4f. 5f. Ionization Energy Trends. Electronegativity. Increasing Electronegativity. Decreasing Electronegativity. Lewis Structure Assumptions. Only valance electrons are involved in bonding. Atoms react to form molecules, so to achieve stable noble gas electron configurations.
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Step 1: Count the total number of valance electrons.
Step 2: Identify the central atom (the first atom written unless that atom
is hydrogen). Place all terminal atoms around that atom.
Hydrogen atoms NEVER have more than one bond.
Step 3: Complete the octet for all atoms in the Lewis structure with
lone pairs of electrons (except hydrogen).
Step 4: Check your structure by counting the number of valance
electrons used (they will match step 1 if the structure is
correct). If your valance electrons don’t match you will need
to tweak your structure as follows.
Too Many Electrons Initially: Redraw the
Lewis structure from step 2 adding a double
bond. If you still have too many electrons
add another multiple bond and repeat.
Always add double bonds before triple
bonds. Every double bonds effectively
remove two electrons from the structure while
triple bonds effectively remove 4 electrons.
Two Few Electrons: If you only have four or
six valance electrons initially you can’t
possibly fill the octet rule (usually BeH2 or
BH3). Just place hydrogens around central
atoms and call it done.
Odd number of electrons: One atom will
have to have less than eight electrons.
Draw the Lewis structure as if it had one
more valance electrons than it actually does.
Then subtract one electron from the least
electronegative element (often the central
Exceeding the Octet Rule: If the central atom is in
the third row or below it may be possible to exceed
eight electrons around the central atom. But only if
you absolutely have no choice. If you can give
everything eight and only eight do that first. When
you must break the octet rule draw the structure as
you would in steps 1-4 and the place the extra
electrons around the central atom. Even when
breaking the octet rule no atom will ever have
more than 12 electrons.