractive forces have little effect on them. When two molecules in a gas collide, the repelling force sends them apart again. Therefore, gas molecules fill a container completely, because they move freely through all the space available.
Most substances can be changed into solids, liquids, or gases by either raising or lowering their temperatures. The temperature at which these changes occur--and also other characteristics of a substance--depends on the size, shape, and mass of the molecules and also on the strength of the Van der Waals forces between them.
Under certain conditions, two molecules may collide with enough energy to react and form one or more new molecules. The process by which many small molecules combine chemically to produce a large molecule is called polymerization. Molecules can also break down into smaller molecules. Causes of molecular disintegration include ultraviolet light, fast-moving electrons, and nuclear radiation.
Scientists can study some molecules directly with an electron microscope. This method provides a picture of a molecule, but the picture is often too blurred to see fine details. A scanning tunneling microscope produces an image of some individual atoms in a solid substance. Scientists also study molecules indirectly. For example, they study solids by X-ray diffraction. The way a solid deflects X rays tells them about the size, shape, and arrangement of its molecules. Scientists also use neutron diffraction and electron diffraction to study solids. They pass a beam of neutrons (uncharged particles) or electrons through a solid, and observe how the beam is affected. Electron diffraction can also be used to study gases.
Scientists also learn about molecules by studying the way they absorb or give off light. Each kind of molecule absorbs or gives off certain colors of light. This group of colors makes up the molecule's spectrum. By studying the spectrum of a substance, scientists can find the sizes and shapes of its molecules, the strength of the forces that hold the atoms together in the molecules, and the way the electrons move about in the molecules.