1931年，化學(xué)家鮑林提出一種分子理論：共振論，共振論是價(jià)鍵論的發(fā)展。

共振論使用情況：當(dāng)一個(gè)分子、離子、自由基的結(jié)構(gòu)不能用經(jīng)典的路易斯結(jié)構(gòu)式正確描述的時(shí)候，可以采用多個(gè)路易斯結(jié)構(gòu)式表示，這些路易斯結(jié)構(gòu)式稱(chēng)為共振結(jié)構(gòu)式。

分子的真是結(jié)構(gòu)是這些共振結(jié)構(gòu)體共振得到的共振雜化體，每一個(gè)共振結(jié)構(gòu)對(duì)共振雜化體的貢獻(xiàn)不同。通常而言，共振結(jié)構(gòu)越穩(wěn)定，對(duì)共振雜化體的貢獻(xiàn)越大。（是否也是符合能量最低原理？）

值得注意的但是：共振結(jié)構(gòu)的書(shū)寫(xiě)除了要符合價(jià)鍵規(guī)則之外，還應(yīng)該遵循每隔共振結(jié)構(gòu)的原子核位置不發(fā)生改變，每個(gè)共振結(jié)構(gòu)的配對(duì)電子或者未共享電子數(shù)目不變的原則。

總而言之，共振理論的優(yōu)勢(shì)在于通過(guò)利用電子式對(duì)共軛體系中的電荷分布位置進(jìn)行定性描述，有應(yīng)用方便，實(shí)用性強(qiáng)的特點(diǎn)。

In1931, thechemist Pauling proposed a molecular theory: resonance theory, which is thedevelopment of valence bond theory.

Resonancetheory usage: When the structure of a molecule, ion, and free radical cannot becorrectly described by the classical Louis structural formula, it can beexpressed by multiple Louis structural formulas, which are called resonantstructural formulas.

The truestructure of the molecule is the resonance hybrid obtained by the resonance ofthese resonance structures, and each resonance structure contributes adifferent contribution to the resonance hybrid. In general, the more stable theresonance structure, the greater the contribution to the resonance hybrid. (Isit also in line with the minimum energy principle?) ）

It is worthnoting, however, that the writing of the resonant structure, in addition toconforming to the valence bond rule, should also follow the principle that theposition of the nucleus of each resonant structure does not change, and thenumber of paired electrons or unshared electrons of each resonant structure isunchanged.

All in all, theadvantage of resonance theory is that it is convenient to apply and has thecharacteristics of strong practicality by using electronic formula toqualitatively describe the charge distribution position in the conjugatesystem.

分子軌道理論

價(jià)鍵理論認(rèn)為共價(jià)鍵是由兩個(gè)自旋方向相反的電子配對(duì)形成，分子中的價(jià)電子被定域在兩個(gè)參與成鍵原子之間，分子軌道理論能夠很好的解釋共價(jià)鍵的二飽和性和方向性，但是分子理論也有著其固有缺陷，比如在解釋不飽和鍵的分子，或者離域的共軛體系以及氧分子的順磁現(xiàn)象，價(jià)鍵理論都不能給出令人滿(mǎn)意的解釋。

分子軌道理論認(rèn)為形成共價(jià)鍵的電子是分布在整個(gè)分子之中的，這樣可以將全部原子軌道之間的相互作用都考慮進(jìn)來(lái)。

Molecularorbital theory

Valence bondtheory believes that covalent bonds are formed by pairing two electrons withopposite spin directions, the valence electrons in the molecule are localizedbetween two participating bonded atoms, molecular orbital theory can explainthe diacturality and directionality of the covalent bond very well, but themolecular theory also has its inherent defects, such as in the interpretationof unsaturated bond molecules, or out-of-domain conjugate systems andparamagnetic phenomena of oxygen molecules, valence bond theory can not give asatisfactory explanation.

Molecularorbital theory holds that the electrons that form covalent bonds aredistributed throughout the molecule, which allows for the interaction betweenall atomic orbitals.

分子軌道理論的而主要內(nèi)容如下：

1. 1.           首先分子理論論認(rèn)為，分子中的電子不從屬于某一個(gè)特定的原子，而是在整個(gè)分子范圍內(nèi)運(yùn)動(dòng)，每一個(gè)電子的運(yùn)動(dòng)狀態(tài)可以用波函數(shù)∮來(lái)描述。與原子軌道相比較，分子軌道是多中心的，電子云分布在多個(gè)原子核周?chē)?，而原子軌道是單中心的，電子云分布在一個(gè)原子核周?chē)?/span>

2. 2.           分子軌道有形成分子的原子軌道線性組合，分子軌道數(shù)目和原子軌道數(shù)目相等。在這里我們可以舉一個(gè)例子：我們假設(shè)分別用∮1和∮2來(lái)表示兩個(gè)原子軌道，當(dāng)它們重疊的時(shí)候，可以形成兩個(gè)分子軌道，這其中，一個(gè)分子軌道是由兩個(gè)原子軌道的波函數(shù)相加組成∮=∮1+∮2，稱(chēng)為成鍵軌道；另外一個(gè)分子軌道是由兩個(gè)原子軌道的波函數(shù)相減組成∮=∮1-∮2，稱(chēng)為反鍵軌道。

3. 3.           進(jìn)一步，由原子軌道線性組合成為分子軌道還應(yīng)當(dāng)滿(mǎn)足以下幾個(gè)原則：1）原子軌道能量相近原則；2）最大重疊原則；3）對(duì)稱(chēng)性原則即必須是位相相同的部位相互重疊才能形成穩(wěn)定的分子軌道，這又被稱(chēng)為對(duì)稱(chēng)型匹配。

The maincontents of molecular orbital theory are as follows:

1. 1.           First of all,molecular theory holds that the electrons in a molecule are not subordinate toa specific atom, but move throughout the entire molecular range, and the stateof motion of each electron can be described by the wave function ∮. In contrastto atomic orbitals, molecular orbitals are polycentric, with electron cloudsdistributed around multiple nuclei, while atomic orbitals are uncentric, andelectron clouds are distributed around one nucleus;

2. 2.           Molecular orbitalshave linear combinations of atomic orbitals that form molecules, and the numberof molecular orbitals and atomic orbitals is equal. Here we can give anexample: we assume that ∮1and ∮2are used to represent two atomic orbitals, respectively, and when theyoverlap, two molecular orbitals can be formed, of which a molecular orbital iscomposed of the sum of the wave functions of the two atomic orbitals∮=∮1+∮ 2, called a bonded orbital;another molecular orbital is composed of subtracting the wave functions of twoatomic orbitals∮=∮1-∮2, called an inverse bonding orbital.

3. 3.           Further, the linearcombination of atomic orbitals into molecular orbitals should also meet thefollowing principles: 1) the principle of energy proximity of atomic orbitals;2) the principle of maximum overlap;3) the principle of symmetry, that is, parts of the same phase must overlapeach other to form stable molecular orbitals, which is also known as symmetricmatching.