
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

In section 3.1.3 we learned about mass numbers. The mass number of an atom is a whole number, and is obtained by adding the number of protons and neutrons in the nucleus of the atom. Since protons and neutrons are the heaviest of the three types of subatomic particles, the mass number of an atom is very close to the mass of the atom.

The periodic table of elements SHOWS you the atomic mass of each element. Note that atomic mass is NOT a whole number! Take a look at the periodic table closely to verify the atomic mass for the first 10 elements by clicking on the symbol of the element.

Periodic Table of the Elements

The unit of atomic mass of an atom is atomic mass unit. If you are unfamiliar with this unit of mass, it can be converted to the SI unit of gram.

1 amu = 1.67 x 10^-24 gram

6.02 x 10²³ amu = 1 gram

Atomic mass unit is used to describe the mass of very tiny objects like an atom.

the atomic mass of ONE carbon atom = 12.011 amu
the atomic mass of ONE magnesium atom = 24.3050 amu
the atomic mass of ONE copper atom= 63.546 amu ..... etc ...

If you click on the above atomic mass links you'll get the amu equivalent in grams.

How did I convert the above masses in amu to gram?

Section 4.5
Isotopes ..p85

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ATOMIC STRUCTURE 3.5 - Atomic Mass

ATOMIC STRUCTURE

3.5 - Atomic Mass