Chapter 3. Compounds

What are compounds?

Ions

Atom is electrically neutral. When it loses or gains electron, it becomes ion.

Ions are the atom or a group of atoms with electrical charge.

  • Positively charged: Cation
  • Negatively charged: Anion

    In the following picture, how sodium cation is made from sodium atom. This is achieved by losing the valence electron of sodium atom.

    The outermost shell electron (or valence electron) is gone when sodium ion is made.

    Similarly, The outermost shell electrons are gone in order to make magnesium ion.

    In the case of anion, a parent atom gains electron. In the case of chlorine the outermost shell gains an electron to complete the shell to become 8 electrons in the valence shell.

    Naming of Ions

    Groups 1 and 2 - Either +1 and +2 ions - in these cases, the name is the name of the element. For example, Mg2+ is magnesium ion.

    Groups 15, 16, and 17 - They are, respectively, -3, -2 and -1 ions. We add -ide to the name of the element. For example, O2- is oxide ion. And, Br1- is bromide ion.

    In the transition metals, nearly all are cations, there are several ionic forms that are available. For example, copper, Cu, has several ionic forms, Cu+1, Cu+2, and Cu+3. They are represented as, respectively, copper(I) ion, copper(II) ion, and copper(III) ion.

    There are many polyatomic ions, which contains more than one element in the ion. Here are the list:

    ChargeFormulaName
    +1NH4+Ammonium ion
    -1OH-hydroxide ion
    -1CN-cyanide ion
    -2CO3-2carbonate ion
    -1NO3-nitrate ion
    -2SO4-2sulfate ion

    the Octet Rule

    If you recall that elements of Group 1 are likely to become +1 charged ions, and elements of Group 17 are likely to beocome -1 charged. Below is the relationship between group and the charge picked up by the element in each group.
    GroupChargeExample# protons# electrons# valnece electrons
    1+1Na11108
    2+2Mg12108
    13+3Al13108
    15-3N7108
    16-2O8108
    17-1F9108
    180Ne10108
    1+1K19188
    2+2Ca20188
    15-3P15188
    16-2S16188
    17-1Cl17188
    180Ar18188
    The last column is the number of valence electrons. For Na+ ion, when you get rid of the valence electron, you have 10 electrons left. It means that it is the same as the neon electron configuration. This is why there are 8 valence electrons in Na+ ion. Similar arguments hold for other cations.

    It means that 8 electrons in the valence shell signifies the stability.

    We say that the atoms and ions that has 8 electrons in the valence shell is called octet.

    Ionic Compounds

    Compound = matter constructed of two or more elements.

    Ionic Compounds = Compounds made from ions

    Ionic compounds are made of Cation and Anion, but when the compound is made it is electrically neutral!

    So, the case of sodium chloride, NaCl

    The parent Na atom loses an electron to become +1 charge and chlorine gains one electron to be -1 charged ion. When combined, overall there is no charge.

    In the case of MgCl2,

    Because Mg belongs to group 2 element, it loses two electrons, gaining +2 charge. In order to cancel the +2, one needs two chloride ions.

    In the following is Na2S.

    Here we know sodium loses one electron, while S which is group 16 elements, so it can gain two electrons to become -2 ion. In order to cancel the charges, we need two Na+ ions added to S-2.

    Naming Ionic Compounds

    Polyatomic Ions

    Polyatomic ions — ion composed of more than one element.

    Often encountered ions are listed in the following table:

    NonmetalFormulaName
    HydrogenOH-Hydroxide
    NitrogenNH4+Ammonium
    NO3-Nitrate
    Carbon CO32-Carbonate
    Sulfur SO42-Sulfate
    PhosphorusPO43-Phosphate

    Naming of Ions containing polyatomic ions

    Exactly the same way we name them. Just remember that the compounds made from ion still has neutral charge!

    NaNO3 = Sodium nitrate

    Al(OH)3 = Aluminum hydroxide

    (NH4)2CO3 = Ammonium carbonate

    Covalent Bonds

    We saw that each group has unique number of valence electrons and we know how to draw them, as shown below:

    Covalent bond is made by sharing of two electrons between two atoms. So,

    From now on, we will represent covalent bond in terms of Lewis structure.

    The Lewis structures are important in understanding the properties of molecules.

    Let us try drawing a Lewis structure for methane, CH4.

    Example: Draw Lewis dot structure of a) phosphorus atom, b) bromine atom, c) barium atom and d) hydrogen atom.

    Since phosphorus is group 15 or group 5A, there are five valence electrons. Similarly, bromine is in group 17 or group 7A. Therefore, there are 7 valence electrons. Barium is in group 2 or group 2A and so there are two valence electrons. Hydrogen has only one valence electron since it is in group 1 or group 1A.

    phosphorusbrominebariumhydrogen

    Molecules

    Covalent compound with no charge overall is called molecule.

    Naming Binary Molecules

    Basically the same as the naming of binary ionic compounds.

    The naming of molecular compounds are done in the similar way as we did for ionic compounds. In some cases there are many different numbers of the same elments can exist in molecular compounds, we need to use prefixes to show their numbers.

    NumberPrefix
    1mono
    2di
    3tri
    4tetra
    5penta
    6hexa
    7hepta
    8octa
    9nona
    10deca

    Formula Weight, Molecular Weight, and Molar Mass

    We use Formula Weight, Molecular Weight, and Molar Mass synonymously.

    Let us calculate molar mass of C6H14 molecule. You need to look at the Periodic Table to see what the masses are. Since the molecule only contains C and H, we have them on the P.T. as

    We need the average mass information. Then, the molar mass is the total mass of all atoms present in the molecule. Here we have 6 carbons and 14 hydrogens. The average mass of hydrogen is 1.008 amu or g/mol, and for carbon it is 12.011 amu or g/mol. Then, add all masses by multiplying 14 for hydrogen and 6 for carbon with their masses.

    They are the mass of molcule composed of masses of component elements.

    How can you tell if compound is ionic or binary molecule? Obviously if the ions are involved, the compound is ionic. What about binary covalent compound? Is there any way to tell?

    If the binary compound is made only of main group elements, it is likely that the compound is binary covalent compound.

    If the transition metals are involved, it is likely ionic compound.