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Electron Dot Structures - Helpful tools in thinking about bonding.

Pictorial representation of the valence electron configuration around an atom.  We will only consider s and p block elements (main group) and consequently only be concerned with s and p orbitals. 

Carbon - 1s22s22p2  - four valence electrons

 

Electron dot structure - valence electrons are represented by dots placed around the chemical symbol.  Electrons are placed up to two on each side of the elemental symbol for a maximum of eight, which is the number of electrons in a filled s and p shell.  We place a single electron on each side before pairing them up (this is related to Hund's rule).  Period one (hydrogen and helium) represents an exception where only a maximum of two electrons are placed on one side of the element (why is this so?).

Electron dot structures for the first two periods - Number of valence electrons related to group number.

Covalent Bonds - Sharing electrons, unlike ionic compounds where electrons are thought of being gained or lost.

Non-metals exhibit both covalent and ionic bonds

compounds with metals, non-metals generally gain en electron and become negatively charged

compounds with other non-metals, valence electrons are generally shared to achieve filled valence shell. 

sharing electrons - covalent bonding - the sharing of electrons is the glue that binds atoms together. 

Examples:

H2

Sometimes we use an x instead of a dot to represent an electron.  This allows us to track the electrons better. 

 

Cl2

 

Octet rule (for s- and p-block elements) - atoms combine and form bonds either by transferring electrons to form ions or by sharing electrons in covalent bonds until each atoms is surround by 8 valence electrons.  Electrons in bonds are considering in counting the electrons in both atoms involved in the bond.   H and He are the exception.

 

H2O

 

How would you put together N and H to form a compound? (ammonia)

How about C and H? (methane - natural gas)

Multiple covalent bonds

octet rule not satisfied - 7 electrons on each oxygen
octet rule not satisfied on both oxygen atoms - 6 electrons on one and 8 electrons on the other
Multiple bonds - have two sets of shared electrons.  The octet rule is satisfied on both oxygen atoms.  Again, the lone pairs are often omitted, but if you understand electron configuration you know that they must be there!

 

N2  Triple bond

Polyatomic Ions

NH4+

CO32-

Element # of electrons
C 4
O x 3 6 x 3 = 18
Sub-total of electrons 22
+ 2 electrons because of 2- charge +2
Total electrons available 24
   
To satisfy octet rule need 4 x 8 electrons 32
   
Only 24 available, can satisfy octet rule if we have 32-24 = 8 shared electrons - 4 bonds

 

open circles represent two extra electrons from 2- charge.