In a polar covalent bond the sharing of electrons be

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In a polar covalent bond , the sharing of electrons be- tween atoms is unequal—one atom attracts the shared elec- trons more strongly than the other. The partial charges are indicated by a lowercase Greek delta ( ) with a minus or plus sign. For example, when polar covalent bonds form, the resulting molecule has a partial negative charge, writ- ten , near the atom that attracts electrons more strongly. At least one other atom in the molecule then will have a partial positive charge, written . A very important exam- ple of a polar covalent bond in living systems is the bond between oxygen and hydrogen in a molecule of water ( Figure 2.5e ). Hydrogen Bonds The polar covalent bonds that form between hydrogen atoms and other atoms can give rise to a third type of chem- ical bond, a hydrogen bond. A hydrogen bond forms when a hydrogen atom with a partial positive charge ( ) attracts the partial negative charge ( ) of neighboring electronega- tive atoms, most often oxygen or nitrogen. Thus, hydrogen bonds result from attraction of oppositely charged parts of molecules rather than from sharing of electrons as in cova- lent bonds. Hydrogen bonds are weak when compared to ionic and covalent bonds. Thus, they cannot bind atoms into molecules. However, hydrogen bonds do establish im- portant links between molecules, such as water molecules, or between different parts of large molecules, such as pro- teins and deoxyribonucleic acid (DNA), where they add strength and stability and help determine the molecule’s three-dimensional shape (see Figure 2.15 ). Chemical Reactions A chemical reaction occurs when new bonds form and/or old bonds break between atoms. Through chemical reactions, body structures are built and body functions are carried out, processes that involve transfers of energy. 30 Chapter 2 Introductory Chemistry
2.1 Introduction to Chemistry 31 H Hydrogen atoms + O Oxygen atoms + Oxygen molecule Hydrogen molecule H H H H H H 2 O O O O O O 2 N Nitrogen atoms Hydrogen atoms + Nitrogen molecule N N N N N C Carbon atom + C H H H H Methane molecule H C CH 4 H H H O H 2 O H H STRUCTURAL FORMULA MOLECULAR FORMULA (a) DIAGRAMS OF ATOMIC AND MOLECULAR STRUCTURE (b) (c) (d) (e) H H H H Hydrogen atoms O Oxygen atom O Water molecule + H H H H δ + δ + δ N 2 Figure 2.5 Covalent bond formation. T he red electrons are shared equally in (a)–(d) and unequally in (e). T o the right are simpler ways to represent these molecules. In a structural formula, each covalent bond is denoted by a straight line between the chemical symbols for two atoms. In a molecular formula, the number of atoms in each molecule is noted by subscripts. What is the main difference between an ionic bond and a covalent bond? In a covalent bond, two atoms share one, two, or three pairs of electrons in the outer shell.
Forms of Energy and Chemical Reactions Energy ( en - in; -ergy work) is the capacity to do work.

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