CHEM 1406 Concept Review: Ionic & Molecular Compounds

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Octet rule: The rule that states that all atoms will attempt to gain, lose, or share electrons in order to have the same number of electrons as the closest noble gas.  (Noble gases, except He, always have 8 valence electrons, hence the name “octet”.)

Example:  Sodium (Na) has 11 electrons.  The closest noble gas is Neon (Ne), which has 10 electrons.  Sodium (Na) will lose one electron to become Na⁺.  Since electrons are negative, when sodium loses an electron (a negative charge), it becomes more positive.  For a summary see chart below:

 

(Elements in the shaded area are mostly transition metals and can have multiple different charges.)

 

 

Ionic Compounds: Bonding and Naming		Molecular Compounds: Bonding and Naming
Ionic bonding:  This type of bonding involves the complete giving up of 1 or more electrons from one atom to another, resulting in a positively charged ion called a “cation” and a negatively charged ion called an “anion”.  This giving and receiving of electrons allows each atom to obtain a “noble gas” electron configuration, making each ion more stable than the original atoms.  These oppositely charged ions then stick together like magnets in a repeating pattern.   Naming Ionic Compounds:  When naming ionic compounds,  follow the following guidelines: Name the cation first. Then, name the anion.   For a metal cation, it is named the same as the element. If the metal can have multiple charges (i.e. a transition metal), you must indicate its charge with a Roman numeral.  (See above chart.  The shaded area indicates where transition elements are.)  Example:  Al3+ is simply called Aluminum, but Pb4+                      would be Lead (IV) and Cu+ would be Copper (I)   For non-metal monatomic ions (one-atom ions), the name of the anion is simply the first syllable of the non-metal followed by an “-ide” ending.       Example:  O2- is oxide, Cl- is chloride, P3- is                                          phosphide.   4.  For polyatomic ions, there is no change in the name of the ion.		Molecular (or Covalent) bonding:  This type of bonding involves the sharing of electrons between atoms so that each atom can have 8 electrons in its outermost (or valence) shell.  These electrons are shared in pairs, with the two nuclei of the “bonded” atoms pulling on them in a sort of tug-of-war fashion.  Atoms that are more electronegative have a stronger “pull”.  (See more about electronegativity on the next page.)   Covalent Compound Naming:  When naming covalent compounds, follow the following guidelines: For covalent compounds, the same two elements can have numerous combinations due to the fact that there are many ways to share electrons. As such, covalent naming uses the following prefixes: Prefix Meaning Prefix Meaning Mono- 1 Hexa- 6 Di- 2 Hepta- 7 Tri- 3 Octa- 8 Tetra- 4 Nona- 9 Penta- 5 Deca- 10   Name the first element with its appropriate prefix (unless the prefix is mono. No “mono-“ prefix for the first element.)  Then, name the second element with its appropriate prefix AND change the ending to an “-ide”.       Example:  N2O is Dinitrogen monoxide, CO2 is Carbon                          dioxide.

 

 

Determining the Charge of a Transition Metal

1. Calculate the total negative charge from all the anions. Then, recognize that there will be an equal amount of positive charge.
   Example Problem:  Name the compound with the formula Fe₂(SO₄)₃.
   
   
   
2. Divide the total positive charge by the number of transition metal ions to get the charge per transition metal.

                  $\frac{+6}{2\:Fe\:ions}=+3\:charge\:per\:Fe\:ion$                    

The name of the above compound is therefore “Iron(III) sulfate”.

 

 

Writing Ionic Formulas

1. Identify the ions present along with their charges. 
   Example:  Magnesium Phosphate would have the ions Mg²⁺  and PO₄^3-.

 

2. Find the lowest common multiple of the two different charges.

      Example:   The lowest common multiple is 6.

 

3. Determine how many cations would be needed to give you a total positive charge equal to the LCM (lowest common multiple) and how many anions would be needed to give you a total negative charge equal to the LCM.
   Example:         3 Mg²⁺ =+6                and                  2 PO₄^3- = -6

 

4. Write the formula for the compound using chemical symbols and indicate how many of each ion with subscripts. If there is more than 1 polyatomic ion needed, use parentheses.
   Example:  Mg₃(PO₄)₂

 

 

Polarity of Bonds and Molecules

Electronegativity:  A measure of the ability of an atom to attract the shared electrons of a bond to itself.

Polar bonds:  Bonds between atoms with significantly different electronegativities causing an unequal sharing of electrons, giving one of the atoms a partial positive charge and the other atom a partial negative charge.

Non-polar bonds:  Bonds between atoms with the same or very similar electronegativities, causing electrons to be shared equally between the atoms.

 

 

Simplified Rules for Determining Molecule Polarity (Requires Lewis Dot Structure to be drawn.)

1. If a molecule has one lone pair of electrons on the central atom, the molecule is polar. If a molecule has 2 or more lone pairs on the central atom, the molecule will be polar UNLESS those lone pairs are in geometrically opposite positions AND all the atoms bonded to the central atom are the same.
   
   
2. If a molecule has no lone pairs and the atoms bonded to the central atom are all the same, the molecule will be non-polar. If a molecule has no lone pairs and the atoms bonded to the central atom are NOT the same, the molecule will usually be polar.
   
   
3. For any diatomic (2 atom) molecule, the polarity of the bond is the same as the polarity of the molecule.