Periodic table
Groups – Columns down
Alkali Metals
Alkaline Earth Metals
Halogens
Noble gases
Periods – Rows across
Lanthanides
Actinides

Figure 02.16

Diatomic Gases

Element Symbols
Atomic Number=number of protons (+)the Z number
Atomic Mass=number of protons and neutrons (+/-) the A number
Z
A
O
= 16
8
O

Isotopes
 There are three different types of oxygen atoms shown above.
 All have same # protons, but diff # neutrons, and diff mass #
 Chemically alike because have identical # protons and electrons, which are subatomic particles responsible for chemical behavior.

Symbol Practice Problems
 Write the symbol for each of the three isotopes for oxygen (oxygen-16, oxygen-17 and oxygen-18) including atomic # and mass #
 Three isotopes of chromium are Cr-50,
Cr-52, and Cr-53. How many neutrons are in each isotope? Atomic number of
Cr is 24.

Atomic Mass (amu)
 In nature, most elements occur as mixture of one or two isotopes.
 Each isotope has a fixed mass and natural percent abundance.
 Consider three isotopes of Hydrogen
 Hydrogen 1 (99.985%)
 Hydrogen 2 and 3 present in trace amounts.
 Look at atomic mass of H on periodic table
(1.0079)
 This is very close to the mass of hydrogen-1
(1.0078).
 The slight difference takes into account the larger masses, but the smaller amounts of the other two isotopes of hydrogen.

Atomic Mass
 Atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of an element.
 Weighted average mass accounts for both mass and relative abundance of isotopes.

Atomic Mass
 You can calculate the atomic mass of an element if you are given:
 The number of isotopes
 The mass of each isotope
 The percent abundance of each isotope.
 To calculate the atomic mass, multiply the mass of each isotope by its natural abundance (in decimal form) then add the products.

Example - Average Atomic
Mass
 Copper has two stable isotopes: carbon-12 has abundance of
98.89% and carbon-13 has natural abundance of 1.11%. The mass of
Carbon-12 is 12.00 amu and the mass of Carbon-13 is 13.003 amu.
Calculate atomic mass of Carbon.

Answer
 Atomic mass of carbon =
(12.00amu X .9889) + (13.003 X
0.0111) = 12.011 amu

Example
 Calculate the atomic mass of bromine. The two isotopes of bromine have atomic masses and relative abundance of 78.92 amu
(50.69%) and 80.92 amu
(49.31%).

Answer
 Atomic mass of Bromine: (78.92 amu X .5069) + (80.92 amu X
.4931) = 79.91 amu
 Homework: Complete isotopes worksheet