o = c = o
因為氧(O) 的陰電性 3.0 高於 碳 2.5
因此當任何其中一邊 C=O 共用電子成為共價鍵 的時候
電子實際上不是平均的分享
電子會偏向 O 稍微遠離 C
所以會形氧帶有負電的傾向 -2
碳被電子遠離 因此會有帶偏正電 +4
对共价化合物来说，元素的化合价等于原子形成共价键时所提供的电子数或所共用的电子对数，这种化合价叫做共价。共用电子对数本来无所谓正、负，只是人为地规定化合物分子中电负性较小的原子的化合价为正价（因共用电子对偏离它一方而使它显正电性），电负性较大的原子的化合价为负价（因共用电子对偏向它一方而使它显负电性）。例如，C和O2作用生成CO2的反应中，C原子最外层的4个电子是分别和2个O原子通过共用电子对而形成CO2分子的，C的共价数为4，O的共价数为2。但由于O原子的电负性比C原子的大，共用电子对偏向O原子一方，于是在CO2分子中把C看做+4价，O看做-2价。这种正、负价之分，不过是反映电子偏移的倾向，并不表示它们实际上所带的电荷数。象这样化合物分子中各原子形式上或外观上所带有的电荷数，叫做氧化数。确定氧化数的规则如下：
化合价曾被用来表示一种元素在化学反应里形成化合物时化合能力的大小。自从原子结构理论创立以后，化合价的现代概念是和化学键密切联系的。一般地说，化合价是表示各原子间化学键的数量关系。根据化学键的类型，化合价可分为电价和共价两种。
对离子化合物来说，元素的化合价等于离子所带的电荷数，这种化合价叫做电价。例如Na和Cl2作用生成NaCl的反应中，Na原子失去最外层1个电子而成带一个正电荷的Na+离子，所以Na是+1价，Cl原子最外层得到1个电子而成带一个负电荷的Cl-离子，所以Cl是-1价，这是一目了然的。
对共价化合物来说，元素的化合价等于原子形成共价键时所提供的电子数或所共用的电子对数，这种化合价叫做共价。共用电子对数本来无所谓正、负，只是人为地规定化合物分子中电负性较小的原子的化合价为正价（因共用电子对偏离它一方而使它显正电性），电负性较大的原子的化合价为负价（因共用电子对偏向它一方而使它显负电性）。例如，C和O2作用生成CO2的反应中，C原子最外层的4个电子是分别和2个O原子通过共用电子对而形成CO2分子的，C的共价数为4，O的共价数为2。但由于O原子的电负性比C原子的大，共用电子对偏向O原子一方，于是在CO2分子中把C看做+4价，O看做-2价。这种正、负价之分，不过是反映电子偏移的倾向，并不表示它们实际上所带的电荷数。象这样化合物分子中各原子形式上或外观上所带有的电荷数，叫做氧化数。确定氧化数的规则如下：
(1)单质原子的氧化数为零；
(2)除NaH等金属氢化物外，一般化合物中氢原子的氧化数是+1；
(3)除OF2、过氧化物等外，一般化合物中氧原子的氧化数是-2；
(4)化合物中各原子的氧化数的代数和为零。
在离子化合物中，离子的氧化数和离子的电价数相等，者是一致的。例如：
在CaCl2中，Ca2+离子的电价数=+2，氧化数=+2。Cl-离子的电价数=-1，氧化数=-1。
在Cl2中，Cl原子的共价数=1，氧化数=0。
在H2O2中，O原子的共价数=2，氧化数=-1。综上所述，氧化数与电价或共价是不相同的概念，它们之间既有联系，也有区别，不能混淆。氧化数只是表示单质或化合物中原子的氧化还原状态，但它不一定和物质的结构相符合，而电价和共价只有了解了物质的结构，才能确定。
在中学化学课本中，没有严格区分氧化数和化合价这两个概念，只是笼统地叫做化合价或正、负化合价。
参考文献：中小学资源网
In biology, one of the factors that defines a living thing from an inanimate object is the organism's ability to carry out chemical reactions that are crucial for its survival. Even one-celled organisms are capable of hundreds of chemical reactions within their cell walls. Imagine the infinite amount of reactions that a large organism such as a human carries out. None of these reactions are possible without enzymes.
Enzymes are biological catalysts or assistants. Enzymes consist of various types of proteins that work to drive the chemical reaction required for a specific action or nutrient. Enzymes can either launch a reaction or speed it up. The chemicals that are transformed with the help of enzymes are called substrates. In the absence of enzymes, these chemicals are called reactants.
To illustrate the speed and efficiency of enzymes, substrates can be transformed to usable products at the rate of ten times per second. Considering that there are an estimated 75,000 different enzymes in the human body, these chemical reactions are performed at an amazing rate. On the other hand, in the absence of enzymes, reactants may take hundreds of years to convert into a usable product, if they are able to do so at all. This is why enzymes are crucial in the sustenance of life on earth.
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Generally, enzymes work on substrates in one of three ways: substrate orientation, physical stress, and changes in substrate reactivity. Substrate orientation occurs when an enzyme causes substrate molecules to align with each other and form a bond. When an enzyme uses physical stress on a substrate, it in effect grips the substrate and forces the molecule to break apart. An enzyme that causes changes in substrate reactivity alters the placement of the molecule’s electrons, which influences the molecule’s ability to bond with other molecules.
Enzymes have active sites where they come into contact with particular substrates. The catalytic properties of enzymes are a cyclic process. Once a substrate has come into contact with the active site of an enzyme, it is modified by the enzyme to form the end product. Once the process is complete, the enzyme releases the product and is ready to begin the process with new substrates. Enzymes are never wasted and always recycled.
The absence of enzymes is responsible for many diseases. In humans, a tragic disease called phenylketonuria (PKU), which causes severe mental retardation and even death in infants, is the result of the absence of one type of enzyme. Tay-Sachs disease is a similarly tragic result of an enzyme deficiency. It causes retardation, paralysis, and often death in early childhood when left untreated.
Our ability to alter enzymes by inhibiting their functioning abilities has resulted in hundreds of life saving drugs. One example is penicillin, a well-known antibiotic that can cure syphilis, pneumonia, and other illnesses. Penicillin works by bonding to the active sites of the disease-causing bacteria’s enzymes, ultimately destroying the bacteria’s ability to survive and reproduce.
Enzymes have active sites where they come into contact with particular substrates. The catalytic properties of enzymes are a cyclic process. Once a substrate has come into contact with the active site of an enzyme, it is modified by the enzyme to form the end product. Once the process is complete, the enzyme releases the product and is ready to begin the process with new substrates. Enzymes are never wasted and always recycled.
The absence of enzymes is responsible for many diseases. In humans, a tragic disease called phenylketonuria (PKU), which causes severe mental retardation and even death in infants, is the result of the absence of one type of enzyme. Tay-Sachs disease is a similarly tragic result of an enzyme deficiency. It causes retardation, paralysis, and often death in early childhood when left untreated.
Our ability to alter enzymes by inhibiting their functioning abilities has resulted in hundreds of life saving drugs. One example is penicillin, a well-known antibiotic that can cure syphilis, pneumonia, and other illnesses. Penicillin works by bonding to the active sites of the disease-causing bacteria’s enzymes, ultimately destroying the bacteria’s ability to survive and reproduce.
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