Although individual hydrogen bonds are weak, many of them can form within one molecule or between two molecules. (B) Hydrogen bonds can form between different parts of the same large molecule.Ī hydrogen bond is much weaker than a covalent bond (see Table 2.1). (Electronegativity is calculated to produce a dimensionless quantity, meaning that it has no unit of measurement such as for length, time, mass, etc.)įigure 2.4: Hydrogen Bonds Can Form between or within Molecules (A) A hydrogen bond forms between two molecules because of the attraction between an atom with a partial negative charge on one molecule and a hydrogen with a partial positive charge on a second molecule. TABLE 2.3 shows the electronegativities of some elements important in biological systems. The electronegativity of a nucleus depends on how many positive charges it has (nuclei with more protons are more positive and thus more attractive to electrons) and on the distance between the electrons in the bond and the nucleus (the closer the electrons, the greater the electronegative pull). The attractive force that an atomic nucleus exerts on electrons in a covalent bond is called its electronegativity. One nucleus may exert a greater attractive force on the electron pair than the other nucleus, so that the pair tends to be closer to that atom. However, when the two atoms are different elements, the sharing is not necessarily equal. If two atoms of the same element are covalently bonded, there is an equal sharing of the pair(s) of electrons in their outermost shells. TABLE 2.2 shows the covalent bonding capacities of some biologically important elements. FIGURE 2.3B shows several different ways to represent the molecular structure of methane. Four covalent bonds-four shared electron pairs-hold methane together. The single shell of each hydrogen atom is also filled. As a result of electron sharing, the outer shell of the carbon atom is now filled with eight electrons-a stable configuration. Methane forms when an atom of carbon reacts with four hydrogen atoms. Because of the octet rule, carbon is most stable when it shares electrons with four other atoms- it can form four covalent bonds ( FIGURE 2.3A). The carbon atom has six electrons: two electrons fill its inner shell, and four electrons are in its outer shell. Let’s see how covalent bonds are formed in the somewhat more complicated methane molecule (CH 4). A covalent bond forms when the electron shells of the two atoms overlap in an energetically stable manner. Figure 2.2: Electrons Are Shared in Covalent Bonds Two hydrogen atoms can combine to form a hydrogen molecule.
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