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How do you tell what kind of bond a compound is

I've been given some homework in chemistry and I'm not sure how to tell if a compound or ion is an ionic, covalent or polar covalent bond. I also have to identify what kind of molecular shape a compound has and I'm not sure how to do that.

An example problem on the paper is carbon tetrafluoride. 

 

Anyhelp would be appreciated.

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2 Answers

There is a couple different ways to determine if a bond is ionic or covalent.  By definition, an ionic bond is between a metal and a nonmetal, and a covalent bond is between 2 nonmetals.  So you usually just look at the periodic table and determine whether your compound is made of a metal/nonmetal or is just 2 nonmetals.  The exception is a compound made with ammonium (NH4+)  Since ammonium is an ion, it forms ionic compounds.  If the compound begins with H, it's an acid.


You can use determine the difference in electronegativty.  This is a value that rates how attracted an electron is to a particular atom.  This value is often stated in a chart or on the periodic table.  Find both elements in the bond, and find the electronegativity values.  For example, Na is 0.9 and Cl is 3.0  Then subtract to find the difference (3.0-0.9 = 2.1)  Using this method, you can determine the value of each bond.

 

If the difference is between 0.0-0.3: The bond is nonpolar covalent

If the difference is between 0.4-1.7 (Some books say 1.9): The bond is polar covalent

If the difference is greater than 1.7 (or above 2.0 in some books): The bond is ionic.

 

As for shapes, you need to first draw a lewis dot structure (LDS) for the molecule.  Based on the LDS, you can determine the shape.  If the LDS has 4 surrounding atoms/4 single bonds and no lone dots on the central atom, the shape is tetrahedral.  If it has 3 single bonds and one pair of lone dots, the shape is trigonal pyramidal.  If it has 2 single bonds and 2 lone dot pairs, it's bent (bent also goes for 1 single, 1 double, and 1 lone dot pair).

If the LDS has 3 single bonds (or 2 single/1 double) and NO lone dot pairs, it's trigonal planar. 

And if the LDS has 2 double bonds and NO lone dot pairs, or is just 2 atoms, the shape is linear.

Comments

The definition above is fairly simplistic, but works much of the time.  However, the ionic bond is not "defined" as a bond between a metal and nonmetal.  An ionic bond is one in which two particles, of opposite charges (positive and negative) are held together by the attraction of those two opposite charges.  

For instance, sodium chloride consists of sodium ions (positively charged) and chloride ions (negatively charged).  When these two ions approach one another, they are attracted to each other.  The attraction between two particles is generally called a bond, and this type of bond is called an ionic bond because it is formed between ions.

 

To add one thing to the discussion on molecular geometries, a single, unpaired electron counts  just like a lone pair of electrons (at a first approximation).

 

Finally, you have to be careful with terminology in this section.  Different teachers and different books will use different terminology.  Shape can mean two completely different things to two different teachers or textbooks.

Comment

CF4 makes 109.5° angle between any two F atoms and C.  Due to symmetry all bond angles are equal.

For an asymmetric molecute, i.e. CHCl3, there may be bond "strain" and not all angles are equal, due to

the difference in size between the terminal atoms. 

However, they will be near to 109.5°

For CF4 the bonds will be covalent.

Comments

You neglected to mention the fact that the bonds are polar covalent but due to the molecule shape the overall molecule is non polar.

Comment