Ammonium ions formal charge

Verify that the formal charges on nitrogen in ammonium ion... 

Formal charges are based on Lewis structures m which electrons are considered to be shared equally between covalently bonded atoms Actually polarization of N—H bonds m ammonium ion and of B—H bonds m borohydride leads to some transfer of positive and negative charge respectively to the hydrogens... 

The electron counts of nitrogen in ammonium ion and boron in borohydride ion are both 4 (half of eight electrons in covalent bonds) Because a neutral nitrogen has five electrons in its valence shell an electron count of 4 gives it a formal charge of +1 A neutral boron has three valence electrons so that an electron count of 4 in borohydride ion corresponds to a formal charge of -1... 

As a result there is an increased partial positive charge on each of the four ammonium ion hydrogens. The operation of the formal charge effect thus can disiribute the net plus charge of the ion over the periphery of the molecule rather than having it centered on the nitrogen atom. 

Figure 17.5 reminds us that the oxidation numbers of the atoms in ions and molecules are determined in the same way as in inorganic chemistry. In the ammonium ion, the four H atoms again possess the oxidation number +1, and the N atom has the oxidation number -3. But the same atom has a formal charge of +1. 

Calculation of the formal charges on the atoms of the ammonium ion is shown in Figure 1.7. Note that the sum of the formal charges on all the atoms of a covalent molecule gives the total charge on that molecule. 

The singly positively charged nitrogen N+ is isoelectronic with carbon and thus this will form bonds of the same kind. Methane is isoelectronic with the tetrahedrally built ammonium ion. These bonds are of quite the same nature as those between formally neutral atoms (see 25, onium compounds). 

Formal charge can also help answer the question where is the charge located that is frequently asked about polyatomic ions. Thus by writing out the Lewis structure for the ammonium ion NH4+, you should be able to convince yourself that the nitrogen atom has a formal charge of +1 and each of the hydrogens has 0, so we can say that the positive charge is localized on the central atom. 

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