Page 30 - Materials Chemistry, Second Edition

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2.2. Types of Bonding in Solids

metals being easily oxidized by the surrounding environmental conditions.
This explains why metals are found in nature as complex geological formations of
oxides, sulfates, silicates, aluminates, etc. It should be noted that metals or alloys
may also exist as liquids. Mercury represents the only example of a pure metal
that exists as a liquid at STP. The liquid state of Hg is a consequence of the
electronic configurations of its individual atoms. The 6s valence electrons are
shielded from the nuclear charge by a filled shell of 4f electrons. This shielding
causes the effective nuclear charge (Z eff ) to be higher for these electrons,
resulting in less sharing/delocalization of valence electrons relative to other
metals. Further, relativistic contraction of the 6s orbital causes these electrons to
be situated closer to the nucleus, making them less available to share with neighbor-
ing Hg atoms. [3] In fact, mercury is the only metal that does not form diatomic
molecules in the gas phase. Energetically, the individual atoms do not pack into a
solid lattice since the lattice energy does not compensate for the energy required to
remove electrons from the valence shell.

Metallic bonding
The most simplistic model for the bonding in metallic solids is best described via the
free electron model. This considers the solid as a close-packed array of atoms, with
valence electrons completely delocalized throughout the extended structure. Since
the delocalization of electrons occurs more readily for valence electrons farther from
the nucleus (experiencing a lesser Z eff ), metallic character increases going down a
Group of the Periodic Table. Perhaps the best example of this phenomenon is
observed for the Group 14 congeners. As you move from carbon to lead, the
elemental properties vary from insulating to metallic, through a transitional range
of semiconducting behavior for Si and Ge.
The close chemical association among neighboring metal atoms in the solid
gives rise to physical properties such as high melting points and malleability. The
nondirectional bonding in metals allows for two modes of deformation to occur
when a metal is bent. Either the atomic spacing between neighboring metal atoms in
the crystal lattice may change (elastic deformation), or planes of metal atoms may
slide past one another (plastic deformation). Whereas elastic deformation results in a
material with “positional memory” (e.g., springs), plastic deformation results in a
material that stays malformed. We will consider the bonding modes of metals and
non-metals in more detail later in this chapter.
Although metals are mostly characterized by crystalline structures, amorphous
alloys may also be produced, known as metallic glasses. A recent example of such a
material is the multicomponent alloy Zr 41 Ti 14 Ni 12 Cu 10 Be 23 . [4] These materials
combine properties of both plastics and metals, and are currently used within electric
transformers, armor-piercing projectiles, and even sports equipment. The media
™
has recently been focused on this latter application, for LiquidMetal golf clubs,
tennis racquets, and baseball bats. Unlike window glass, metallic glass is not brittle.
Many traditional metals are relatively easy to deform, or bend permanently out of