5.8: Naming Molecular Compounds (2024)

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    Learning Objectives
    • Determine the name of a simple molecular compound from its chemical formula.

    Molecular Compounds

    Molecular compounds are inorganic compounds that take the form of discrete molecules. Examples include such familiar substances as water \(\left( \ce{H_2O} \right)\) and carbon dioxide \(\left( \ce{CO_2} \right)\). These compounds are very different from ionic compounds like sodium chloride \(\left( \ce{NaCl} \right)\). Ionic compounds are formed when metal atoms lose one or more of their electrons to nonmetal atoms. The resulting cations and anions are electrostatically attracted to each other.

    So what holds the atoms of a molecule together? Rather than forming ions, the atoms of a molecule share their electrons in such a way that a bond forms between a pair of atoms. In a carbon dioxide molecule, there are two of these bonds, each occurring between the carbon atom and one of the two oxygen atoms.

    5.8: Naming Molecular Compounds (1)

    Larger molecules can have many, many bonds that serve to keep the molecule together. In a large sample of a given molecular compound, all of the individual molecules are identical.

    Naming Binary Molecular Compounds

    Recall that a molecular formula shows the number of atoms of each element that a molecule contains. A molecule of water contains two hydrogen atoms and one oxygen atom, so its formula is \(\ce{H_2O}\). A molecule of octane, which is a component of gasoline, contains 8 atoms of carbon and 18 atoms of hydrogen. The molecular formula of octane is \(\ce{C_8H_{18}}\).

    5.8: Naming Molecular Compounds (2)

    Naming binary (two-element) molecular compounds is similar to naming simple ionic compounds. The first element in the formula is simply listed using the name of the element. The second element is named by taking the stem of the element name and adding the suffix -ide. A system of numerical prefixes is used to specify the number of atoms in a molecule. Table \(\PageIndex{1}\) lists these numerical prefixes.

    Table \(\PageIndex{1}\): Numerical Prefixes for Naming Binary Covalent Compounds
    Number of Atoms in Compound Prefix on the Name of the Element
    1 mono-*
    2 di-
    3 tri-
    4 tetra-
    5 penta-
    6 hexa-
    7 hepta-
    8 octa-
    9 nona-
    10 deca-

    *This prefix is not used for the first element’s name.

    5.8: Naming Molecular Compounds (3)

    Note
    • Generally, the less electronegative element is written first in the formula, though there are a few exceptions. Carbon is always first in a formula and hydrogen is after nitrogen in a formula such as \(\ce{NH_3}\). The order of common nonmetals in binary compound formulas is \(\ce{C}\), \(\ce{P}\), \(\ce{N}\), \(\ce{H}\), \(\ce{S}\), \(\ce{I}\), \(\ce{Br}\), \(\ce{Cl}\), \(\ce{O}\), \(\ce{F}\).
    • The a or o at the end of a prefix is usually dropped from the name when the name of the element begins with a vowel. As an example, four oxygen atoms, is tetroxide instead of tetraoxide.
    • The prefix is "mono" is not added to the first element’s name if there is only one atom of the first element in a molecule.

    Some examples of molecular compounds are listed in Table \(\PageIndex{2}\).

    Table \(\PageIndex{2}\)
    Formula Name
    \(\ce{NO}\) nitrogen monoxide
    \(\ce{N_2O}\) dinitrogen monoxide
    \(\ce{S_2Cl_2}\) disulfur dichloride
    \(\ce{Cl_2O_7}\) dichlorine heptoxide

    Notice that the mono- prefix is not used with the nitrogen in the first compound, but is used with the oxygen in both of the first two examples. The \(\ce{S_2Cl_2}\) emphasizes that the formulas for molecular compounds are not reduced to their lowest ratios. The o of the mono- and the a of hepta- are dropped from the name when paired with oxide.

    Exercise \(\PageIndex{1}\)

    Write the name for each compound.

    1. CF4
    2. SeCl2
    3. SO3
    Answer a:
    carbon tetrafluoride
    Answer b:
    selenium dichloride
    Answer c:
    sulfur trioxide

    Simple molecular compounds with common names

    For some simple covalent compounds, we use common names rather than systematic names. We have already encountered these compounds, but we list them here explicitly:

    • H2O: water
    • NH3: ammonia
    • CH4: methane
    • H2O2: hydrogen peroxide

    Methane is the simplest organic compound. Organic compounds are compounds with carbon atoms and are named by a separate nomenclature system.

    Some Compounds Have Both Covalent and Ionic Bonds

    If you recall the introduction of polyatomic ions, you will remember that the bonds that hold the polyatomic ions together are covalent bonds. Once the polyatomic ion is constructed with covalent bonds, it reacts with other substances as an ion. The bond between a polyatomic ion and another ion will be ionic. An example of this type of situation is in the compound sodium nitrate. Sodium nitrate is composed of a sodium ion and a nitrate ion. The nitrate ion is held together by covalent bonds and the nitrate ion is attached to the sodium ion by an ionic bond.

    Summary

    • A molecular compound is usually composed of two or more nonmetal elements.
    • Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix -ide. Numerical prefixes are used to specify the number of atoms in a molecule.
    5.8: Naming Molecular Compounds (2024)

    FAQs

    What is the rule for naming molecular compounds? ›

    Summary
    • A molecular compound is usually composed of two or more nonmetal elements.
    • Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix -ide. Numerical prefixes are used to specify the number of atoms in a molecule.
    Aug 2, 2022

    What is 5 in naming compounds? ›

    Rules for Naming Molecular Compounds:

    “Mono-” indicates one, “di-” indicates two, “tri-” is three, “tetra-” is four, “penta-” is five, “hexa-” is six, “hepta-” is seven, “octo-” is eight, “nona-” is nine, and “deca” is ten.

    What is a molecular compound that starts with CO? ›

    carbon monoxide

    Do the compounds combine metals with metals, metals with nonmetals, or nonmetals with nonmetals? ›

    Metals often react with nonmetals to form ionic compounds. These compounds are composed of positive and negative ions formed by adding or subtracting electrons from neutral atoms and molecules. Nonmetals combine with each other to form covalent compounds, which exist as neutral molecules.

    What are the three rules for naming compounds? ›

    Rules for Naming Molecular Compounds

    Write the name for both elements. Change the ending of the second element to ide. Place prefixes in front of each element based on the number of atoms present.

    What is the correct order of naming compounds? ›

    Lesson Summary
    1. Locate the longest carbon chain in our compound.
    2. Name that parent chain (find the root word)
    3. Figure out the ending.
    4. Number your carbon atoms.
    5. Name the side groups.
    6. Put the side groups in alphabetical order.

    How to easily name compounds? ›

    How to Name Compounds
    1. Determine whether the compound is ionic or molecular. ...
    2. Determine how many atoms of each element are in the compound. ...
    3. Add ''-ide'' or ''-ate'' suffix to the element names. ...
    4. Determine whether or not oxygen is an element in the compound. ...
    5. Determine if Roman numerals should be added.
    Sep 27, 2022

    How to solve nomenclature? ›

    IUPAC Rules for Alkane Nomenclature
    1. Find and name the longest continuous carbon chain.
    2. Identify and name groups attached to this chain.
    3. Number the chain consecutively, starting at the end nearest a substituent group.
    4. Designate the location of each substituent group by an appropriate number and name.

    What is 7 when naming compounds? ›

    Naming Binary Covalent Compounds
    1 - mono2 - di3 - tri
    6 - hexa7 - hepta8 - octa

    What are 5 examples of molecular compounds? ›

    3. Examples of Molecular Compounds
    • A few examples of molecular compounds include:
    • Water (H2O)
    • Carbon dioxide (CO2)
    • Methane (CH4)
    • Carbon monoxide (CO)
    • Ammonia (NH3)
    • Oxygen (O2)
    • Nitrogen (N2)
    Feb 15, 2023

    How to identify a molecular compound? ›

    As a general rule of thumb, compounds that involve a metal binding with either a non-metal or a semi-metal will display ionic bonding. Compounds that are composed of only non-metals or semi-metals with non-metals will display covalent bonding and will be classified as molecular compounds.

    What is the simplest way to tell if something is an element? ›

    An element is a substance that cannot be broken down into a simpler format. They are distinguished by a unique atomic number.

    What are the simplest chemical substances composed of only? ›

    Elements are the simplest complete chemical substances. Each element corresponds to a single entry on the periodic table. An element is a material that consists of a single type of atom. Each atom type contains the same number of protons.

    What are two metals in a compound called? ›

    Alloys can be either solid solutions of two or more metals or mixtures of metals.

    How to tell if a compound is ionic or covalent? ›

    Classifying compounds as ionic or covalent
    1. If a compound is made from a metal and a non-metal, its bonding will be ionic.
    2. If a compound is made from two non-metals, its bonding will be covalent.

    What are the rules for naming a molecular covalent compound? ›

    Rules for naming simple covalent compounds:
    • Name the non-metal furthest to the left on the periodic table by its elemental name.
    • Name the other non-metal by its elemental name and an -ide ending.
    • Use the prefixes mono-, di-, tri-.... to indicate the number of that element in the molecule.

    What is the rule of molecular formula? ›

    A molecular formula tells us which atoms and how many of each type of atom are present in a molecule. If only one atom of a specific type is present, no subscript is used. For atoms that have two or more of a specific type of atom present, a subscript is written after the symbol for that atom.

    What is the general rule for naming organic compounds? ›

    In summary, the name of the compound is written out with the substituents in alphabetical order followed by the base name (derived from the number of carbons in the parent chain). Commas are used between numbers and dashes are used between letters and numbers. There are no spaces in the name.

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