http://www.ck12.org Chapter 4. Atomic Structure
4.3 Isotopes and Atomic Mass
Lesson Objectives
- Explain the workings of a mass spectrometer and describe how this device is used to determine average atomic
mass. - Define atomic number and mass number.
- Define isotope.
Lesson Vocabulary
- atomic number: The number of protons in the nucleus of each atom of an element.
- mass number: The total number of protons and neutrons in an atom.
- A-Z notation: The shorthand notation of the composition of any atom using the atomic number (Z) and the
mass number (A). - isotopes: Atoms that have the same atomic number but different mass numbers due to a difference in the
number of neutrons. - mass spectrometer: An instrument that determines the masses of atoms, molecules, and molecular fragments.
- percent abundance: The percentage of atoms of a particular isotope in a naturally occurring sample of the
pure element. - atomic mass: The weighted average of the atomic masses of the naturally occurring isotopes of an element.
Check Your Understanding
- Describe the composition of an atom.
- What are the three subatomic particles and what are their properties?
Introduction
Atoms are the fundamental building blocks of all matter and are composed of protons, neutrons, and electrons.
Because atoms are electrically neutral, the number of positively charged protons must be equal to the number of
negatively charged electrons. One of Dalton’s points in his atomic theory was that all atoms of a given element are
identical in mass. In this section, we will see how this is not strictly true, due to variability in the number of neutrons
that an atom may contain.