How molecules are symbolized
Cl2
2Cl
2Cl2
• Molecules may also have brackets to
indicate numbers of atoms. E.g. Ca(OH)2
• Notice that the OH is a group
O Ca O
H
• The 2 refers to both H and O H
• How many of each atom are in the following?
a) NaOH
Na = 1, O = 1, H = 1
b) Ca(OH)2 Ca = 1, O = 2, H = 2
c) 3Ca(OH)2 Ca = 3, O = 6, H = 6
Balancing equations: MgO
• The law of conservation of mass states that
matter can neither be created or destroyed
• Thus, atoms are neither created or destroyed,
only rearranged in a chemical reaction
• Thus, the number of a particular atom is the
same on both sides of a chemical equation
• Example: Magnesium + Oxygen (from lab)
• Mg + O2  MgO Mg + O O  Mg O
• However, this is not balanced
• Left:
Mg = 1, O = 2
• Right: Mg = 1, O = 1
Balance equations by “inspection”
Mg + O2  MgO
2Mg + O2  2MgO is correct
Mg + ½O2

MgO
is
incorrect
Mg2 + O2  2MgO is incorrect
4Mgwith
+ 2elements
O2  4MgO
is incorrect
Hints: start
that occur
in one
compound on each side. Treat polyatomic ions
that repeat as if they were a single entity.
a) P4 + 5 O2  P4O10
b) 2 Li + 2 H2O  H2 + 2 LiOH
c) 2 Bi(NO3)3 + 3 K2S  Bi2S3 + 6 KNO3
d) C2H6 +3.5 O2  2 CO2 + 3 H2O
2 C2H6 + 7 O2  4 CO2 + 6 H2O
From
Balance these skeleton equations:
a)
b)
c)
d)
e)
f)
g)
h)
i)
Mg + 2HCl  MgCl2 + H2
3Ca + N2  Ca3N2
NH4NO3  N2O + 2H2O
2BiCl3 + 3H2S  Bi2S3 + 6HCl
2C4H10 + 13O2  8CO2 + 10H2O
6O2 + C6H12O6  6CO2 + 6H2O
3NO2 + H2O  2HNO3 + NO
Cr2(SO4)3+ 6NaOH  2Cr(OH)3+ 3Na2SO4
Al4C3 + 12H2O  3CH4 + 4Al(OH)3
Returning to reaction types
• We have looked at several types of
reactions without worrying about balancing
• However, all equations should be balanced
• Predict the products and balance these:
(recall, metals above replace metals below,
reactions with water yield metal hydroxides)
2 Fe + 3 CuSO4  3 Cu +
Fe2(SO
)3
Ni + NaCl  NR
(no 4reaction)
2 Al + 3 CuCl2  3 Cu + 2AlCl3
2 Li + ZnCO3  Zn +
Li2CO3+ ½H2
2 Li + 2 H2O
 2 LiOH
 2 Al2O
4 Al + 3O2
K
Na
Li
Ca
Mg
Al
Zn
Fe
Ni
Sn
Pb
H
Cu
Hg
Ag
Au
Discovery of Radioactivity
• Radioactivity is the release of energy or
particles when an atom disintegrates (demo)
• Radioactivity was discovered when minerals
were exposed to film through an opaque cover
• The 3 types of radioactivity can be shown by
passing emissions through an electrical field:
Lead block
+
Radioactive
substance


–

Phosphorescent
zinc sulfide
detection screen
Strong magnetic or
electrostatic field
Types of Radioactivity
Types of radiation: 1) , 2) , 3) 
1. Alpha () particles are symbolized as 42He
2. Beta () particles (essentially electrons) are 0–1e
3. Gamma () rays are symbolized as 00
• You can determine the composition of each:
: mass of 4 u, charge of +2 (2 p+, 2 n0, 0 e–)
• Other symbols: proton = 11p, neutron = 10n
• There are different terms to describe the
different types of nuclear reactions
• “alpha decay” means an  particle is given off.
• Other: beta decay, fusion (meaning to bring
together), fission (meaning to break apart)
Nuclear equations
Q. Write the beta
decay for C-14
Q. Write the alpha
decay for 209Po
14
6C
209
84

0
-1 e
Po 
+
14
7N
4
205
2 He + 82 Pb
Q. Complete this fission reaction
235
1
1
139
94
+
0 n  3 0 n + 56 Ba + 36 Kr
92 U
In all cases, charge and mass must be balanced
• Practice: pg. 222-3, Q6, Q3
6. a) 32 b) 99 c) 187 d)
38Sr
75Re
15P
3. a) 226
88
b)
84
35
c)
242
94
Ra 
4
222
2 He + 86 Rn
Br 
0
-1 e
Pu 
4
238
2 He + 92 U
1
14
+
p
1
7N
e) 18 + 1
0n
8O
d)
e) 99
4
2He
43Tc


+
84
36
Kr
4
11
2 He + 6 C
0
19
-1 e + 9 F
Here are some more to balance:
a)
b)
c)
d)
e)
f)
g)
h)
i)
2KNO3  2KNO2 + O2
2Pb(NO3)2  2PbO + 4NO2 + O2
P4 + 6I2  4PI3
3MgO + 2H3PO4  Mg3(PO4)2 + 3H2O
Br2 + 2KI  I2 + 2KBr
Ca(OH)2 + 2HNO3  Ca(NO3)2 + 2H2O
Bi2O3 + 3H2  2Bi + 3H2O
3Fe + 2O2  Fe3O4
2CaO + 5C  2CaC2 + CO2
Question 3 pg. 252
a)
b)
c)
d)
e)
f)
g)
h)
2Li + 2H2O  H2 + 2LiOH
P4 + 5O2  P4O10
2C2H6 + 7O2  4CO2 + 6H2O
CS2 + 3O2  CO2 + 2SO2
2AsCl3 + 3H2S  As2S3 + 6HCl
3AgNO3 + FeCl3  3AgCl + Fe(NO3)3
2KClO3  2KCl + 3O2
2SO2 + O2  2SO3
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