Section outline

  • Colorful model of a water molecule showing red oxygen and blue hydrogen atoms.

    The colorful red and blue model in the opening image represents a water molecule . The molecule’s one oxygen is colored red, and its two hydrogen atoms are colored blue. Can you guess why? The red color represents negative electric charge , and the blue color represents positive electric charge. The colors show that water is a polar .
    ::开张图像中的多彩红色和蓝色模型代表了水分子。 分子的一个氧气是红色的,它的两个氢原子是蓝色的。 你能猜到原因吗? 红色代表负电荷,蓝色代表正电荷。 颜色显示水是极的。

    What Are Polar Compounds?
    ::什么是极地化合物?

    Polar compounds, such as water, are compounds that have a partial negative charge on one side of each molecule and a partial positive charge on the other side. All polar compounds contain polar bonds (although not all compounds that contain polar bonds are polar.) In a polar bond, two atoms share electrons unequally. One atom attracts the shared electrons more strongly, so it has a partial negative charge. The other atom attracts the shared electrons less strongly, so it is has a partial positive charge. In a water molecule, the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. This explains why the oxygen side of the water molecule has a partial negative charge and the hydrogen side of the molecule has a partial positive charge.
    ::极地化合物,例如水,是每个分子的一面有部分负电荷的化合物,而另一面则有部分正电荷的化合物。所有极地化合物都含有极性联结(尽管并非所有含有极性联结的化合物都是极性联结。)在极地联结中,两个原子不平均分享电子。一个原子吸引共享电子的强度更大,因此它有部分负电荷。另一个原子吸引共享电子的强度较小,因此它有部分正电荷。在水分子中,氧原子比氢原子更强烈地吸引共享电子。这解释了为什么水分子的氧面有部分负电荷,而分子的氢面则有部分正电荷。

    Q: If a molecule is polar, how might this affect its interactions with nearby molecules of the same compound?
    ::问题:如果分子是极性分子,这将如何影响分子与同一化合物附近分子的相互作用?

    A: Opposite charges on different molecules of the same compound might cause the molecules to be attracted to each other.
    ::A:同一化合物不同分子的对面电荷可能导致这些分子相互吸引。

    Hydrogen Bonding
    ::氢联结

    Because of water’s , individual water molecules are attracted to one another. You can see this in the Figure . The positively charged hydrogen side of one water molecule is attracted to the negatively charged oxygen side of a nearby water molecule. This force of attraction is called a hydrogen bond . 
    ::由于水的缘故,单个水分子相互吸引。您可以从图中看到这一点。一个水分子的正充电氢面被附近的水分子的负充氧氧面吸引。这种吸引力被称为氢联结。

    Hydrogen bonds between water molecules with labeled hydrogen and oxygen atoms.

    Hydrogen bonds are intermolecular forces (“between-molecule”), rather than intramolecular (“within-molecule”) forces . They occur not only  between water molecules, but between any polar molecules containing an  H−N, H−O, or H−F  bond. The  positive hydrogen atoms are attracted to negative atoms (nitrogen, hydrogen, or fluorine) in nearby molecules.  These bonds are extremely polar because of the high electronegativity difference between the atoms. This strong polarity causes very strong dipole-dipole interactions between molecules, called .  Hydrogen bonds are weaker than . For example, they are much weaker than the holding atoms together within molecules of covalent compounds. 
    ::氢离子是分子的内在力量(“分子之间”),而不是分子内部(“分子内部”)的力量,它们不仅发生在水分子之间,而且发生在含有H-N、H-O或H-F联结的任何极分子之间。正氢原子被附近分子中的负原子(氮、氢或氟)吸引。这些联结极极极之大,因为原子之间的电子密度差异很大。这种极分性导致分子(称为.氢联结)之间非常强烈的dipoole-二极相互作用。例如,它们比共价化合物分子内积聚的原子要弱得多。

    Hydrogen Bonds and Changes of State
    ::氢债券和国家变化

    from to and from liquid to occur when matter gains . The energy allows individual molecules to separate and move apart from one another. It takes more energy to bring about these changes of state for polar molecules. Although hydrogen bonds are weak, they add to the energy needed for molecules to move apart from one another, so it takes higher temperatures for these changes of state to occur in polar compounds. This explains why polar compounds have relatively high and points. The Table compares melting and boiling points for some polar and nonpolar covalent compounds.
    ::当物质增加时, 就会从液体中产生。 能量可以让个别分子分离和分离。 需要更多能量来改变极分子的状态。 虽然氢的联结很弱, 它们增加了分子分离所需的能量, 因此极地化合物中发生这种状态变化需要更高的温度。 这解释了极地化合物相对高和点的原因。 表格比较了某些极地和非极地共价化合物的熔点和沸点。

    Name of Compound (Chemical Formula) Polar or Nonpolar? Melting Point(°C) Boiling Point (°C)
    Methane (CH 4 ) nonpolar -182 -162
    Ethylene (C 2 H 2 ) nonpolar -169 -104
    Ammonia (NH 3 ) polar -78 -33
    Water (H 2 O) polar 0 100

    Q: Which compound in the Table do you think is more polar, ammonia or water?
    ::问题:你认为表中哪一种化合物更极、氨或水?

    A: Water is more polar than ammonia. Its strong polarity explains why its melting and boiling points are high even for a polar covalent compound .
    ::甲:水比氨更极极。它的强烈极性解释了为什么它的熔点和沸点都很高,即使是极地共价化合物也是如此。

    Summary
    ::摘要

    • Polar covalent compounds have molecules with a partial negative charge on one side and a partial positive charge on the other side. This occurs because the compounds contain polar bonds. In a polar bond, one atom attracts the shared electrons more strongly than the other atom does.
      ::极共价化合物的分子在一边有部分负电荷,在另一边则有部分正电荷。这是因为该化合物含有极性联结。在极性联结中,一个原子比另一个原子更能吸引共享电子。
    • In some polar molecules that contain hydrogen atoms, the partial positive charge of the hydrogen atoms of one molecule are attracted to the partial negative charge of an atom of a nearby molecule. This force of attraction is called a hydrogen bond.
      ::在一些含有氢原子的极分子中,一个分子的氢原子的部分正电荷被附近一个分子原子的部分负电荷所吸引。这种吸引力被称为氢联结。
    • Hydrogen bonds are relatively weak, but they add to the energy needed for molecules to move apart from each other when matter changes state from a solid to a liquid or from a liquid to a gas. This explains why polar covalent compounds have relatively high melting and boiling points.
      ::氢键相对薄弱,但当物质从固体变成液体或从液体变成气体时,分子分离所需的能量会增加。 这就解释了极地共价化合物熔点和沸点相对较高的原因。

    Review
    ::回顾

    1. What are polar covalent compounds?
      ::什么是极性共价化合物?
    2. Define hydrogen bond.
      ::界定氢联结。
    3. Explain why hydrogen bonds increase melting and boiling points of polar covalent compounds such as water.
      ::解释为什么氢联结会增加水等极地共价化合物的熔点和沸点。

    Explore More
    ::探索更多

    Watch the video about hydrogen bonds and then answer the questions below.
    ::观看关于氢债券的视频,然后回答下面的问题。

     

    1. What is electronegativity?
      ::什么是电子能力?
    2. In water molecules, why do shared electrons spend more time orbiting the oxygen atom than the hydrogen atoms?
      ::在水分子中,为什么共享电子在氧原子轨道上花费的时间比氢原子多?
    3. Hydrogen forms hydrogen bonds with oxygen. What is another element besides oxygen that may be involved in hydrogen bonds?
      ::除了氧气之外,还有哪些元素可能与氢气结合有关?
    4. Water has the properties of cohesion and adhesion. Define these two properties, and explain why they occur in water.
      ::水具有凝聚和粘合的特性。 定义这两个特性, 并解释为什么它们出现在水中。
    5. Why are hydrogen bonds extremely important in biology?
      ::为什么氢联结在生物学中极为重要?