http://www.ck12.org Chapter 22. The Nucleus
Example Problem:
The element boron consists of two isotopes, boron-10 and boron-11. The abundance of boron-10 is 20.0% and the
abundance of boron-11 is 80.0%. What is the atomic weight of boron?
Solution:
Atomic weight= ( 0. 20 )( 10 .0 amu)+( 0. 80 )( 11 .0 amu) = 2. 0 + 8. 8 = 10 .8 amu
Summary
- Since the identification of the neutron, the model of the atomic nucleus has changed very little. According to
this model, a nucleus is considered to be an aggregate of protons and neutrons. - The number of protons in a nucleus is called the atomic number and is designated by the symbolZ.
- The total number of nucleons, neutrons and protons, is designated by the symbolAand is called the mass
number. - The number of neutrons,N, isN=A−Z.
- Nuclei that have the same number of protons in the nucleus but a different number of neutrons are called
isotopes. - Each element has its own square in the periodic table.
- Each elemental square on the periodic table contains the chemical symbol for the element, the atomic number,
and the atomic weight. - The atomic weight of an element is a weighted average of its isotopes.
Practice
Questions
http://www.blinkx.com/#v/isotopes/GaG9ZHLyovqma9Z9nADYktg
Follow up questions:
- Nucleus #1 has 6 protons and a mass number of 14 while nucleus #2 has 7 protons and a mass number of
14. Are these isotopes or are they nuclei of different elements? - Nucleus #1 has 5 protons and a mass number of 12 while nucleus #2 has 5 protons and a mass number of
13. Are these isotopes or are they nuclei of different elements?