IsotopesAverage Atomic Mass

Isotopes Atoms of the same element (same atomic number)

with different mass numbers

Atoms with the same number of protons, but different

numbers of neutrons.

Isotopes of chlorine

35Cl 37Cl17 17

Chlorine - 35 Chlorine - 37

Facts About Isotopes

Almost each element is found in the Universe as a mixture of isotopes

Every isotope is found in nature in a fixed percentage called Isotopic Abundance

For example: Magnesium can be found as a mixture of three isotopes 79% Mg-24, 10% Mg-25 and 11% Mg-26

Facts About Isotopes These percentages are different for

each element Each element has a particular number

of isotopes The isotope notation includes the name

or symbol of the element followed by the atomic mass.

Example: Magnesium-25 or Mg-25

The existence of isotopes explains the fact that atomic mass is a decimal for most elements

Isotopes of one element have the same chemical behaviour but slightly different physical properties (e.g. boiling point, melting point)

They have lots of real life applications (medical field, agriculture,..)

There are almost 2000 isotopes found The isotopes of radioactive elements are

called Radioisotopes

Learning Check Naturally occurring carbon consists of three

isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#p _______ _______ _______

#n _______ _______ _______

#e _______ _______ _______

Atomic Mass on the Periodic Table

11

Na22.99

Atomic Number

Symbol

Atomic Mass

Atomic Mass

Atomic mass is the weighted average mass of all the atomic masses of the isotopes of that atom correlated with their isotopic abundances.

where: Aav = average atomic mass (%) = isotopic abundance

nnav AAAA (%)...........(%)(%) 2211

Example 1: Average Atomic Mass

Cl-35 is about 75.5 % and Cl-37 about 24.5% of natural chlorine.

Calculate the average atomic mass for Chlorine.

Example 2: Average Atomic Mass

Assume that the element 108Uno is synthesized and that the sample contains 25.2% 272Uno (271.4 amu), 30.8% 273Uno (272.3 amu) and 44.0% 275Uno (274.2 amu).

What is the value of the atomic weight?

Calculating Isotopic Abundance

Steps: Assign variables for the isotopic

abundances; x and y Write the eq. for isotopic abundances: x + y = 1 Write the eq. for the Average atomic

mass: Set both eq. as a linear system and solve

for x and y by substitution

yAxAAav 21

Practice!

Boron exists as two naturally occurring isotopes:

B-10.01a.m u and B-11.01a.m.u. Calculate the isotopic abundance of

each isotope of Boron.

Practice!

Silver (Atomic weight 107.868) has two naturally-occurring isotopes with isotopic weights of 106.90509 and 108.90470.

What is the percentage abundance of each isotope in Silver?