(covalent bonds)
When two atoms bond together, and neither one donates or gives up an electron, they form a covalent bond. In a covalent bond the atoms share the electrons in their outer orbital shells. The sharing of electrons in covalent bonding creates a very strong bond because each atom participating in the electron share has a full outer shell and a neutral charge.
An example of a covalent bond is found in a molecule of water, H2O. As illustrated in Figure 5-5, the two atoms of hydrogen (H) share electrons with the atom of oxygen (O).
FIGURE 5-4: The ionic bond between sodium and chloride to form a molecule of NaCl.
Migrating electrons (metallic bonds)
Metallic bonds occur between atoms that have very few electrons in their outermost electron shells. Instead of donating or sharing these electrons, the electrons are released from the orbital shell and available for a nearby cluster of atoms to use. Some scientists describe the metallically bonded atoms as floating in a sea of electrons because the electrons are not specifically attached to the orbital shell of any atom in particular. The electrons in a metallic bond move freely from one atom to another. This idea is illustrated in Figure 5-6.
FIGURE 5-5: Covalent bonding in a water molecule.
FIGURE 5-6: Metallic bonding between atoms in a sea of electrons.
Formulating Compounds
The bonding of elements to form compounds is fundamental to understanding the formation of rocks and minerals (which I describe in Chapter 6). When scientists discuss the processes of rock formation, as well as other earth processes involving chemical changes (such as weathering, described in Chapter 7), they use a shorthand of chemical formulas.
The chemical formula of a compound describes the number of different atoms of each element that are combined into a compound. For example, the chemical formula for quartz is as follows:
SiO2
This formula indicates that one atom of silicon (Si) and two atoms of oxygen (O) have bonded together, forming the compound.
(Mg, Fe)2SiO4
Two atoms of either magnesium (Mg) or iron (Fe) will combine with one atom of silicon (Si) and four atoms of oxygen (O). Either magnesium or iron can create the mineral olivine, so when you write the chemical formula, you put a parenthesis around and a comma separating the possible atoms that can form that particular chemical compound.
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