Covalent Bonds A covalent bond is the sharing of a pair of valence electrons by two atoms. For example, lets consider what happens when two hydrogen atoms approach each other. Recall that hydrogen has 1 valence electron in the rst shell, but the shells capacity is 2 electrons. When the two hydrogen atoms come close enough for their 1s orbitals to overlap, they can share their electrons (Figure 2.11). Each hydrogen atom is now associated with 2 electrons in what amounts Hydrogen atoms (2 H) 1 In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus. + + 2 When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton in the other nucleus. + + 3 The two electrons become shared in a covalent bond, forming an H2 molecule. + + Hydrogen molecule (H2) (cid:2) Figure 2.11 Formation of a covalent bond. 38

U N I T O N E The Chemistry of Life to a completed valence shell. Two or more atoms held together by covalent bonds constitute a molecule, in this case a hydrogen molecule. Figure 2.12a shows several ways of representing a hydrogen molecule. Its molecular formula, H2, simply indicates that the molecule consists of two atoms of hydrogen. Electron sharing can be depicted by an electron distribution diagram or by a Lewis dot structure, in which element symbols are surrounded by dots that represent the valence electrons (H:H). We can also use a structural formula, HH, where the line represents a single bond, a pair of shared electrons. A space-lling model comes closest to representing the actual shape of the molecule. Oxygen has 6 electrons in its second electron shell and therefore needs 2 more electrons to complete its valence shell. Two oxygen atoms form a molecule by sharing two pairs of valence electrons (Figure 2.12b). The atoms are thus joined by a double bond (OO).

Name and Molecular Formula Electron Distribution Diagram Lewis Dot Structure and Structural Formula SpaceFilling Model (a) Hydrogen (H2). Two hydrogen atoms share one pair of electrons, forming a single bond. H H H H(cid:129) (cid:129) H H (b) Oxygen (O2). Two oxygen atoms share two pairs of electrons, forming a double bond. O O (cid:129)(cid:129) (cid:129)(cid:129) O (cid:129) (cid:129) (cid:129) (cid:129) O (cid:129)(cid:129) (cid:129)(cid:129) O O (c) Water (H2O). Two hydrogen atoms and one oxygen atom are joined by single bonds, forming a molecule of water. O H H (cid:129)(cid:129) O H (cid:129)(cid:129) H(cid:129) (cid:129) O H H (d) Methane (CH4). Four hydrogen atoms can satisfy the valence of one carbon atom, forming methane. H H C H H (cid:129) (cid:129) H C H (cid:129)(cid:129) (cid:129)(cid:129) H (cid:129) (cid:129) H H H C H

H (cid:2) Figure 2.12 Covalent bonding in four molecules. The number of electrons required to complete an atoms valence shell generally determines how many covalent bonds that atom will form. This gure shows several ways of indicating covalent bonds.