1.6 Formal Charge to both nitrogen and hydrogen, and the third has a single bond to nitrogen and a tive charge. Nitrogen is positively charged. The positive and negative charges are alled formal charges, and the Lewis structure of nitric acid would be incomplete were they to be omitted le calculate formal charges by counting the number of electrons""owned"by each atom in a Lewis structure and comparing this electron count with that of a neutral atom. The r Figure 1.6 illustrates how electrons are counted for each atom in nitric acid. counting group element such electrons for the purpose of computing the formal charge differs from counting electrons nitrogen is equal to its group to see if the octet rule is satisfied. A second-row element has a filled valence shell if the gen this is 5 sum of all the electrons, shared and unshared, is 8. Electrons that connect two atoms by a covalent bond count toward filling the valence shell of both atoms. when calculating the formal charge, however, only half the number of electrons in covalent bonds can b considered to be"owned"by an atom To illustrate, let's start with the hydrogen of nitric acid. As shown in Figure 1.6, It will always be true that ydrogen is associated with only two electrons--those in its covalent bond to oxygen. covalently bonded hydrogen It shares those two electrons with oxygen, and so we say that the electron count of each has no formal charge(formal hydrogen is 2(2)=1. Since this is the same as the number of electrons in a neutral hydrogen atom, the hydrogen in nitric acid has no formal charge Moving now to nitrogen, we see that it has four covalent bonds(two single bonds t one double bond), and so its electron count is ) 4. A neutral nitrogen has five will always be true that a electrons in its valence shell. The electron count for nitrogen in nitric acid is I less than bonds has a formal charge of that of a neutral nitrogen atom, so its formal charge is +1 +1. (A nitrogen with four e Electrons in covalent bonds are counted as if they are shared equally by the atoms unshared pairs, because of they connect, but unshared electrons belong to a single atom. Thus, the oxygen which the octet rule is doubly bonded to nitrogen has an electron count of 6(four electrons as two unshared pairs+ two electrons from the double bond). Since this is the same as a neutral oxy- gen atom, its formal charge is 0. Similarly, the OH oxygen has two bonds plus two It will always be true that an unshared electron pairs, giving it an electron count of 6 and no formal charge The oxygen highlighted in yellow in Figure 1.6 owns three unshared pairs(six bonds and two unshared electrons)and shares two electrons with nitrogen to give it an electron count of 7. This is I more than the number of electrons in the valence shell of an oxygen atom, and so its formal charge is -l reasoning oxygen with one covalent The method described for calculating formal charge has been one of reasoning through a series of logical steps. It can be reduced to the following equation: pairs has a formal charge of Formal charge s group number in number of bonds -number of unshared electrons -Electron count(O)=1(4)+4=6 Electron count(H)=1(2)=1- Electron count(N)=5(8)=4 Electron count(O)=1(4)+4=6 Electron count(O)=1(2)+6=7 FIGURE 1.6 Counting electrons in nitric acid. The electron count of each atom is equal to half the number of electrons it r of electrons in its own unshared pairs. Back Forward Main MenuToc Study Guide ToC Student o MHHE Websiteto both nitrogen and hydrogen, and the third has a single bond to nitrogen and a nega￾tive charge. Nitrogen is positively charged. The positive and negative charges are called formal charges, and the Lewis structure of nitric acid would be incomplete were they to be omitted. We calculate formal charges by counting the number of electrons “owned” by each atom in a Lewis structure and comparing this electron count with that of a neutral atom. Figure 1.6 illustrates how electrons are counted for each atom in nitric acid. Counting electrons for the purpose of computing the formal charge differs from counting electrons to see if the octet rule is satisfied. A second-row element has a filled valence shell if the sum of all the electrons, shared and unshared, is 8. Electrons that connect two atoms by a covalent bond count toward filling the valence shell of both atoms. When calculating the formal charge, however, only half the number of electrons in covalent bonds can be considered to be “owned” by an atom. To illustrate, let’s start with the hydrogen of nitric acid. As shown in Figure 1.6, hydrogen is associated with only two electrons—those in its covalent bond to oxygen. It shares those two electrons with oxygen, and so we say that the electron count of each hydrogen is 1 2 (2) 1. Since this is the same as the number of electrons in a neutral hydrogen atom, the hydrogen in nitric acid has no formal charge. Moving now to nitrogen, we see that it has four covalent bonds (two single bonds  one double bond), and so its electron count is 1 2 (8) 4. A neutral nitrogen has five electrons in its valence shell. The electron count for nitrogen in nitric acid is 1 less than that of a neutral nitrogen atom, so its formal charge is 1. Electrons in covalent bonds are counted as if they are shared equally by the atoms they connect, but unshared electrons belong to a single atom. Thus, the oxygen which is doubly bonded to nitrogen has an electron count of 6 (four electrons as two unshared pairs  two electrons from the double bond). Since this is the same as a neutral oxy￾gen atom, its formal charge is 0. Similarly, the OH oxygen has two bonds plus two unshared electron pairs, giving it an electron count of 6 and no formal charge. The oxygen highlighted in yellow in Figure 1.6 owns three unshared pairs (six electrons) and shares two electrons with nitrogen to give it an electron count of 7. This is 1 more than the number of electrons in the valence shell of an oxygen atom, and so its formal charge is 1. The method described for calculating formal charge has been one of reasoning through a series of logical steps. It can be reduced to the following equation: Formal charge group number in  number of bonds  number of unshared electrons periodic table 1.6 Formal Charge 17 H±O±N O O   œ ± Electron count (O) (4)  4 6 1 2 Electron count (N) (8) 4 1 2 Electron count (O) (2)  6 7 1 Electron count (O) (4)  4 6 2 1 2 Electron count (H) (2) 1 1 2 FIGURE 1.6 Counting electrons in nitric acid. The electron count of each atom is equal to half the number of electrons it shares in covalent bonds plus the number of electrons in its own unshared pairs. The number of valence elec￾trons in an atom of a main￾group element such as nitrogen is equal to its group number. In the case of nitro￾gen this is 5. It will always be true that a covalently bonded hydrogen has no formal charge (formal charge 0). It will always be true that a nitrogen with four covalent bonds has a formal charge of 1. (A nitrogen with four co￾valent bonds cannot have unshared pairs, because of the octet rule.) It will always be true that an oxygen with two covalent bonds and two unshared pairs has no formal charge. It will always be true that an oxygen with one covalent bond and three unshared pairs has a formal charge of 1. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website