Читать книгу Materials for Biomedical Engineering - Mohamed N. Rahaman - Страница 55

The formation of an ionic bond involves the transfer of one electron or more from one atom to the other, creating ions with discrete positive and negative charges (Figure 2.1a). On the other hand, the covalent bond involves sharing of electrons between two atoms (Figure 2.1b). When the atoms forming the covalent bond are the same, such as two hydrogen atoms or two chlorine atoms to form a hydrogen molecule (H₂) or a chlorine molecule (Cl₂), the bonding electrons are shared equally between the two atoms (Figure 2.5a). However, when the two atoms are different, such as a hydrogen atom and a chlorine atom forming a hydrogen chloride molecule (HCl), the bonding electrons in the covalent bond are not shared equally between the atoms. The chlorine atom has a greater capacity to attract the shared electrons to itself, so due to its higher electronegativity, the shared electrons are attracted more strongly to the chlorine atom than the hydrogen atom (Figure 2.5b). Because of this unequal sharing, the chlorine atom develops a slightly negative character whereas the hydrogen atom develops a slightly positive character. The covalent bond is said to be polar, to consist of a dipole or to have a dipole moment. When appropriate to emphasize the polar nature of the molecule, the more electronegative atom in the bond is shown with a partial negative charge δ ⁻ symbol while the less electronegative atom is shown with a δ + symbol (Figure 2.5).

Figure 2.5 Illustration of (a) nonpolar bond in hydrogen molecule (H₂) and (b) polar bond in hydrogen chloride molecule (HCl).