Question

isotopes and atomic mass
model 1: make isotopes
open the isotopes and atomic mass simulation http://phet.colorado.edu/en/simulation/isotopes - and - atomic - mass
play with the \make isotopes\ tab of the simulation for a few minutes and then answer the following questions.

1. what particles determine the mass number?
2. why is mass number always a whole number?
3. one isotope of carbon (c) has exactly the same mass number and atomic mass since it was used as the definition of the atomic mass unit (amu). which isotope is it and what is its atomic mass?
4. what is the approximate mass of one proton? ______ amu
5. what is the approximate mass of one neutron? ______ amu
6. look at 3 or 4 other atoms using the simulation. do any of them have a whole number for atomic mass?

model 2: mix isotopes
play with the \mix isotopes\ tab for a few minutes, then answer the following questions.

1. what are the factors that affect the average atomic mass of a mixture of isotopes?
2. beryllium (be) and fluorine (f) have only one stable isotope. use the sim and the periodic table to complete the following table:

element	mass of 1 atom	average mass of 2 atoms (sim)	average mass of 3 atoms (sim)	atomic mass (periodic table)
beryllium (be)	9.01218 amu
fluorine (f)	18.99840 amu

1. why are all the values in each row of the table above the same?

Explanation:

1. The mass number of an atom is determined by protons and neutrons. They are located in the nucleus and have a mass of approximately 1 amu each.
2. Mass number is the sum of protons and neutrons, both of which are counted as whole - entities, so it's always a whole number.
3. The isotope of carbon used as the definition of the atomic mass unit is carbon - 12, and its atomic mass is 12 amu.
4. The approximate mass of one proton is 1 amu.
5. The approximate mass of one neutron is 1 amu.
6. Most elements have isotopes with different masses, so their atomic masses (weighted averages) are usually not whole numbers. But some elements with a single stable isotope may have an atomic mass close to a whole number.

For Model 2:

1. The factors affecting the average atomic mass of a mixture of isotopes are the relative abundances of the isotopes and the mass of each isotope.
2. For beryllium, if the mass of 1 atom is 9.01218 amu, the average mass of 2 atoms (sim) is 9.01218 amu (since all atoms have the same mass), the average mass of 3 atoms (sim) is 9.01218 amu, and from the periodic table, the atomic mass is 9.01218 amu. For fluorine, if the mass of 1 atom is 18.99840 amu, the average mass of 2 atoms (sim) is 18.99840 amu, the average mass of 3 atoms (sim) is 18.99840 amu, and from the periodic table, the atomic mass is 18.99840 amu.
3. Since beryllium and fluorine have only one stable isotope, all atoms of these elements have the same mass, so the mass of one atom, the average mass of multiple atoms, and the atomic mass from the periodic table are the same.

Answer:

1. Protons and neutrons.
2. Because it is the sum of whole - numbered protons and neutrons.
3. Carbon - 12; 12 amu.
4. 1
5. 1
6. Some elements with a single stable isotope may have an atomic mass close to a whole number, but generally most do not.

Model 2:

1. Relative abundances of isotopes and mass of each isotope.
2. Beryllium: Mass of 1 atom: 9.01218 amu, Average mass of 2 atoms (sim): 9.01218 amu, Average mass of 3 atoms (sim): 9.01218 amu, Atomic mass (periodic table): 9.01218 amu; Fluorine: Mass of 1 atom: 18.99840 amu, Average mass of 2 atoms (sim): 18.99840 amu, Average mass of 3 atoms (sim): 18.99840 amu, Atomic mass (periodic table): 18.99840 amu
3. Since they have only one stable isotope, all atoms have the same mass.