Name: lewis structures with formal charge
Uploaded: 2020-09-03T13:38:28.000Z
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Formal Charges

一些分子或多原子离子 可以由多个路易斯结构表示，但是如何确定哪个是主导结构呢？通过计算原子的形式电荷，可以确定最接近分子实际结构 的路易斯结构。每个原子都分配了一个称为形式电荷 的假想电荷，如果分子中所有其他原子 具有相同的电负性 则该电荷就是原子上的电荷。假定每个成键电子 被两个原子均匀共享。以氯化氢为例。为了确定每个原子上的形式电荷 首先将非成键电子的数量 加上成键电子数量的一半 然后用价电子数量 减去所得值。分子或离子中所有形式电荷的总和 等于分子或离子的净电荷。例如，笑气可以由三个可能的 路易斯结构表示—一个结构有两个双键，一个结构二个氮原子之间有三键，另一个结构在氮和氧之间有三键 所有结构都满足八隅体规则。最好的路易斯结构是通过 形式电荷计算确定的。氮有五个价电子，而氧气有六个价电子。基于非成键电子数 和一半成键电子数的计算 得出了每种结构的形式电荷。因为一氧化二氮是中性分子，所以所有形式电荷的总和必须为零。一般来说，主导路易斯结构中各单个原子的 形式电荷最接近于零。因此，具有较高形式电荷的第三种结构 可以忽略。另外，负形式电荷（如果存在的话）应由电负性最高的原子携带。由于氧比氮电负性更高，因此第二种结构，氧带有负形式 电荷，被确定为一氧化二氮的 主导结构。形式电荷不是分子或原子的 实际电荷，而是记录惯例。分子的实际电荷 取决于多个因素，包括 组成原子之间的 电负性差异。

形式电荷

在某些情况下，分子和多原子离子离子似乎有多个有效的路易斯结构。 当存在多个合理的结构时，形式电荷的概念可用于帮助预测最合适的路易斯结构。 
 
 <h4>正在计算形式电荷 </h4>
 
 分子中原子的形式电荷是如果键中的电子在原子之间均匀分布时原子会产生的假设电荷。 或者，当从中性原子的价电子数量开始，非粘接电子首先被减少，然后在路易斯结构中减去与该原子连接的键数量时，形式电荷会产生结果。 
 
 因此，形式电荷的计算方法如下：  
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065_Equations-01.png" style="height: 60px;" />
 
 形式电荷计算可以由确定对整个结构的形式电荷总和进行双重检查。 分子中所有原子的形式电荷总和必须为零；离子中的形式电荷总和应等于离子的荷电。 请记住，为原子计算的形式电荷不是原子在分子中的实际电荷。 形式电荷只是一个有用的簿记程序；它不表示存在实际费用。 
 
 <h4>根据路易斯结构计算形式电荷 </h4>
 
 按照以下步骤为卤素离子 ICl4− 中的每个原子分配形式电荷。 
 
 步骤 1. 所有 I – CL 键的键电子线对平均除法： 
 
 <img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_1.svg" style="height: 120px;" />
 
 步骤 2. 为原子分配单对电子。 现在每个氯原子都有七个电子分配给它，而碘原子有八个电子分配给它。 
 
 步骤 3. 从中性原子的价电子数中减去该数值： 
 碘： 7 – 8 = – 1 
 氯： 7 – 7 = 0 
 所有原子的形式电荷总和等于– 1 ，这与离子荷电 (– 1) 相同。 
 
 <h4>使用形式电荷预测分子结构 </h4>
 
 分子或离子中原子的排列称为其分子结构。 在许多情况  下，按照编写路易斯结构的步骤可能会导致多个可能的分子结构——多个不同的键和单对电子放置或原子的不同排列。 一些涉及形式电荷的指南有助于确定哪种可能的结构最有可能用于特定分子或离子： 
 
 <ol>
 <li>所有形式电荷均为零的分子结构比某些形式电荷并非为零的分子结构更可取。 </li>
 <li>如果路易斯结构必须具有非零形式电荷，则最好采用最小非零形式电荷的排列方式。 </li>
 <li>当相邻的形式电荷为零或相反符号时，路易斯结构更可取。 </li>
 <li>当从多个具有类似形式电荷分布的路易斯结构中进行选择时，在更具电子性的原子上使用负形式电荷的结构更可取。 </li>
 </ol>
 
 为了了解这些准则的适用情况，请考虑二氧化碳和二氧化碳的一些可能结构。 众所周知，电离较少的原子通常占据中心位置，但形式电荷有助于了解为什么会发生这种情况。 结构有三种可能：碳在中心有两个双键，碳在中心有一个和三个键，氧气在中心有双键。  
 
 <img alt="Figure2" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065image_2.svg" style="height: 80px;" />
 
 通过比较三个形式电荷，可以将左侧的结构确定为最佳结构，因为它只有形式电荷为零。 
 
 另一个例子是，硫氰酸酯离子，一种由碳原子，一种氮原子和一种硫原子形成的离子，可能有三种不同的分子结构： NCS ， CNS 或 CSN。 这些分子结构中的形式电荷可以帮助选择最可能的原子排列。 硫氰酸酯离子的三种可能结构中的每一种可能的路易斯结构和形式电荷是：碳在中心，双键，氮在中心，双键，硫在中心，双键。  
 
 <img alt="Figure3" src="https://www.jove.com/files/core/CDNSource/lessons/1065/1065Image_3.svg" style="height: 95px;" />
 
 请注意，每种情况下形式电荷的总和等于离子 (– 1) 的电荷。 但是，首选的是原子在中心使用碳的第一种排列方式，因为原子的非零形式电荷数量最少。 此外，它还将电离原子置于中心位置，负电荷则置于电离更强的元件上。 
 
 本文改编自 <a href="https://openstax.org/books/chemistry-2e/pages/7-4-formal-charges-and-resonance">Openstax, 化学 2e, 章节 7.4 形式电荷与共振。</a>

在某些情况下，分子和多原子离子似乎存在不止一种有效的路易斯结构。形式电荷的概念可用于……

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lewis structures with formal charge

Lewis structures can sometimes show more than one reasonable arrangement for a molecule or polyatomic ion. Formal charge helps chemists choose the best Lewis structure when that happens. It is a useful way to compare possible structures in chemistry.

Formal charge is the hypothetical charge an atom would have if bonding electrons were shared evenly between the atoms. It can also be found by starting with the valence electrons of the neutral atom, subtracting the nonbonding electrons, and then subtracting the number of bonds on that atom in the Lewis structure. This is a bookkeeping tool, not the actual charge on the atom.

The formal charge of a full structure should also be checked as a whole. For a neutral molecule, the sum of all formal charges must be zero. For an ion, the sum of all formal charges should match the ion’s overall charge.

A common example is the interhalogen ion ICl4−. To assign formal charges, the bonding electron pairs are divided equally among the I–Cl bonds. Then lone pairs are assigned to their atoms. In this structure, each chlorine has seven electrons assigned to it, and iodine has eight. That gives iodine a formal charge of −1 and each chlorine a formal charge of 0, which adds up to the −1 charge of the ion.

Formal charge also helps predict molecular structure when several Lewis structures are possible. A structure with all formal charges equal to zero is preferred over one with nonzero charges. If nonzero charges are needed, the best choice has the smallest charges possible. Structures are also preferred when adjacent formal charges are zero or opposite in sign. When several choices are still possible, the negative charge should be on the more electronegative atom.

Carbon dioxide shows why these rules matter. One possible structure has carbon in the center with two double bonds. Other possibilities place carbon between a single and a triple bond, or place oxygen in the center with double bonds. The structure with carbon in the center and two double bonds is preferred because all of its formal charges are zero.

The thiocyanate ion, NCS−, gives another clear example. It can be written with carbon, nitrogen, or sulfur in the center. The formal charges in each possible Lewis structure help identify the most likely arrangement of atoms. The carbon-centered structure is preferred because it has the fewest atoms with nonzero formal charges. It also places the least electronegative atom in the center and the negative charge on the more electronegative element.

Lewis Structures with Formal Charge

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Lewis Structures with Formal Charge

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