Question

average atomic mass
how are the masses on the periodic table determined?
model 1: a strip of magnesium metal

1. what is the atomic number for each mg atom in model 1?
2. what are the mass numbers of the naturally occurring isotopes of magnesium shown in model 1?
3. do all of the atoms of magnesium in model 1 have the same atomic mass? explain.
4. for the sample of 20 atoms of magnesium shown in model 1

a. draw a table indicating the mass numbers of the three isotopes and the number of atoms of each isotope present.
b. circle the most common isotope.

Explanation:

Step1: Recall atomic number concept

The atomic number of an element is a fixed property and for magnesium (Mg), it is 12. Each Mg atom in Model 1 has an atomic number of 12.

Step2: Identify mass - numbers from isotopes notation

The mass numbers of the naturally - occurring isotopes of magnesium shown in Model 1 are 24, 25, and 26 (from $^{24}Mg$, $^{25}Mg$, $^{26}Mg$).

Step3: Analyze atomic mass variation

No, all of the atoms of magnesium in Model 1 do not have the same atomic mass. This is because they are different isotopes. Isotopes of an element have the same number of protons but different numbers of neutrons, resulting in different mass numbers and thus different atomic masses.

Step4a: Create a table

Isotope	Mass Number	Number of Atoms
$^{25}Mg$	25	3
$^{26}Mg$	26	3

Step4b: Determine the most common isotope

The most common isotope is $^{24}Mg$ since it has the highest number of atoms (14) among the three isotopes in the sample of 20 atoms.

Answer:

1. 12
2. 24, 25, 26
3. No. They are different isotopes with different numbers of neutrons and thus different atomic masses.
4. a.

Isotope	Mass Number	Number of Atoms
$^{25}Mg$	25	3
$^{26}Mg$	26	3

b. $^{24}Mg$ (circle it)