Transcript Lewis structures

Chemical bonding
Chemical
The name’s Bond…….chemical bond.
Ionic Bonding
• In ionic bonding, electrons are completely
transferred from one atom to another.
• In the process of either losing or gaining
negatively charged electrons, the reacting atoms
form ions.
• The oppositely charged ions are attracted to each
other by electrostatic forces, which are the basis
of the ionic bond.
For example, during the reaction of sodium with chlorine:
sodium (on the left) loses its one valence electron to
chlorine (on the right), resulting in a positively charged
sodium ion (left) and a negatively charged chlorine ion
(right).
+1 charge
-1 charge
Covalent Bonding
• The second major type of atomic bonding occurs
when atoms share electrons. As opposed to ionic
bonding in which a complete transfer of electrons
occurs, covalent bonding occurs when two (or more)
elements share electrons.
• Covalent bonding occurs because the atoms in the
compound have a similar tendency for electrons
(generally to gain electrons).
• This most commonly occurs when two nonmetals
bond together. Because both of the nonmetals will
want to gain electrons, the elements involved will
share electrons in an effort to fill their valence shells.
Lewis structures
(aka: electron dot structures).
•The diagrams are named for Gilbert N. Lewis, who
described them in his 1916 article entitled The Atom
and the Molecule.
•Lewis structures depict the bonds between
atoms of a molecule as well as any unbonded
electron pairs.
•You can draw a Lewis dot structure for any
covalent molecule or coordination
compound.
Lewis Structure Basics
•A Lewis structure is a type of shorthand notation.
•Atoms are written using their element symbols.
•Lines are drawn between atoms to indicate
chemical bonds.
•Single lines are single bonds.
•Double lines are double bonds.
•Triple lines are triple bonds.
•Dots are drawn next to atoms to show unbonded
electrons. A pair of dots is a pair of excess electrons.
Steps to Drawing a Lewis Structure
• Pick a Central Atom Start your structure by picking
a central atom and writing its element symbol. This
atom will be the one with the lowest
electronegativity. Sometimes it's difficult to know
which atom is the least electronegative, but you
can use the periodic table trends to help you out.
• Electronegativity typically increases as you move
from left to right across the periodic table and
decreases as you move down the table, from top
to bottom.
• For the compounds we will encounter
in this class, O, N, C, P and S are the
best candidates to start with as the
central atom.
• Once you have selected the central atom,
write it down and connect the other
atoms to it with a single bond.
• You may change these bonds to become
double or triple bonds as you progress.
Count Electrons
• Lewis electron dot structures show the
valence electrons for each atom. You don't
need to worry about the total number of
electrons, only those in the outer shells.
• The number indicated over each family
(vertical columns) indicates the number of
valence electrons.
Practice
How many valence electrons do atoms (neutral) of
the following elements have?
Carbon
Nitrogen
Oxygen
Fluorine
Helium
Sodium
Neon
How many electrons do atoms (neutral) of the
following elements have?
Carbon = 4
Nitrogen = 5
Oxygen = 6
Fluorine = 7
Helium = 2
Sodium = 1
Neon = 8
Practice
The octet rule
• The octet rule states that atoms with 8
electrons in their outer shell are stable.
• This rule applies well up to period 4, when it takes
8 electrons to fill the outer orbitals.
Place Electrons around Atoms
• Once you have determined how many electrons to
draw around each atom, start placing them on the
structure.
• Start by placing one pair of dots for each pair of valence
electrons. Once the lone pairs are placed, you may find
some atoms, particularly the central atom, don't have a
complete octet of electrons.
• This indicates there are double or possibly triple bonds.
Remember, it takes a pair of electrons to form a bond.
Practice
Write the Lewis structure for the following
compound
• CHCl3 (chloroform)
Practice: homework
Write the Lewis structure for the following molecules:
1. CHCl3 (chloroform)
2.
3.
4.
5.
6.
7.
8.
H2
O2
H2O
CH4 (methane)
C2H6 (ethane)
HBrO (Hypobromous Acid)
C5H12 (pentane)
2 points each = 18 pts.
1attempted 1correct