they can share their electrons (Figure 2.11). Each hydrogenatom is now associated with 2 electrons in what amountsto a completed valence shell. Two or more atoms held to-gether by covalent bonds constitute a molecule, in thiscase a hydrogen molecule.Figure 2.12ashows several ways of representing a hydrogenmolecule. Its molecular formula, H2, simply indicates that themolecule consists of two atoms of hydrogen. Electron sharingcan be depicted by an electron distribution diagram or by aLewis dot structure, in which element symbols are surroundedby dots that represent the valence electrons (H:H). We can alsouse a structural formula, H¬H, where the line represents asingle bond, a pair of shared electrons. A space-filling modelcomes closest to representing the actual shape of the molecule.Oxygen has 6 electrons in its second electron shell andtherefore needs 2 more electrons to complete its valenceshell. Two oxygen atoms form a molecule by sharing twopairs of valence electrons (Figure 2.12b). The atoms are thusjoined by a double bond(O“O).Figure 2.12Covalent bonding in four molecules.Thenumber of electrons required to complete an atom’s valence shellgenerally determines how many covalent bonds that atom will form.This figure shows several ways of indicating covalent bonds.OOHHHHCHHOHHHElectronDistributionDiagramName andMolecularFormulaLewis DotStructure andStructuralFormulaSpace-FillingModelHOHHCHHHHOO(a) Hydrogen (H2).Two hydrogen atoms share one pair of electrons, forming a single bond.(b) Oxygen (O2).Two oxygen atoms share two pairs of electrons, forming a double bond.(c) Water (H2O).Two hydrogen atoms and one oxygen atom are joined by single bonds, forming a molecule of water.(d) Methane (CH4).Four hydrogen atoms can satisfy the valence ofone carbonatom, forming methane.H H••OHH••••••••CHHHH••••••••O••••••••O••••
CHAPTER 2The Chemical Context of Life39Each atom that can share valence electrons has a bondingcapacity corresponding to the number of covalent bonds theatom can form. When the bonds form, they give the atom afull complement of electrons in the valence shell. The bond-ing capacity of oxygen, for example, is 2. This bonding capac-ity is called the atom’s valenceand usually equals the numberof unpaired electrons required to complete the atom’s outer-most (valence) shell. See if you can determine the valences ofhydrogen, oxygen, nitrogen, and carbon by studying the elec-tron distribution diagrams in Figure 2.9. You can see that thevalence of hydrogen is 1; oxygen, 2; nitrogen, 3; and carbon, 4.However, the situation is more complicated for elements inthe third row of the periodic table. Phosphorus, for example,can have a valence of 3, as we would predict from the pres-ence of 3 unpaired electrons in its valence shell. In some mol-ecules that are biologically important, however, phosphoruscan form three single bonds and one double bond. Therefore,it can also have a valence of 5.