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3.20: Covalent Bonding: Diatomic Molecules

  • Page ID
    213170
  • Learning Objectives
    • Distinguish between homonuclear and heteronuclear diatomic molecules.
    • Draw Lewis structures of homonuclear and heteronuclear diatomic molecules.
    • Write the chemical formulas and chemical names of homonuclear and heteronuclear diatomic molecules.

    The previous sections have presented and applied the process for drawing the Lewis structures of covalent molecules.  These two-dimensional pictures were then used as the basis for deriving the corresponding chemical formulas and chemical names for a variety of covalent molecules.  In every example that was presented, the number of unpaired electrons contained within both of the given elements was determined, in order to assign the relative placement of these elements within the final covalent molecule.  Specifically, the element with more unpaired electrons became the central atom, and the other element was used as a surrounding atom.  One surrounding atom was paired with each of the central atom's unpaired electrons, in order to create a shared pair of electrons.  Because the central atoms in all of the previous examples had more than one unpaired valence electron, multiple surrounding atoms were used in the corresponding pairing processes.

    However, some covalent molecules consist of only two atoms, in total, and, therefore, have no true central atom.  These diatomic molecules can be classified as either homonuclear, meaning that they contain two atoms of the same element, or heteronuclear, which requires that they be comprised of one atom of two different elements.  The following paragraphs will detail a modified process for drawing Lewis structures that can be applied to generate homonuclear and heteronuclear diatomic molecules.  As before, the resultant structures will be used as the basis for deriving the chemical formulas and chemical names of the corresponding covalent molecules.

    Drawing Lewis Structures of Diatomic Molecules

    A modified version of the procedure that was presented in Section 3.15 can be used to draw the Lewis structures of both homonuclear and heteronuclear diatomic molecules, as described below.

    For example, consider hydrogen and iodine.  

    Based on the combinations listed in Section 3.14, hydrogen and iodine, which are both non-metals, will combine to form a covalent molecule.  Since hydrogen is found in Group 1A of the periodic table, it contains 1 valence electron.  Iodine, which is located in Group 7A, has 7 valence electrons.  A chemically-correct electron dot structure for each of these elements is shown below.

    Hydrogen Iodide 1.png

    Based on the structures shown above, hydrogen has 1 unpaired electron, as does iodine.  Typically, the element with more unpaired electrons becomes the central atom in a Lewis structure, and the other element is used as the surrounding atom.  However, in the current example, hydrogen and iodine have the same number of unpaired electrons.  As a result, neither can be designated as the central atom.  Therefore, in order to satisfy the valences of each of these elements, hydrogen's unpaired electron must be paired with iodine's unpaired electron, in order to create a shared pair of electrons.  When executing this pairing step, the electron dot structure for one of the atoms should be rotated so that its unpaired electron aligns with the unpaired electron on the other atom.  Because the dots on an electron dot structure can be placed on any "side" of the elemental symbol, this rotation changes the orientation of the structure, but does not alter its meaning.  The structure that results upon correctly executing this pairing process is shown below.

    Hydrogen Iodide 2.png

    This structure contains one shared pair of electrons, which was created by pairing hydrogen's and iodine's unpaired electrons.  As this electron pair is located in between hydrogen's and iodine's electron dot structures, these electrons contribute to the overall electron configuration of both atoms.  The remaining electrons only impact the electron configuration of the atom on which they are drawn.

    Since all of the electrons shown above are paired, this structure represents the most stable bonding arrangement that can be achieved by combining hydrogen and iodine.  Note that by correctly executing the pairing process, iodine is surrounded by a total of eight, fully-paired dots.  However, as explained in Section 3.18, hydrogen is unable to achieve an octet configuration when bonding.  Instead, hydrogen is associated with two total electrons in the structure shown above and achieves a duet configuration, as expected.  This information is visually-highlighted in the structure shown below using a blue circle around hydrogen and a green box around iodine.

    Hydrogen Iodide 3.png

    Finally, in order to generate a structure that is more visually-appealing, the shared pair of electrons is replaced with a line that connects the adjacent elemental symbols.  The remaining electrons are redrawn as dots on the resultant structure, as shown below.

    Hydrogen Iodide 4.png

    In the structure that is generated upon the completion of this final step, the line represents a covalent bond, or a shared pair of electrons, and the remaining pairs of dots are called lone pairs.  The structure shown above, which is a chemically-correct representation of a covalent compound, is the Lewis structure that represents the molecule that is formed when hydrogen and iodine bond with one another.  Because this molecule only contains one atom of two different elements, it is classified as a heteronuclear diatomic molecule.

    Example \(\PageIndex{1}\)

    Draw the Lewis structure that represents the compound that is formed when two chlorine atoms bond with one another.

    Solution

    Based on the combinations listed in Section 3.14, two chlorine atoms will combine to form a covalent molecule, because chlorine is a non-metal.  Since chlorine is found in Group 7A of the periodic table, it contains 7 valence electrons.  Two chemically-correct electron dot structures for this element are shown below.

    Molecular Chlorine 1.png

    Based on the structures shown above, each chlorinatom has 1 unpaired electron.  Typically, the element with more unpaired electrons becomes the central atom in a Lewis structure, and the other element is used as the surrounding atom.  However, in the current example, each chlorinatom has the same number of unpaired electrons.  As a result, neither can be designated as the central atom.  Therefore, in order to satisfy the valences of each of these atoms, the unpaired electrons on each chlorine atom must be paired, in order to create a shared pair of electrons.  When executing this pairing step, the electron dot structure for one of the atoms should be rotated so that its unpaired electron aligns with the unpaired electron on the other atom.  Because the dots on an electron dot structure can be placed on any "side" of the elemental symbol, this rotation changes the orientation of the structure, but does not alter its meaning.  The structure that results upon correctly executing this pairing process is shown below.

    Molecular Chlorine 2.png

    This structure contains one shared pair of electrons, which was created by pairing the unpaired electrons on each chlorine atom.  As this electron pair is located in between both chlorine electron dot structures, these electrons contribute to the overall electron configuration of both atoms.  The remaining electrons only impact the electron configuration of the atom on which they are drawn.  By correctly executing the pairing process, each chlorine atom is surrounded by a total of eight, fully-paired dots.  This information is visually-highlighted in the structure shown below using a blue circle around one chlorine atom and a green box around the other chlorine atom.  As an octet configuration is the most stable electron arrangement that can be achieved by an atom, this structure represents the most stable bonding arrangement that can be achieved by combining two chlorine atoms.

    Molecular Chlorine 3.png

    Finally, in order to generate a structure that is more visually-appealing, the shared pair of electrons is replaced with a line that connects the adjacent elemental symbols.  The remaining electrons are redrawn as dots on the resultant structure, as shown below.

    Molecular Chlorine 4.png

    In the structure that is generated upon the completion of this final step, the line represents a covalent bond, or a shared pair of electrons, and the remaining pairs of dots are called lone pairs.  The structure shown above, which is a chemically-correct representation of a covalent compound, is the Lewis structure that represents the molecule that is formed when two chlorine atoms bond with one another.  Because this molecule only contains two atoms of the same element, it is classified as a homonuclear diatomic molecule.

    Writing Chemical Formulas of Diatomic Molecules

    For a covalent molecule, the information represented in its chemical formula must be a direct reflection of its Lewis structure.  Elemental symbols are incorporated into a chemical formula by counting the number of times that each symbol appears in the corresponding Lewis structure.  In order to ensure consistent formatting, the elemental symbol that appears fewer times is written first in a covalent chemical formula, and subscripts are used to indicate how many times each elemental symbol appears in the Lewis structure.  As indicated previously, values of "1" are usually implicitly-understood in chemistry and, therefore, should not be written in a chemical formula.  The subscripts must not be reduced to the lowest-common ratio of whole numbers, even if it is mathematically-possible to do so, as dividing the subscripts would cause their values to be inconsistent with the number of times that each elemental symbol appears in the Lewis structure.

    Heteronuclear Molecules

    The chemical formula of a heteronuclear diatomic molecule can be determined using a modified version of the rules presented above.  For example, consider the Lewis structure shown below, which represents the covalent molecule that is formed when hydrogen and iodine bond with one another.

    Hydrogen Iodide 4.png

    This Lewis structure contains one hydrogen atom and one iodine atom.  As stated above, the elemental symbol that appears fewer times is typically written first in a covalent chemical formula.  However, in the current example, the elements are present in equal quantities.  As a result, the order in which the elemental symbols are written in the corresponding chemical formula must be determined using a secondary rule:  The elemental symbol for the element that is farther away from fluorine on the periodic table is written first.  Therefore, the elemental symbol for hydrogen, "H," is written before iodine's elemental symbol, "I."  No subscript should be written on either elemental symbol, as values of "1" should not be explicitly-written in a chemical formula.  The resultant chemical formula, HI, accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule.

    Homonuclear Molecules

    The chemical formula of a homonuclear diatomic molecule can be determined using a modified version of the rules presented above.  For example, consider the Lewis structure shown below, which represents the covalent molecule that is formed when two chlorine atoms bond with one another.

    Molecular Chlorine 4 - Uncolored.png

    As only one type of element is present in this Lewis structure, only one elemental symbol, "Cl," is written in the corresponding chemical formula.  Furthermore, because this Lewis structure contains two chlorine atoms, a subscript of "2" should be written on chlorine's elemental symbol.  The resultant chemical formula, Cl2, accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule.

    Naming Diatomic Molecules

    For a covalent molecule, the information represented in its chemical name must also be a direct reflection of its Lewis structure.  Therefore, the chemical name of a covalent molecule must contain information that indicates the identities of its constituent elements and usually reflects how many of each of those elements are present within the molecule.  Note that, if written properly, the chemical formula for a covalent molecule also contains this information and, therefore, can be used as the basis for developing a chemical name.  Elemental names are incorporated into a covalent molecule's chemical name in the order in which their corresponding elemental symbols appear in the chemical formula.  The suffix on the second elemental term is replaced with "-ide," in order to indicate its secondary placement within the chemical formula.  Finally, since the subscripts in a covalent chemical formula are used to indicate how many times each elemental symbol appears in the molecule's Lewis structure, corresponding numerical prefixes are usually incorporated into the molecule's chemical name.  Remember that the prefix "mono-" is never used to change the first elemental term in a covalent chemical name and should only be used as a modifier on the remaining term if the secondary element is oxygen.  Finally, an "a" or "o" at the end of a prefix is usually dropped if the name of the element that is being altered begins with a vowel.

    Heteronuclear Molecules

    The chemical name of a heteronuclear diatomic molecule can be determined using the rules presented above.  For example, consider HI, the molecule that is formed when hydrogen and iodine bond with one another.

    Since the elemental symbol "H" appears first in the given chemical formula, "hydrogen" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "hydrogen."

    Because the elemental symbol "I" is written second in the given chemical formula, "iodide" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is only used as a modifier on the second term in a covalent chemical name if the secondary element is oxygen.  Because the secondary element is iodine, the "mono-" prefix is not applied.  Therefore, the second word in the chemical name of this molecule is "iodide."

    The result of combining these words, "hydrogen iodide," is the chemically-correct name for HI.

    Homonuclear Molecules

    Because a homonuclear diatomic molecule contains only a single element, the rules that are typically used for naming covalent molecules are not applicable.  Instead, the term "molecular" is written as the first word in the chemical name of a homonuclear diatomic molecule, as an indicator that only a single element is present.  The name of the element is written as the second word, and its suffix is not replaced with "-ide," because only a single elemental name is incorporated into the molecule's chemical name.  Finally, as a homonuclear diatomic molecule must contain two atoms of the same element, by definition, referencing a subscript of "2" within the chemical name of this type of molecule is considered redundant.  Therefore, a prefix of "di-" is not incorporated into the name of a homonuclear diatomic molecule.

    For example, consider Cl2, the molecule that is formed when two chlorine atoms bond with one another. 

    Because this molecule contains only a single element, the term "molecular" is written as the first word in its chemical name, and the name of the element, "chlorine," is written as the second word within the molecule's chemical name.  The suffix of this elemental name is not replaced with "-ide," and a prefix of "di-" is not applied.

    The result of combining these words, "molecular chlorine," is the chemically-correct name for Cl2.

    Exercise \(\PageIndex{1}\)

    Consider each of the following Lewis structures.

    Diatomic Exercise Structures.png

    For each,

    1. classify the molecule as either a homonuclear diatomic molecule or a heteronuclear diatomic molecule,
    2. write the chemical formula for the corresponding molecule, and
    3. write the chemical name for the corresponding molecule.
    Answer a
    Because this molecule contains two atoms of hydrogen, it is classified as a homonuclear diatomic molecule.

    As only one type of element is present in this Lewis structure, only one elemental symbol, "H," is written in the corresponding chemical formula.  Furthermore, because this Lewis structure contains two hydrogen atoms, a subscript of "2" should be written on hydrogen's elemental symbol.  The resultant chemical formula, H2, accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule.

    Finally, because this molecule contains only a single element, the rules that are typically used for naming covalent molecules are not applicable.  Instead, the term "molecular" is written as the first word in the chemical name of a homonuclear diatomic molecule, as an indicator that only a single element is present.  The name of the element is written as the second word, and its suffix is not replaced with "-ide," because only a single elemental name is incorporated into the molecule's chemical name.  Finally, as a homonuclear diatomic molecule must contain two atoms of the same element, by definition, referencing a subscript of "2" within the chemical name of this type of molecule is considered redundant.  Therefore, a prefix of "di-" is not incorporated into the name of a homonuclear diatomic molecule.  In this example, the term "molecular" is written as the first word in its chemical name, and the name of the element, "hydrogen," is written as the second word within the molecule's chemical name.  The suffix of this elemental name is not replaced with "-ide," and a prefix of "di-" is not applied.  The result of combining these words, "molecular hydrogen," is the chemically-correct name for H2.
    Answer b
    Because this molecule contains one atom of two different elements, it is classified as a heteronuclear diatomic molecule.

    Specifically, this Lewis structure contains one hydrogen atom and one bromine atom.  The elemental symbol that appears fewer times is typically written first in a covalent chemical formula.  However, in this example, the elements are present in equal quantities.  As a result, the order in which the elemental symbols are written in the corresponding chemical formula is determined using a secondary rule:  The elemental symbol for the element that is farther away from fluorine on the periodic table is written first.  Therefore, the elemental symbol for hydrogen, "H," is written before bromine's elemental symbol, "Br."  No subscript should be written on either elemental symbol, as values of "1" should not be explicitly-written in a chemical formula.  The resultant chemical formula, HBr, accurately summarizes the information in the Lewis structure shown above and, therefore, is the chemically-correct formula for this covalent molecule.

    Finally, since the elemental symbol "H" appears first in the given chemical formula, "hydrogen" is the basis of the first word in the molecule's chemical name.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is not used to alter the first elemental term in a covalent chemical name.  Therefore, the first word in the chemical name of this molecule is "hydrogen."  Because the elemental symbol "Br" is written second in the given chemical formula, "bromide" becomes the basis of the second word in the molecule's chemical name.  The suffix on this elemental term is "-ide," as an indicator of its secondary placement within the chemical formula.  The subscript on this elemental symbol, an unwritten "1," corresponds to prefix of "mono-."  However, this prefix is only used as a modifier on the second term in a covalent chemical name if the secondary element is oxygen.  Because the secondary element is bromine, the "mono-" prefix is not applied.  Therefore, the second word in the chemical name of this molecule is "bromide."  The result of combining these words, "hydrogen bromide," is the chemically-correct name for HBr.
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