Download Chapter 2 - Westmount High School

Chapter 2: Molecules & Solutions
What is a molecule?
 Definition: a molecule is a group of two or more
chemically bonded atoms.
 The two or more atoms joined together, can be the
same element or different elements.
E.g. molecule of water = H2O, molecule of oxygen = O2
Why do atoms bond with each other?
Atoms bond with one another so that they
can fulfill the octet rule. An atom will try to
have the same electron configuration as
that of noble gases (full last shell).
many
 E.g. two oxygen atoms How
electrons
does each 6
oxygen
atom have?
What do the yellow dots represent?
****VALENCE ELECTRONS****
 What atom is this? ARGON
 Will this atom bond with any other atom? NO
 When an atom’s last shell is complete/full, it will not want to
react or bond easily with another atom. E.g. all noble gases
Valence Electrons: Bonding
When 2 atoms form a bond, it is their
valence electrons that “react” with one
another to create that bond.
Potassium (K) & Chlorine (Cl)
What is the best way for each atom to
acquire a full last shell?
Ionic
K loses 1 eCl gains 1 e-
Chemical Formulae
 Symbols of elements in a chemical formula
represent the elements present. Subscript
numbers represent mole proportions of the
proceeding elements.
 In this chemical formula, there are two
atoms of chlorine and one atom of calcium.
 CaCl2
 Note that if there is no subscript number, it
means a subscript of 1.
 In this example, Ca has no subscript. It
means that there is one atom of calcium.
 CaCl2
“Ca1Cl2”
CaCl2
Number of molecules
 To show the number of molecules, a full sized
number is located in front of the molecule. For
example 4 molecules of carbon dioxide is
designated as:
4CO2
 This means there are a total of 4 C atoms and 8
O atoms in the combination. A way to remember
this is to think of it as 4 x (CO2).
4 CO2
Complex formulas
 Just as in Algebra, you can use parentheses to
separate parts in a complex formula. One
example is the formula for nitroglycerin, a highly
explosive substance.
C3H5(NO3)3
 This formula shows that nitroglycerin consists of
3 atoms of C, 5 atoms of H and then 3 NO3
nitrate ions. If the parentheses were not used,
you might have a formula like:
C3H5N3O9
 Is this formula correct?
How many atoms of sulfur (S) are there in
Na2SO4?
a. 1
b. 2
c. 3
d. 4
How many atoms of oxygen (O) are there
in Al2(SO4)3 ?
a. 2
b. 3
c. 4
d. 12
Which of the following chemical formulae
represents the following diagram?
H3PO4
C6H11OH
S(CH3)4(NH2)2
CH3CH2OH
Exceptions
Lithium, Beryllium, Boron tend to lose their
valence electrons to resemble Helium 
Duet rule
Ions
 Atoms are normally neutrally charged because
they contain as many protons as they have
electrons.
Protons have a positive charge
Electrons have a negative charge
 E.g. Carbon has 6 protons = +6 (proton charge)

Carbon has 6 electrons = - 6 (electron charge)
0 (neutral)
Definition: an ion is an atom that has
become electrically charged by losing
or gaining one or more electrons.
Gaining and Losing Electrons
When does an atom gain or lose
electrons?
When an atom forms a bond it can gain or lose
electrons to fulfill the octet rule.
17 protons
E.g. Chlorine atom = ___
17 electrons
___
1 eTo fulfill the octet rule, chlorine will gain/lose __
17 protons
- 18 electrons
-1
Cl
When an atom loses 1 or more electrons,
it becomes positively charged = positive
ion.
When an atom gains 1 or more electrons,
it becomes negatively charged = negative
ion.
Exercise: if oxygen gains 2 electrons, what is its
net charge? -2
Review
Negative Ion
- gained one or
more electrons
- more electrons
than protons
- negative net
charge
Positive Ion
- lost one or more
electrons
-more protons than
electrons
- positive net
charge
Number of
Charge Tendency Valence Example
electrons Element
Group
Number
Ionic Charges Table
IA
Li
1
IIA
IIIA
IVA
Be
2
B
3
C
4
N
5
Gain
or lose
4e
Lose
1e
Lose
2e
Lose
3e
1+
2+
3+ +/- 4
VA
VIA
VIIA
VIIIA
O
6
F
7
Ne
8
Gain
3e
Gain
2e
Gain
1e
None
-3
-2
-1
None
Cation: positively charged atom
Anion: negatively charged atom