Читать книгу Astrobiology - Charles S. Cockell - Страница 42

In ionic bonding, the central feature of the bond is the transfer of an electron from one atom to another to generate two ions. In covalent bonds, an electron is shared. Covalent bonds take place between atoms that are generally close to each other in the Periodic Table and have a small difference in electronegativity (the tendency of an atom to attract electrons). Similarly to ionic bonds, the sharing of an electron allows the noble gas electron configuration to be attained by both atoms.

The simplest example of a covalent bond is found in the hydrogen molecule, shown in Figure 3.7. To return to the electron configurations that we discussed earlier, a hydrogen atom has an electron configuration of 1s¹: one electron in the first shell, the s subshell. It would like to have two in this subshell so that the subshell is full. In the hydrogen molecule, the sharing of each electron between two hydrogen atoms allows for two electrons in this subshell.

Figure 3.7 The covalent bond in the hydrogen molecule. The two electrons are shared between both hydrogen atoms.

Covalent bonds are very strong. An example is diamond, a covalent network of carbon. Taking the same logic that we used for ionic bonds, the energy per bond is ∼6 × 10⁻¹⁹ J. That's equivalent to 150 times the thermal energy at room temperature. In other words, the thermal energy in the bonds of diamond is much lower than the energy needed to break carbon–carbon bonds. Diamond is very stable at room temperature.