共价键——定义、类型、性质、例子

当来自两个参与原子的电子均等共享时，就会形成共价键。参与这种键合的电子对称为共享对或键合对。分子键是共价键的另一个名称。键对的共享将确保原子在其外壳中实现稳定性，类似于惰性气体原子。

电离能极高的元素不能转移电子，电子亲和能极低的元素不能吸收电子。这些元素的原子倾向于与其他元素的原子或同一元素的原子共享电子，从而使两个原子在它们各自的价壳中实现八位组构型，从而实现稳定性。共价键是指通过在不同或相似种类之间共享电子对而形成的这种结合。

共价键的性质

如果原子之间共享单个电子对不满足原子的正常化合价，则原子之间可能共享多个电子对。共价键具有以下性质：

• 新电子的形成不是共价键合的结果。债券只连接它们。
• 它们是存在于原子之间的极强的化学键。
• 共价键形成后很少自行断裂。
• 共价键是定向的，因为键合的原子相对于彼此具有不同的方向。
• 大多数共价结合的化合物具有相对较低的熔点和沸点。
• 具有共价键的化合物通常具有较低的汽化焓和熔化焓。
• 由于缺乏自由电子，由共价键形成的化合物不导电。
• 共价化合物不溶于水。

Octet Rule

Except for noble gases, all atoms have fewer than eight electrons in their valence shell. In other words, these atoms’ valence shells do not have stable configurations. As a result, they combine with one another or with other atoms to form stable electronic configurations.

The tendency of atoms of different elements to achieve a stable configuration of eight electrons in their valence shells is the cause of chemical combination, and the principle of achieving a maximum of eight electrons in an atom’s valence shell is known as the octet rule.

共价键的类型

根据共享电子对的数量，共价键可分为单双键和三键共价键。

单一债券

When only one pair of electrons is shared by the two participating atoms, a single bond is formed. It is denoted by a single dash (-). Despite having a lower density and being weaker than a double or triple bond, this type of covalent bond is the most stable. 

例如，一个 HCl 分子包含一个具有 1 个价电子的氢原子和一个具有 7 个价电子的氯原子。在这种情况下，氢和氯通过共享一个电子形成单键。

双键

When two pairs of electrons are shared by two participating atoms, a double bond is formed. It’s denoted by two dashes (=). Although double covalent bonds are much stronger than single bonds, they are also less stable.

例如，在二氧化碳分子中，一个碳原子有六个价电子，两个氧原子有四个价电子。碳与一个氧原子共享其两个价电子，与另一个氧原子共享两个价电子以完成其八位字节。因为每个氧原子与碳共享两个电子，所以 CO2 包含两个双键。

氧分子：氧分子形成中的每个氧原子在其价壳中都有六个电子。为了完成它们的八位字节，每个原子还需要两个电子。结果，原子共享两个电子以形成氧分子。因为两个电子对是共享的，所以两个氧原子形成双键。

三重债券

When three pairs of electrons are shared by two participating atoms, a triple bond is formed. Triple covalent bonds are the least stable type of covalent bond and are represented by three dashes. 

例如，在氮分子的形成过程中，每个具有五个价电子的氮原子贡献三个电子，形成三个电子对以供共享。结果，在两个氮原子之间形成三键。

极性共价键

This type of covalent bond exists when the electronegativity of combining atoms differs, resulting in unequal electron sharing. Electrons will be drawn to more electronegative atoms. The atoms’ electronegative difference is greater than zero but less than 2. 

结果，共享电子对将更接近该原子。例如，考虑由于不平衡的静电势而形成氢键的分子。在这种情况下，氢原子与带负电的氟、氢或氧相互作用。

非极性共价键

This type of covalent bond is formed when two atoms share an equal number of electrons. The difference in electronegativity between two atoms is zero. 

每当结合的原子具有相似的电子亲和力时，就会发生这种情况。例如，非极性共价键可以在氢气、氮气等气体分子中找到。

Ionic键与共价键的比较

Covalent Bond	Ionic Bond
A covalent bond is formed when two similar electronegative nonmetals come together.	This type of bond is formed when a metal and a non-metal combine.
Bonds formed by covalent bonding have a distinct shape.	No definite shape
Low melting and boiling point	High melting and boiling point
low polarity	High Polarity
More flammable	Less flammable
They are in the liquid or gaseous state at room temperature	They are in solid-state
e.g. HCl, etc..	e.g. NaCl, etc.

Coordinated or Dative Covalent Bond: This type of bond occurs when one of the atoms in the bond provides electrons for sharing. This is accomplished through the reaction of ammonia and boron trifluoride. Nitrogen has two free electrons, whereas boron lacks electrons. They complete their last shell with eight electrons by combining nitrogen and boron.

示例问题

问题 1：原子为什么会发生反应以及如何反应？

回答：

Atoms with eight electrons in their final orbit are stable and do not react. Atoms with fewer than eight electrons react with other atoms to gain eight electrons in their outermost orbit and thus become stable. Atoms with slightly more than eight electrons may lose them to atoms with fewer than eight. Atoms that cannot lose or gain may share to obtain an octet configuration. Even after the reaction, molecules lacking an octet configuration may accept a lone pair of electrons from other atoms or molecules.

问题2：什么是杂化轨道？

回答：

Relatively similar energy sub-orbitals can merge to form a new set of the same number of orbitals, with the property of all contributing orbitals being proportional to their numbers. These orbitals are called hybridised orbitals.

问题3：什么是配位共价键？

回答：

This type of bond occurs when one of the atoms in the bond provides electrons for sharing. This is accomplished through the reaction of ammonia and boron trifluoride. Nitrogen has two free electrons, whereas boron lacks electrons. They complete their last shell with eight electrons by combining nitrogen and boron.

问题4：什么是键焓？

回答：

The chemical bonding between atoms causes energy to be released when they come close together. Bond dissociation enthalpy or Bond enthalpy is the amount of energy required to break one mole of a type of bond in order to separate the molecule into individual gaseous atoms.