SCH3U Bonding Notes 2013

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SCH3U Bonding Notes
Ionic Bonding
 An ionic bond is a chemical bond formed by the electrostatic attraction between
positive and negative ions. Therefore, ionic bonds form between metals and
non-metals.
 We can represent these ions using Lewis-dot diagrams
e.g. sodium:
chlorine:
Note: Dots are placed one to each side of a letter symbol until all four sides are
occupied. Then the dots are written two to a side until all valence electrons are
accounted for.
 During ionic bond formation, electrons are transferred:
Na
+
Cl

Na+
+
 Cl -
Instead of showing the process, we can simply just show the Lewis-dot diagram of the
compound formed:
 An ionic compound exists as an array of oppositely charged ions- a crystal lattice.
We only show the simplest unit (since ionic compounds are reduced to lowest
terms) of an ionic compound when drawing Lewis-dot diagrams. This simplest
unit is called a formula unit.
e.g. 1) AlCl3
e.g. 2) Ca3N2
Ionic Bonding: Practice Drawing Formula Units
Name
potassium
bromide
magnesium
oxide
sodium
sulphide
aluminum
oxide
barium
nitride
rubidium
fluoride
gallium
bromide
sodium
nitrate
ammonium
chloride
Lewis Dot
Diagram of
Element Losing e-
Lewis Dot Diagram
of Element Gaining
e-
Lewis-dot Diagram of Compound
*remember, compounds are neutral overall!
Formula
Covalent Bonding
 Occurs when one atoms nucleus is unable to completely pull an electron away from
the other atoms nucleus.
 As a result, the atoms must share the electrons between them.
 Covalent bonding occurs between two or more non-metals since non-metals have
high electron affinities.
 The smallest unit of a covalent compound is called a molecule.
Examples- representing molecules:
1. Single Covalent bonds *show by one dash in between bonding atoms
Note: one dash represents two shared electrons!
Hydrogen and all of the Halogens are found as diatomic molecules in nature-they form
single covalent bonds with each other in order to be more stable (having 8 valence
electrons, or 2 in the case of H)
H2
Lewis-dot diagram of bond (show bonding electrons as dots or x’s):
Structural diagram of bond (show bonding electrons as dashes):
NOTE: when drawing Lewis-dot diagrams, let dots represent the valence electrons of
one atom and x’s represent the valence electrons of the other atom. If more than two
atoms are involved, represent the central atom’s valence electrons as x’s and the others
as dots.
F2
Lewis diagram of bond:
Structural diagram of bond:
SiCl4 Lewis diagram of bond:
Structural diagram of bond:
* Check that each atom has 8 valence electrons (except H) and that the valence
electrons for each atom add up: Si (4) + 4 x Cl (7) = a total of 32
2. Double and Triple Covalent Bonds:
CO2- carbon has 4 valence electrons and thus needs 4 more in order to be stable. Each
oxygen atoms requires 2 more electrons to obtain the stable octet. In order for each
atom to have 8 valence electrons they must share two pairs of electrons.
Lewis diagram of bond:
Structural diagram of bond:
Note: two dashes (=) represent a double bond or 4 shared electrons
N2-each nitrogen atom needs three more electrons to become stable. When two
nitrogen atoms form a bond, they must share 3 pairs of electrons in order to satisfy the
octet rule.
Lewis diagram of bond:
Structural diagram of bond:
Note: three dashes () represent a triple bond or 6 shared electrons
Covalent Bonding Practice
NOTE: It is often difficult to determine how to arrange the atoms in a molecule or where to
start. Often trial and error is how one might come up with the correct Lewis diagram. There
are however, some helpful hints to get you started:




Molecules tend to be symmetrical- thus the atom found by itself in the compound is
usually written in the centre (e.g. SiCl4- Si is in the centre)
The least electronegative atom is usually found in the centre of a molecule and is usually
written first in the formula (e.g. NF3O- nitrogen is the central atom, not oxygen)
Carbon is always in the centre!
Remember the octet rule (exception: Hydrogen only needs two electrons to be stable)
The bonding capacity of atoms will also help you to know what atoms tend to form single,
double, or triple covalent bonds- fill in this table: (one is done for you)
Atom/family
# of valence e- # of bonding e(unpaired electrons)
H
Halogens (e.g. F)
Group IVA (e.g. C)
Group VA (e.g. N)
Group VIA (e.g.O)
5
Bonding
capacity
3
3
Types of bonds
-, =, and 
Try These:
Molecule
Lewis-dot
Diagram
Structural
Diagram
Molecule
NF3
H2CO
SO3
OF2
HCN
ClF
Lewis-dot
Diagram
Structural
Diagram
Molecule
Lewis-dot
Diagram
Structural
Diagram
Molecule
Lewis-dot
Diagram
Structural
Diagram
NF3O
H2S
O3
CS2
Note: Some of the above molecules contain a co-ordinate covalent bond. This is a covalent
bond where the bonding electrons are provided by only one of the atoms. Circle any coordinate covalent bonds (ccb) in your Lewis-dot Diagrams.
Still having trouble? Try these step by step rules which will always give you the correct Lewis
Structure. Use SO3 & SO32- examples.
1) Arrange the symbols of the elements of the compound as you
would expect the atoms to be arranged in the molecule. The
element with the highest bonding capacity is generally
written in the centre.
2) Add up the number of valence electrons available in each of
the atoms. If the structure is a polyatomic ion- add one
electron for each unit of negative charge and subtract one
electron for each unit of positive charge.
3) Place one pair of electrons between each adjacent pair of
elements- these are the bonding electrons
4) Place any pairs of remaining electrons as lone pairs on the
peripheral atoms (not the central atom) – no more than 8
electrons (2 for hydrogen) allowed
5) If any electrons remain, place these on the central atom
6) If octets are not complete, move lone pairs into bonding
position between those atoms to make double or triple
bonds
7) To give the structural formula, remove the dots between
atoms and draw in dashes. Remember two electrons (single
bond) are represented by one dashed line.
Try these on a separate sheet of paper: SO2, CO, CO32-, PO43-, Cl03-1, NH4+, SOCl2, Ca(NO3)2