If lithium ions are brought near negatively charged fluoride ions (F ), the opposite electrostatic charges attract. The ions can pack into a three-dimensional lattice, as shown opposite, to form a solid known as a salt. Common table salt is sodium chloride, in which sodium ions (Na ) and chloride ions (Cl ) are arranged in the same way as the Li and F ions in the diagram.

If many lithium atoms are brought together into a solid, their loosely held outer electrons are free to wander through the entire metal block under the influence of external electrostatic forces. One can think of a metal as a closely packed collection of positive ions held together by a "glue" of mobile electrons (see opposite). These mobile electrons are responsible for the physical properties that we associate with a metal: conduction of electricity and heat, metallic luster or sheen, and mechanical deformability. We will look at metals in more detail in Chapter 5. Lithium is a relatively soft metal because it has only one electron per positive ion to serve as "glue" in holding the metal together. In contrast, beryllium has two electrons outside the inner helium core. Beryllium metal has twice as many electrons per ion, and therefore is harder than lithium. For boron, the energy required to remove its three outer electrons is too great. Boron atoms do not lose three electrons and form boron ions (B  ) . Instead, they share electrons with neighbor atoms in covalent, or electron-pair, bonds. Lithium and beryllium are metals; boron lies on the borderline between these metallic elements and the nonmetals carbon, nitrogen, oxygen, and fluorine.