they can share their electrons Figure 211 Each hydrogen atom is now associated

They can share their electrons figure 211 each

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they can share their electrons (Figure 2.11) . Each hydrogen atom is now associated with 2 electrons in what amounts to a completed valence shell. Two or more atoms held to- gether 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 , H 2 , 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 , H ¬ H, where the line represents a single bond , a pair of shared electrons. A space-filling 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 (O O). Figure 2.12 Covalent bonding in four molecules. The number of electrons required to complete an atom’s valence shell generally determines how many covalent bonds that atom will form. This figure shows several ways of indicating covalent bonds. O O H H H H C H H O H H H Electron Distribution Diagram Name and Molecular Formula Lewis Dot Structure and Structural Formula Space- Filling Model H O H H C H H H H O O (a) Hydrogen (H 2 ). Two hydrogen atoms share one pair of electrons, forming a single bond. (b) Oxygen (O 2 ). Two oxygen atoms share two pairs of electrons, forming a double bond. (c) Water (H 2 O). Two hydrogen atoms and one oxygen atom are joined by single bonds, forming a molecule of water. (d) Methane (CH 4 ). Four hydrogen atoms can satisfy the valence of one carbon atom, forming methane. H H O H H C H H H H O O
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CHAPTER 2 The Chemical Context of Life 39 Each atom that can share valence electrons has a bonding capacity corresponding to the number of covalent bonds the atom can form. When the bonds form, they give the atom a full 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 valence and usually equals the number of unpaired electrons required to complete the atom’s outer- most (valence) shell. See if you can determine the valences of hydrogen, oxygen, nitrogen, and carbon by studying the elec- tron distribution diagrams in Figure 2.9. You can see that the valence of hydrogen is 1; oxygen, 2; nitrogen, 3; and carbon, 4. However, the situation is more complicated for elements in the 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, phosphorus can form three single bonds and one double bond. Therefore, it can also have a valence of 5.
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