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Biology LibreTexts

3.7: Chemical Bonding

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    Figure 1

    Chemical Bonding

    When you think of bonding, you may not think of ions. Like most of us, you probably think of bonding between people. Like people, molecules bond — and some bonds are stronger than others. It's hard to break up a mother and baby, or a molecule made up of one oxygen and two hydrogen atoms!

    chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons. Valence electrons are the electrons in the outer energy level of an atom that may be involved in chemical interactions. Valence electrons are the basis of all chemical bonds. 

    Why Bonds Form

    To understand why chemical bonds form, consider the common compound known as water, or H2O. It consists of two hydrogen (H) atoms and one oxygen (O) atom. As you can see in the on the left side of the Figure below, each hydrogen atomhas just one electron, which is also its sole valence electron. The oxygen atom has six valence electrons. These are the electrons in the outer energy level of the oxygen atom.



    Figure 2

    Bonds between hydrogen and oxygen atoms in a water molecule


    In the water molecule on the right in the Figure above, each hydrogen atom shares a pair of electrons with the oxygen atom. By sharing electrons, each atom has electrons available to fill its sole or outer energy level. The hydrogen atoms each have a pair of shared electrons, so their first and only energy level is full. The oxygen atom has a total of eight valence electrons, so its outer energy level is full. A full outer energy level is the most stable possible arrangement of electrons. It explains why elements form chemical bonds with each other.

    Types of Chemical Bonds

    Not all chemical bonds form in the same way as the bonds in water. There are actually three different types of chemical bonds that we will discuss here are polar covalent , hydrogen , non-polar covalent, and ionic bonding.  Each type of bond is described below. 

    • Polar Covalent bond: A covalent bond is the force of attraction that holds together two nonmetal atoms that share a pair of electrons. One electron is provided by each atom, and the pair of electrons is attracted to the positive nuclei of both atoms. The water molecule represented in the Figure above contains polar covalent bonds.

      The attractive force between water molecules is a dipole interaction.  The hydrogen atoms are bound to the highly electronegative oxygen atom (which also possesses two lone pair sets of electrons, making for a very polar bond.  The partially positive hydrogen atom of one molecule is then attracted to the oxygen atom of a nearby water molecule (see Figure below).



      Figure 3

      A hydrogen bond in water occurs between the hydrogen atom of one water molecule and the lone pair of electrons on an oxygen atom of a neighboring water molecule.

      hydrogen bond is an intermolecular attractive force in which a hydrogen atom that is covalently bonded to a small, highly electronegative atom is attracted to a lone pair of electrons on an atom in a neighboring molecule. Hydrogen bonds are very strong compared to other dipole interactions.  The strength of a typical hydrogen bond is about 5% of that of a covalent bond.

      Hydrogen bonding occurs only in molecules where hydrogen is covalently bonded to one of three elements: fluorine, oxygen, or nitrogen.  These three elements are so electronegative that they withdraw the majority of the electrondensity in the covalent bond with hydrogen, leaving the H atom very electron-deficient.  The H atom nearly acts as a bare proton, leaving it very attracted to lone pair electrons on a nearby atom.

      The hydrogen bonding that occurs in water leads to some unusual, but very important properties.  Most molecular compounds that have a mass similar to water are gases at room temperature.  Because of the strong hydrogen bonds, water molecules are able to stay condensed in the liquidstate.  Figure below shows how the bent shape and two hydrogen atoms per molecule allows each water molecule to be able to hydrogen bond to two other molecules.



      Figure 4

      Multiple hydrogen bonds occur simultaneously in water because of its bent shape and the presence of two hydrogen atoms per molecule.

      In the liquid state, the hydrogen bonds of water can break and reform as the molecules flow from one place to another.  When water is cooled, the molecules begin to slow down.  Eventually, when water is frozen to ice, the hydrogen bonds become permanent and form a very specific network (see Figure below).



      Figure 5

      When water freezes to ice, the hydrogen bonding network becomes permanent. Each oxygen atom has an approximately tetrahedral geometry – two covalent bonds and two hydrogen bonds.

      The bent shape of the molecules leads to gaps in the hydrogen bondingnetwork of ice.  Ice has the very unusual property that its solid state is less dense than its liquid state.  Ice floats in liquid water.  Virtually all other substances are denser in the solid state than in the liquid state.  Hydrogen bonds play a very important biological role in the physical structures of proteinsand nucleic acids.


    • Non-polar Covalent bond: In the picture of methane (CH4) below (Figurebelow), the carbon atom (with four electrons in its outermost valence energyshell) shares a single electron from each of the the four hydrogens. Hydrogen has one valence electron its first energy shell. Covalent bonding is prevalent in organic compounds. In fact, your body is held together by electrons shared by carbons and hydrogens! Covalent bonds are also very strong, meaning it takes a lot of energy to break them apart. The electrons are equally shared in all directions; therefore, this type of covalent bond in referred to as non-polar.


    Figure 6

    Methane is formed when four hydrogens and one carbon covalently bond.

    • Ionic bond: Electrons are transferred between atoms. An ion will give one or more electrons to another ion. Table salt, sodium chloride (NaCl), is a common example of an ionic compound. Note that sodium is on the left side of the periodic table and that chlorine is on the right side of the periodic table. In the Figure below, an atom of lithium donates an electron to an atom of fluorine to form an ionic compound. This happens to full fill their outermost valence shell. The transfer of the electron gives the lithium ion a net charge of +1, and the fluorine ion a net charge of -1. These ions bond because they experience an attractive force due to the difference in sign of their charges.


    Figure 7

    Lithium (left) and fluorine (right) form an ionic compound called lithium fluoride.



    • In an ionic bond, an atom gives away one or more electrons to another atom.
    • In a covalent bond, two atoms share one or more electrons.
    • A hydrogen bond is a relatively weak bond between two oppositely charged sides of two or more molecules. Water is a polar molecule.
    • chemical bond is a force of attraction between atoms or ions. Bonds form when atoms share or transfer valence electrons.
    • Atoms form chemical bonds to achieve a full outer energy level, which is the most stable arrangement of electrons.
    • There are three different types of chemical bonds: polar covalent, non-polar covalent, ionic, and hydrogen bonds. 



    1. How is a covalent bond different from an ionic bond?
    2. Why is a hydrogen bond a relatively weak bond?
    3. Diagram the polarity of a water molecule
    4. What is a chemical bond?
    5. Explain why hydrogen and oxygen atoms are more stable when they form bonds in a water molecule.
    6. How do ionic bonds and covalent bonds differ?
    7. What is ionic bonding?
    8. How many valence electrons does sodium have? How many valence electrons does chlorine have?
    9. How does chlorine bond with sodium?
    10. What is the charge on a sodium ion? What about the chlorine ion?
    11. When does covalent bonding occur? How does it work?
    12. How many valence electrons does oxygen have?
    13. Why do oxygen and hydrogen bond so well?


    Explore More

    Use this resource to answer the questions that follow.

    1. What is ionic bonding?
    2. How many valence electrons does sodium have? How many valence electrons does chlorine have?
    3. How does chlorine bond with sodium?
    4. What is the charge on a sodium ion? What about the chlorine ion?
    5. When does covalent bonding occur? How does it work?
    6. How many valence electrons does oxygen have?
    7. Why do oxygen and hydrogen bond so well?


    Image Attributions

    [Figure 1] 
    Credit: Thomas; 
    License: CC BY-NC 3.0

    [Figure 2] 
    Credit: Christopher Auyeung; 
    Source: CK-12 Foundation

    [Figure 3] 
    Credit: Ben Mills (Wikimedia: Benjah-bmm27); 
    License: CC BY-NC 3.0

    [Figure 4] 
    Credit: Laura Guerin; 
    Source: CK-12 Foundation
    License: CC BY-NC 3.0

    [Figure 5] 
    Credit: User:Materialscientist/Wikimedia Commons; 
    License: CC BY-NC 3.0

    [Figure 6] 
    Credit: Laura Guerin; 
    Source: CK-12 Foundation

    [Figure 7] 
    Credit: Laura Guerin; 
    Source: CK-12 Foundation
    License: CC BY-NC 3.0