A chemical compound is a combination of two or more elements or molecules, such as quartz, a combination of silicon and oxygen, or dolomite, a compound of calcium, magnesium, carbon, and oxygen. Water is a compound of hydrogen and oxygen.
There are two basic kinds of compounds: ionic and covalent. Ionic compounds are held together by electromagnetic attraction between positive and negative ions, for example NaCl (sodium chloride, halite, rock salt) or CaCO3 (calcium carbonate, calcite, limestone).
Covalent compounds are held together by sharing electrons, such as H2 (hydrogen), O3 (ozone), CH4 (methane), H2O (water).
sharing of free electrons in metals, called metallic bonding, is
similar in concept to ionic bonding. Many compounds have bonding
that is a combination of covalent and ionic.
A mixture is a physical combination of a minimum of two elements or compounds. No chemical reactions take place between the mixed components. For example, sandstone is a mixture of quartz, water and/or oil and/or gas, and/or other constituents such as clay, silt, or any other rock mixtures. Salt dissolved in water is also a mixture.
When a compound is formed from two or more elements, the volume of the resulting molecule may be more or less than the original components. However, the total weight or mass, will not change, providing all gases formed, if any, are retained.
When a physical mixture is created, such as sand grains and water, the volume of the resulting mixture is the sum of the volumes of the original components, provided any gases involved, such as air between sand grains, are retained, and held at a constant temperature and pressure. The mass again will remain the sum of the masses of the individual components.
Valence electrons have the ability to absorb or release energy in the form of photons. This gain or loss of energy can trigger an electron to move (jump) to another shell or even break free from the atom and its valence shell. When an electron absorbs energy in the form of one or more photons, then it moves to a more outer shell, depending on the amount of energy gained. When an electron loses energy (photons), then it moves to a more inner shell.
The number of electrons in an atom's outermost valence shell governs its bonding behavior. Therefore, elements with the same number of valence electrons are grouped together in the periodic table of the elements. As a general rule, atoms of main group elements (except hydrogen and helium) tend to react to form a "closed" or complete shell, corresponding to an s2p6 electron configuration. This tendency is called the octet rule since the bonded atom has or shares eight valence electrons.
The most reactive metallic elements are the alkali metals of Group 1, for example sodium (Na) and potassium (K) whose atoms each have a single valence electron. This is easily lost to form a positive ion (cation) with a closed shell (Na+ or K+), during the formation of an ionic bond which provides the necessary ionization energy. The alkaline earth metals of Group 2, for example magnesium, are somewhat less reactive since each atom must lose two valence electrons to form a positive ion with a closed shell such as Mg2+.
Nonmetal atoms tend to attract
additional valence electrons to attain a full valence shell. This
can be achieved one of two ways: an atom can either share electrons
with neighboring atoms, a covalent bond, or it can remove electrons
from other atoms, an ionic bond. The most reactive non-metals are
the halogens such as fluorine (F) and chlorine (Cl), which have
electron configurations s2p5 and require only one additional valence
electron for a closed shell. To form an ionic bond, a halogen atom
can remove an electron from an other atom to form an anion
In these simple cases where the octet rule is obeyed, the valence of an atom equals the number of electrons gained, lost or shared to form the stable octet. However there are also many molecules which are exceptions, and for which the valence is less clearly defined.
The valence electrons are also responsible for the electrical conductivity of elements, which may be divided into metals, nonmetals, and semiconductors or metalloids.
Metals or metallic elements are elements with high electrical conductivity in the solid state. In each row of the periodic table the metals occur to the left of the nonmetals and thus have fewer valence electrons. The valence electrons which are present have small ionization energies, and in the solid state they are relatively free to leave one atom and move to its neighbour. These “free electrons” can move under the influence of an electric field and their motion constitutes an electric current. They are therefore responsible for the electrical conductivity of the metal. Copper, aluminium, silver and gold are examples of good conductors used widely in industry.
Nonmetallic elements have low electrical conductivity and act as insulators. They are found to the right of the periodic table with valence shells which are at least half full (except for boron). Their ionization energies are large so that electrons cannot leave an atom easily when an electric field is applied, and they conduct only very small electric currents. Examples of solid elemental insulators are diamond (an elemental form of carbon) and sulphur.
Solid compounds containing metals can also be insulators if the valence electrons of the metal atoms are used to form ionic bonds. For example, although elemental sodium is a metal, solid sodium chloride is an insulator because the valence electron of sodium is transferred to chlorine to form an ionic bond and cannot move easily in an electric field.
Semiconductors have an electrical
conductivity intermediate between metals and nonmetals, and also
differ from metals in that their conductivity increases with
temperature. The typical elemental semiconductors are silicon and
germanium with four valence electrons each. Their properties are
best explained using band theory, as a consequence of a small energy
gap between a valence band which contains the valence electrons at
absolute zero, and a conduction band to which valence electrons are
excited by thermal energy.
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