Definition: substance with high electrical conductivity, luster, and
malleability, which readily loses electrons to form positive ions
(cations). Metals are otherwise defined according to their position on
the Periodic Table, including groupings as alkali metals, alkaline earth
metals, transition metals, and rare earth metals.
It's a combination of factors, mainly electron mobility (causing
conductivity and subsequent reflectivity) and a metallic lattice (although
the crystal systems - BCC for example - are about the same, I seem to
remember there's something different relating to the spare electrons).
Crystal deformation, i.e. malleability is also a typical feature.
Some other elements and compounds posess conductivity, graphite for example,
many sulfides (pyrite, galena, etc.) and oxides (cuprous oxide, lead
dioxide, etc.) and so on (gallium arsenide for example is used for LEDs and
as such must have free charge carriers). But none of these are classified
as metals for reasons of luster and malleability.
Some metals, for example tungsten, uranium (IIRC) and bismuth (in my
experience) are on the brittle side (in the case of tungsten however, it is
merely below the brittle-ductile transition, something iron experiences
particularly with high sulfur impurity), but still win out as metals by
appearance, behavior in general, and of course geography on the table.
Then there's the semimetals, such as silicon, arsenic, selenium and
tellurium. They have a metallic appearance, but tend to act as nonmetals,
forming cations most often. So being on the fence, they are merely
classified as such!
"California is the breakfast state: fruits, nuts and flakes."
The main defining feature is that the atoms form metallic bonds with
each other. (How's that for circular?!) Really, though, metallic
bonds are when each atom readily gives up its outer shell of electrons
(aka, the valence electrons). These electrons form a "sea" which
surrounds the atom cores (cations). The atom cores and the sea are
oppositely charged, so they are mutually attracted to each other -->
It's this particular kind of bonding that accounts for the usual
properties of metals. Metals are (usually) ductile because the bonds
are vague and non-directional; thus atoms can slide by each other
without completely *breaking* a bond (ductility is complex and requires
"dislocations", but that's the gist at the dislocation itself). Metals
conduct electricity and heat bc all those free electrons really are
free to move about. Melting temp and modulus of elasticity (stiffness)
are directly related to how strong the bonding is--big clunky atoms
like gold and lead have weak bonds. Strength has more to do w/
macroscopic things like grain boundaries, crystal structure, and
impurities (e.g., carbon in steel, which prevents the iron atoms from
sliding past each other so easily -- that's why steel's ductility is
inversely proportional to it's strength. Pure iron is soft like lead.)
Here are some nice pics and explanations: