Atomic Theory

GENERAL CHEMISTRY
September 2011
GENERAL CHEMISTRY
September 2011
ISOTOPES

Atoms of a given element which have the same number of protons but different numbers of neutrons are called isotopes. Thus, isotopes have the same position in the periodic table, the same chemical properties and the same atomic charge.
The simplest example of an atom with different isotopes is hydrogen. The three isotopes of hydrogen are shown here:
The increasing number of neutrons in the nucleus of the hydrogen atom adds mass to the atom and thus each isotope of a given element has a different mass.

Isotopes can be represented as follows:
For the isotopes of hydrogen, 1H (or hydrogen-1), 2H (or hydrogen-2) and 3H (or hydrogen-3) represent protium (usually just referred to as hydrogen), deuterium and tritium, respectively.
Most of the light elements contain different proportions of at least two isotopes. Usually one isotope is the predominantly abundant isotope.
Average Atomic Mass: The calculation of the average atomic mass of an atom is performed using the relative abundance data from the isotope of each atom.
Average Atomic Mass

Where Mass1 = mass of first isotope in amu Abundance1 = abundance of first isotope as a percentage n = number of isotopes of element in calculation
For example:
The natural abundance for boron isotopes is 19.9% 10B (10.013 amu*) and 80.1% 11B (11.009 amu*). Calculate the atomic mass of boron:

Average atomic mass = = 10.811
(note that this is the value of atomic mass given on the periodic table)

*amu is the atomic mass unit (u, μ or amu), which is defined as 1/12th the mass of a carbon-12 atom. This value is arbitrary and simply provides a reference point for measuring relative atomic masses.
Practice problems: 1. Here are three isotopes of an element: 612C 613C 614C a. The element is: __________________ b. The number 6 refers to the _________________________ c. The numbers 12, 13, and 14 refer to the ________________________ d. How many protons and neutrons are in the first isotope? _________________ e. How many protons and neutrons are in the second isotope? _________________ f. How many protons and neutrons are in the third isotope? _________________

2. Complete the following chart: Isotope name | atomic # | mass # | # of protons | # of neutrons | # of electrons | 92 uranium-235 | | | | | | 92 uranium-238 | | | | | | 5 boron-10 | | | | | | 5 boron-11 | | | | | |

3. Naturally occurring europium (Eu) consists of two isotopes was a mass of 151 and 153. Europium-151 has an abundance of 48.03% and Europium-153 has an abundance of 51.97%. What is the atomic mass of europium?

4. Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%). Calculate the atomic mass of strontium.