Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 53

The carbon (C) and hydrogen (H) atoms have the following electronic configurations:

 C: 1s22s22p2

 H: 1s1

The formation of a CH₄ molecule can be achieved by the carbon atom sharing each of its four electrons in the outermost shell with an H atom. As described later (Section 2.7), the 2s orbital and three 2p orbitals in the carbon atom combine to form four hybrid orbitals (called sp³ orbitals), each occupied by one valence electron. By sharing one electron in each orbital with an electron from the hydrogen atom, the carbon atom achieves an outer shell of eight electrons, corresponding to the neon configuration, while the hydrogen atom has two electrons in its shell, corresponding to the helium configuration.

The octet rule is a useful guideline but there are exceptions to it. Furthermore, it does not consider electrons in the d and f shells of atoms. As only s and p electrons are considered, the octet rule is a useful rule for predicting the type bonding in the main group elements in the periodic table, that is, elements not in the transition metal blocks.

Another guiding principle in interatomic bonding is that when atoms lose, gain, or share electrons to form an octet in their outermost shell, they do so in a manner to minimize the number of charges that have to be transferred or shared. An alternative way to form an ionic bond between sodium and chlorine atoms, for example, is by the transfer of seven electrons from the outer shell of the chlorine atom to the sodium atom, forming Cl⁷⁺ and Na⁷⁻ ions, respectively. However, this does not occur because the energy required to create the Cl⁷⁺ and Na⁷⁻ ions with these many charges in the first place is exorbitant.