![]() 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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