Name: shared electrons in covalent molecules
Uploaded: 2020-09-03T13:38:28.000Z
Duration: 2 min 46 s
Channel: JoVE

K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.1

Covalent Bonding and Lewis Structures

共价键是非金属元素之间 通过价电子的共享而形成的。但是为什么非金属元素喜欢共享电子 而不是像离子键那样转移它们呢？因为非金属元素具有很高的电离能 所以很难把价电子从一个原子 转移到另一个原子。以一个氨分子为例。氮原子需要另外三个电子 才能达到八位态，而氢原子需要一个电子 才能达到二位态。因此，一个氮原子 与三个氢原子结合，这样就能使氮和氢 都达到稳定的电子构型。由于每一个键共用一对电子，所以称之为单键。共价键中的共享电子对 被称为键对。不参与成键的价电子 称为孤对或非键电子。氧原子有 6 个价电子，因此还需要两个电子才能达到八位态。因此，两个氧原子共用两个电子对 形成双键。另一方面，氮气以 双原子的形式共享三个未配对的电子，形成了一个三键。单键和多键在键长和 强度方面是不同的。三键比双键短，而双键 又比单键短。键的强度随键的多重性而增加。这就是为什么在氮气中很难打破三键，使它相对不活跃。路易斯模型有助于预测分子的结构 和稳定性。根据路易斯模型，H2O 是一种稳定的分子；因为氧和氢都获得了稳定的电子 构型。如果氧只与一个氢原子共享电子，则生成的 OH 分子是不稳定的，因为氧只有 7 个 价电子，不能达到八位态。然而，如果在氧中加入一个额外的电子 来完成八位态，所产生的氢氧化物离子 会带着一个负电荷变得稳定。共价键是定向的，因为共享电子 连接两个特定的原子对。相反，离子键是无方向性的 并且会在晶格中留住几个离子。因此，在共价化合物中，单个分子 被认为是基本单位。

共价键和路易斯结构

与离子键相比，离子键是由金属原子与非金属原子之间的电子转移引起的，而共价键则是由于原子相互吸引而形成“共享”原子。对电子。
 
 当两个原子具有相似的吸引电子的倾向时（即，当两个原子具有相同或相当相似的电离能和电子亲和力时），在两个原子之间形成共价键。
 
 <h4>共价化合物的物理性质</ h4>
 
 含有共价键的化合物表现出与离子化合物不同的物理性质。因为电中性的分子之间的吸引力弱于带电离子之间的吸引力，所以共价化合物的熔点和沸点通常比离子化合物低得多。实际上，许多共价化合物在室温下为液体或气体，在固态下，它们通常比离子型固体软得多。此外，尽管离子化合物溶于水时是良好的电导体，但大多数共价化合物不溶于水；由于它们是电中性的，因此在任何状态下都是不良的导体。
 
 <h4>共价键的形成</ h4>
 
 非金属原子经常与其他非金属原子形成共价键。例如，氢分子H 2 在其两个氢原子之间包含一个共价键。两个具有特定势能的氢原子彼此接近，它们的价轨道（1 s ）开始重叠。然后，每个氢原子上的单电子与两个原子核相互作用，占据两个原子周围的空间。每个共享电子对两个原子核的强烈吸引使系统稳定，并且随着键合距离的减小，势能减小。如果原子继续彼此接近，则两个原子核中的正电荷开始相互排斥，势能增加。键的长度取决于获得最低势能的距离。
 
 必须记住，必须添加能量才能破坏化学键（吸热过程），而形成化学键则释放能量（放热过程）。在H 2 的情况下，共价键非常牢固；必须添加大量能量，即436 kJ，才能破坏一摩尔氢分子中的键并导致原子分离：
 
 <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1059/1059_Equations-01.png" style="height: 40px;" />
 
 相反，当一摩尔H 2 分子由两摩尔H原子形成时，会释放出相同量的能量：
 
 <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1059/1059_Equations-02.png" style="height: 40px;" />
 
 <h4>路易斯结构</ h4>
 
 路易斯符号可用于指示共价键的形成，这些键在路易斯结构中显示，该图描述了分子和多原子离子之间的键合。例如，当两个氯原子形成一个氯分子时，它们共享一对电子：
 
 <img alt="Figure1" src="https://www.jove.com/files/core/CDNSource/lessons/1059/1059Image_1.svg" style="height: 90px;" />
 
 路易斯结构表明，每个Cl原子都有三对未用于键合的电子（称为孤对）和一个共享的电子对（写在原子之间）。有时会用破折号（或线）表示一对共享的电子：＆nbsp; Cl＆mdash; Cl。
 
 <ul>
 单个共享的电子对称为单键。每个Cl原子与八个价电子相互作用：孤对中的六个电子和单键中的两个电子。</ li>
 但是，一对原子可能需要共享不止一对电子才能达到必需的八偶体。当一对原子之间共享两个电子对时，例如CH 2 O（甲醛）中的碳原子和氧原子之间以及C 2中的两个碳原子之间，会形成双键 H 4 （乙烯）。 
 <img alt="Figure2" src="https://www.jove.com/files/core/CDNSource/lessons/1059/1059Image_2.svg" style="height: 120px;" /></li>
 <li>当三个电子对被一对原子共享时，形成三键，如一氧化碳（CO）和氰化物离子（CN）。 
 <img alt="Figure3" src="https://www.jove.com/files/core/CDNSource/lessons/1059/1059Image_3.svg" style="height: 90px;" /></li>
 </ul>
 
 元素周期表可用于预测原子中的价电子数量以及将形成的一个八偶体的键数。 18族元素（如氩气和氦气）具有电子结构，因此很少参与化学键合。但是，来自第17组的原子（例如溴或碘）仅需要一个电子即可到达八偶体。因此，属于第17族的原子可以形成单个共价键。第16组原子需要2个电子才能达到一个八偶体；因此它们可以形成两个共价键。同样，属于第14组的碳还需要4个电子才能到达一个八偶体；因此碳可以形成四个共价键。
 
 本文改编自<a href="https://openstax.org/books/chemistry-2e/pages/7-2-covalent-bonding"> Openstax，化学2e，第7.2节：共价键</a>和<a href="https://openstax.org/books/chemistry-2e/pages/7-3-lewis-symbols-and-structures"> Openstax，化学2e，第7.3节：路易斯符号和结构。</a>

与金属原子和非金属原子之间因电子转移形成的离子键相比，共价键是由原子间共享电子对而形成的……

Education

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JoVE Core

化学

Chemical Bonding: Basic Concepts

化学键：基本概念

KEY TERMS AND CONCEPTS

关键术语与概念

shared electrons in covalent molecules

Covalent molecules form when atoms share electrons instead of transferring them. This bonding usually happens between nonmetal atoms with similar tendencies to attract electrons. Because the atoms pull on the electrons in a shared way, the bond is held together by mutual attraction.

Covalent compounds often have physical properties that differ from ionic compounds. The attraction between neutral molecules is weaker than the attraction between charged ions. As a result, many covalent compounds have lower melting and boiling points, and some are liquids or gases at room temperature. When they are solids, they are usually softer than ionic solids. Most covalent compounds are also insoluble in water and are poor conductors of electricity in any state.

Hydrogen gas shows how a covalent bond forms. In H2, two hydrogen atoms approach each other and their 1s valence orbitals overlap. Each electron interacts with both nuclei, which lowers the potential energy and stabilizes the molecule. If the atoms move too close, the positive nuclei repel each other, and the potential energy rises. The bond length is the distance where the potential energy is lowest. Forming a bond releases energy, so it is an exothermic process. Breaking a bond requires energy, so it is endothermic. For H2, 436 kJ must be added to break one mole of molecules, and the same amount is released when one mole forms from hydrogen atoms.

Lewis structures show covalent bonding with Lewis symbols. In chlorine gas, Cl2, the two chlorine atoms share one pair of electrons. Each chlorine atom also has three lone pairs, which are pairs not used in bonding. A shared pair can be shown with a dash, so Cl—Cl represents a single bond. Each chlorine atom then has eight valence electrons around it, including lone pairs and the bonding pair.

Some atoms need more than one shared pair to reach an octet, which is a full outer shell of eight electrons. A double bond forms when two pairs of electrons are shared, as in CH2O and C2H4. A triple bond forms when three pairs are shared, as in CO and CN−. The periodic table helps predict bonding patterns from valence electrons. Group 18 elements such as argon and helium already have filled electron configurations and rarely bond. Group 17 atoms, such as bromine and iodine, usually need one electron and can form one covalent bond. Group 16 atoms need two electrons and can form two covalent bonds. Carbon in group 14 needs four more electrons, so it can form four covalent bonds.

Shared Electrons in Covalent Molecules

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covalent bonding and lewis structures

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lewis structures of molecular compounds and polyatomic ions

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Shared Electrons in Covalent Molecules | JoVE K12

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JoVE Core 化学通过简单明了且通俗易懂的动画视频课程，将普通化学概念带入到现实生活当中。此外，在科学家行动视频中，我们演示了在实验室中进行的经典和原始研究实验。

Physical Science

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Structure and Properties of Matter

3.2.9-12.B

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Study Of Matter

Bonding and compounds

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Chemical bonding

Representing compounds

Physical Geology

Minerals

Chemical bonding (ionic, covalent, metallic)

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High School

Matter and Its Interactions

CHEM1.PS1.12

CHEM1.PS1.14

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CHEM1.PS2.1

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PH.9.e

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9-12.C.2.2.a

9-12.C.2.2.c

Florida Department of Education

SC.912.P.8

SC.912.P.8.6

Chemistry and Physics

IPC.7

IPC.7.A

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New York State Science Learning Standards

HS. Structure and Properties of Matter

HS-PS1-3.

HS. Chemical Reactions

HS-PS1-2.

Georgia Standards of Excellence

SPS2.

Chemical Bonds

SC2.

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Bonding Types: From Nonpolar Covalent to Ionic

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Nuclear Binding Energy

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Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

Nuclear Power

Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

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Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

PS.Chm.1.2

Atomic Orbitals

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The Pauli Exclusion Principle

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The Energies of Atomic Orbitals

In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are ...

The Aufbau Principle and Hund's Rule

To determine the electron configuration for any particular atom, we can build the structures in the order of atomic numbers. Beginning with hydrogen, and ...

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Electron Configuration of Multielectron Atoms

The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

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PS.Chm.1.3

Periodic Classification of the Elements

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PS.Chm.2.1

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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

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Analyze the structure of atoms and isotopes.

Shared Electrons in Covalent Molecules

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