Relative atomic mass - Ar
Just another way of saying how
heavy different atoms are compared
with the mass of one atom of carbon
– 12 (regular carbon!)
Mass number
Atomic number
Number of
protons present
Q. What does the atomic mass number
represent?
A. Total number of protons and
neutrons found in an atoms
nucleus
Another handy trick you can do with the periodic
table
You can work out how many neutrons an
element has by subtracting the proton number
from the mass number!!
Mass number
Atomic number
So how many neutrons:
Relative atomic mass - Ar
Relative atomic mass is easy!! It’s the same value
as the mass number – it just sounds scarier!
So what does this tell us
about Mg and H?
Table.1. Different elements and their different Ar
Element
Ar
H
1
C
12
O
16
Mg
24
Cl
35.
Relative formula mass - Mr
• To find the relative formula mass (Mr) of a
compound, you just add together the
Ar values for all the atoms in its formula.
Example 1: Find the Mr of carbon monoxide
(CO).
The Ar of carbon is 12 and the Ar of oxygen
is 16.
So the Mr of carbon monoxide is 12 + 16 = 28.
16
8
O
Relative formula mass - Mr
• Example 2:Find the Mr of sodium oxide-Na2O
The Ar of sodium is 23 and the Ar of oxygen
is 16.
So the Mr of sodium oxide is (23 x 2) + 16 = 62.
Find the Mr of these:
Carbon dioxide
Sulphur dioxide SO2
Calcium carbonate CaCO3
Sodium hydroxide NaOH
Sulphuric acid H2SO4
Hydrochloric acid HCl
Copper sulphate CuSO4
Magnesium chloride MgCl2
Sodium carbonate Na2CO3
ATOMIC MASS AND AVERAGE ATOMIC MASS

Atomic Mass = used to numerically indicate
the mass of an atom in its ground state, it is
expressed in the non SI unit of u

u = refers to unified atomic mass unit
(formerly known as atomic mass unit or amu)
1 amu = 1/12 the mass of carbon-12 atom,
 therefore the mass of C-12 atom is made
equal to 12 amu

Carbon-12 atom is an isotope of carbon
1 amu = 1.66 x 10-24 g
Note that: Atomic mass of 12C = mass of p + mass
of n + mass of e
Mass of e = 1/1800 of mass of p and n so it is
negligible making the equation
Atomic mass of 12C = mass of p + mass of n
•
Atomic mass
For carbon it is 12 u
not 12.01 u
vs. Average atomic mass
Used to relate the fact that the numerical
value assigned to each element in the
periodic table reflects the average
abundances of the atoms that compose a
naturally occurring element
Related to isotopes
For carbon it is 12.01 u
Chemists often will use the term “atomic
mass” when they are actually referring to
average atomic mass of an atom.
Average Mass of Isotopes
• Isotopes are naturally occurring.
• The mass # of an element (periodic table) is the weighted
avg. of all isotopes that exist in nature.
- abundance of isotope is just as important as mass!
• Ex...
Natural copper (Cu) consists of 2 isotopes ...
Copper - 63 (mass = 62 .930 g/mole)
69%
Copper - 65 (mass = 64 .930 g/mole)
31%
• To calculate avg. mass...
Step 1 : mass x abundance for each isotope
Step 2 : add the two values from step 1 together
43.42
62 .93 x .69 = 43.42
+ 20.13
64 .93 x .31 = 20.13
63.55 g/mole
• The average mass of an element is closest to the isotope
that is most plentiful in nature.
• Ex...
Three isotopes of Oxygen:
Oxygen - 16 99 . 759%
Oxygen - 17 0.037%
Oxygen - 18 0.204%
• The avg. mass (from P.T.) is closest to 16, therefore,
Oxygen-16 is the isotope that is most abundant in nature.
Working with Weighted Averages
&
Calculating Average Atomic Mass
Read page 165 of your text and make short notes.