
UNIT # 3
Introduction
Objectives
Reading
3.1 Structure of the Atom 3.2 Isotopes 3.3 Arrangement of Electrons 3.4 Electron Configura tions 3.5 Atomic Mass
Problems 1 \| 2 \| 3

ATOMIC STRUCTURE

3.5 - Atomic Mass

3.5.1 - Calculation of Atomic Mass

The atomic mass of the elements as seen on the periodic table is a weighted average of the masses of all naturally occurring isotopes for the element. Most elements have several naturally occurring isotopes in varying abundance. Some elements like hydrogen and carbon have 2 naturally occurring isotopes. Others like Magnesium has 3 naturally occurring isotopes, and Strontium has 4 naturally occurring isotopes.

Atomic masses that are on the periodic table are calculated based on knowledge of the following information:

the mass of all the naturally occurring isotopes for the element
the natural abundance of each of these naturally occurring isotopes (usually expressed as a percentage)

The formula that you need is to calculate the atomic mass of an element is:

atomic mass =	(the fraction of isotope 1)(mass of isotope 1) +
	(the fraction of isotope 2)(mass of isotope 2) +
	(the fraction of isotope 3)(mass of isotope 3) + ...

where:

the fractions of the isotopes is the natural abundance of the isotope written as a decimal.
the masses of the isotopes are determined accurately to as many as 4 decimal places.

Section 4.5
Isotopes ..p86

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ATOMIC STRUCTURE 3.5 - Atomic Mass

ATOMIC STRUCTURE

3.5 - Atomic Mass