Chemistry and attraction dating

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In the everyday world around us, we observe three very different types of materials: gases, liquids, and solids.

Closer examination of the physical properties of homogeneous crystalline solids shows that they can be subdivided into four distinct categories according to their physical properties and the different forces holding them together.

(See Figure 1.) In many metals, each atom is in contact with twelve others: six in a plane, three above, and three below.

The outermost valence electrons of the atoms belong to the crystal as a whole, delocalized as a "sea" in which they are freely mobile to flow from atom to atom.

In other metals (e.g., iron), each atom is in contact with eight others at the corners of a cube; such structures are called body-centered cubic.

To a first approximation, we expect that the melting point of the metal should be related to the number of valence electrons that each atom contributes to the "sea." Two series of metals, as outlined in Table 1, illustrate the effect.

The positive nuclei of the atoms are embedded in the sea as a close-packed three-dimensional array. The electrons are constrained about each atom, some atoms with excess positive charge (cations) and some with excess negative charge (anions).

The ions are packed closely together, held by coulomb (electrostatic) forces of attraction. A solid that is extremely hard, that has a very high melting point, and that will not conduct electricity either as a solid or when molten is held together by a continuous three-dimensional network of covalent bonds.

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