History
The idea of covalent bonding can be traced several years prior to 1919 to Gilbert N. Lewis, who in
1916 described the sharing of electron pairs between atoms. He introduced the so called Lewis
notation or electron dot notation or The Lewis Dot Structure in which valence electrons (those in
the outer shell) are represented as dots around the atomic symbols. Pairs of electrons located
between atoms represent covalent bonds. Multiple pairs represent multiple bonds, such as double
and triple bonds. Some examples of Electron Dot Notation are shown in the following figure. An
alternative form, in which bond-forming electron pairs are represented as solid lines, is shown
alongside.Early concepts in covalent bonding arose from this kind of image of the molecule of
methane. Covalent bonding is implied in the Lewis structure that indicates sharing of electrons
between atoms.Early concepts in covalent bonding arose from this kind of image of the molecule
of methane. Covalent bonding is implied in the Lewis structure that indicates sharing of electrons
between atoms.
While the idea of shared electron pairs provides an effective qualitative picture of covalent
bonding, quantum mechanics is needed to understand the nature of these bonds and predict the
structures and properties of simple molecules. Walter Heitler and Fritz London are credited with
the first successful quantum mechanical explanation of a chemical bond, specifically that of
molecular hydrogen, in 1927.[5]. Their work was based on the valence bond model, which
assumes that a chemical bond is formed when there is good overlap between the atomic orbitals
of participating atoms. These atomic orbitals are known to have specific angular relationships
between each other, and thus the valence bond model can successfully predict the bond angles
observed in simple molecules.
Resonance
Many bonding situations can be described with more than one valid Lewis Dot Structure (for
example, ozone, O3). In an LDS diagram of O3, the center atom will have a single bond with one
atom and a double bond with the other. The LDS diagram cannot tell us which atom has the
double bond; the first and second adjoining atoms have equal chances of having the double
bond. These two possible structures are called resonance structures. In reality, the structure of
ozone is a resonance hybrid between its two possible resonance structures. Instead of having
one double bond and one single bond, there are actually two 1.5 bonds with approximately three
electrons in each at all times.
A special resonance case is exhibited in aromatic rings of atoms (for example, benzene). Aromatic
rings are composed of atoms arranged in a circle (held together by covalent bonds) that may
alternate between single and double bonds according to their LDS. In actuality, the electrons tend
to be disambiguously and evenly spaced within the ring. Electron sharing in aromatic structures is
often represented with a ring inside the circle of atoms.Bond order is a number that indicates the
number of pairs of electrons shared between atoms forming a covalent bond. The term is only
applicable to diatomic molecules, but is used to describe bonds within polyatomic compounds as
well.
Metallic bond
The metallic bond accounts for many physical characteristics of metals, such as strength,
malleability, ductility, conduction of heat and electricity, and lustre.Metallic bonding is the
electrostatic attraction between delocalized electrons, called conduction electrons, and the
metallic ions within metals. Because it involves the sharing of free electrons among a lattice of
positively-charged metal ions, metallic bonding may be compared to that within molten salts.
Metallic bonds are non-polar, because in alloys there is little difference among the
electronegativities of the atoms participating in the bonding interaction (and in pure elemental
metals, none at all), and the electrons involved in the interaction are delocalized throughout the
crystalline structure of the metal.
Metal atoms contain few electrons in their valence shells relative to their periods or energy levels.
Such electrons can stray easily from the atoms and become delocalized, forming a sea of
electrons permeating a giant lattice of positive ions. The freedom of conduction electrons to
migrate gives metal atoms, or layers of them, the capacity to slide past each other, giving rise to
metals' typical characteristic phenomena of malleability and ductility.The electrons and positive
ions in metals have a strong attractive force between them. Much energy is required to overcome
it. Therefore, metals often have high melting and boiling points. The principle is similar to that of
ionic bonds.
Because metals' conduction electrons move independently in a sea of negative charge, metals
exhibit electrical conductivity, allowing charge to pass quickly through them, manifested as
current. A few non-metals conduct electricity, notably graphite (which, like metals, has free
electrons) and molten and aqueous ionic compounds, which have free ions.Heat conduction
works on the same principle; free electrons can transfer energy at a faster rate than the fixed
electrons of other substances, such as those which are covalently bonded.
Alloy
Alloys are usually prepared to improve on the properties of other elements. For instance,steel is
stronger than iron, its primary element. The physical properties of an alloy, such as density,
reactivity, Young's modulus, and electrical and thermal conductivity may not differ greatly from
the alloy's elements, but engineering properties, such as tensile strength[1] and shear strength
can be substantially different from those of the constituent materials. This is sometimes due to
the differing sizes of the atoms in the alloy, since larger atoms exert a compressive force on
neighboring atoms, and smaller atoms exert a tensile force on their neighbors. This helps the alloy
resist deformation, unlike a pure metal where the atoms move more freely. Alloys may exhibit
marked difference in behaviour even in the case of small amounts of impurities being one element
of the alloy; for example impurities in semiconducting ferromagnetic alloys lead to different
properties as first predicted by White, Hogan, Suhl and Nakamura.
Alloy is a metal that is made by melting and mixing two or more other metals. Brass is an alloy
made from copper and zinc. Bronze, used for statues, ornaments and church bells, is an alloy of
tin and copper. Unlike pure metals, most alloys do not have a single melting point. Instead, they
have a melting range in which the material is a mixture of solid and liquid phases. The temperature
at which melting begins is called the solidus, and that at which melting is complete is called the
liquidus. However, for most pairs of elements, there is a particular ratio which has a single melting
point; this is termed the eutectic mixture.
Pyrite
The mineral pyrite, or iron pyrite, is iron sulfide, FeS2. It has isometric crystals that usually appear
as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a
result of alternation of the cube and pyritohedron faces. Pyrite also frequently occurs as
octahedral crystals and as pyritohedra (a dodecahedron with pentagonal faces). It has a slightly
uneven and conchoidal fracture, a hardness of 6–6.5, and a specific gravity of 4.95–5.10. It is
brittle, meaning it breaks or powders easily. It can be identified in the field by the sulfur smell of
the powdered mineral. Its metallic luster and pale-to-normal, brass-yellow hue have earned it the
nickname fool's gold due to many miners mistaking it for the real thing, though small quantities of
actual gold are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable ore of gold.
Pyrite is the most common of the sulfide minerals. It is usually found associated with other
sulfides or oxides in quartz veins, sedimentary rock and metamorphic rock, as well as in coal
beds, and as the replacement mineral in fossils.The name pyrite is from the Greek word πυρά
(pura) meaning "fire". This is likely due to the sparks that result when pyrite is struck against steel.
This capacity made it popular for use in early firearms such as the wheellock.
Weathering and release of sulfuric acid
Pyrite exposed to the environment during mining and excavation reacts with oxygen and water to
form sulfuric acid, resulting in acid mine drainage. This drainage results from the action of
Thiobacillus bacteria, which generate their energy by using oxygen to oxidize ferrous iron (Fe2+)
to ferric iron (Fe3+). The ferric iron in turn reacts with pyrite to produce ferrous iron and sulfuric
acid. The ferrous iron is then available for oxidation by the bacteria; this cycle can continue until
the pyrite is exhausted.Pyrite is often confused with the mineral marcasite, a name derived from
the Arabic word for pyrite, due to their similar characteristics. Marcasite is a polymorph of pyrite,
which means it has the same formula as pyrite but a different structure and, therefore, different
symmetry and crystal shapes. The formal oxidation states are, however, the same as in pyrite
because again the sulfur atoms occur in persulfide-like pairs. Marcasite/pyrite is probably the
most famous polymorph pair next to the diamond/graphite pair. Appearance is slightly more
silver.
Marcasite is metastable relative to pyrite and will slowly be changed to pyrite if heated or given
enough time. Marcasite is relatively rare, but may be locally abundant in some types of ore
deposits, such as Mississippi Valley-type Pb-Zn deposits. Marcasite appears to form only from
aqueous solutions.Pyrite is often used in jewellery such as necklaces and bracelets, but although
the two are similar, marcasite cannot be used in jewellery as it has a tendency to crumble into
powder. Adding to the confusion between marcasite and pyrite is the use of the word marcasite as
a jewellery trade name. The term is applied to small polished and faceted stones that are inlaid in
sterling silver, but even though they are called marcasite, they are actually pyrite.
Formal oxidation states for pyrite,
marcasite, and arsenopyrite
From the perspective of classical inorganic chemistry, which assigns formal oxidation states to
each atom, pyrite is probably best described as Fe2+S22-. This formalism recognizes that the
sulfur atoms in pyrite occur in pairs with clear S-S bonds. These persulfide units can be viewed as
derived from hydrogen persulfide, H2S2. Thus pyrite would be more descriptively called iron
persulfide, not iron disulfide. In contrast, molybdenite, MoS2, features isolated sulfide (S2-)
centers. Consequently, the oxidation state of molybdenum is Mo4+. The mineral arsenopyrite has
the formula FeAsS. Whereas pyrite has S2 subunits, arsenopyrite has AsS units, formally derived
from deprotonation of H2AsSH. Analysis of classical oxidation states would recommend the
description of arsenopyrite as Fe3+AsS3-. Of course these formalisms ignore covalency, which is
strongly implied by the semi-conducting behavior of this family of inorganic solids.Pyrite is used
commercially for the production of sulfur dioxide, for use in such applications as the paper
industry, and in the manufacture of sulfuric acid, though such applications are declining in
importance. It is also used for costume jewelry.