Question

1. why is the atomic mass unit (amu), rather than the gram, usually used to express atomic mass?
2. what isotope of carbon has been chosen as the reference isotope for atomic mass units? what is the defined atomic mass in amu of this isotope?
3. is the following sentence true or false? the atomic mass of an element is always a whole number of atomic mass units.
4. circle the letter of each statement that is true about the average atomic mass of an element and the relative abundance of its isotopes.

a. in nature, most elements occur as a mixture of two or more isotopes.
b. isotopes of an element do not have a specific natural percent abundance.
c. the average atomic mass of an element is usually closest to that of the isotope with the highest natural abundance.
d. because hydrogen has three isotopes with atomic masses of about 1 amu, 2 amu, and 3 amu, respectively, the average atomic mass of natural hydrogen is 2 amu.

1. circle the letter of the correct answer. when chlorine occurs in nature, there are three atoms of chlorine - 35 for every one atom of chlorine - 37. which atomic mass number is closer to the average atomic mass of chlorine?

a. 35 amu b. 37 amu

Explanation:

1. Atoms have extremely small masses. Using grams would result in very small decimal numbers (e.g., \(10^{-24}\) g range). The atomic mass unit (amu) is a more convenient scale for expressing atomic - scale masses.
2. The isotope of carbon chosen as the reference is carbon - 12. By definition, the atomic mass of carbon - 12 is \(12\) amu.
3. The atomic mass of an element is a weighted average of the masses of its isotopes. Since isotopes have different abundances and non - whole number contributions (in terms of mass and abundance), the atomic mass of an element is not always a whole number. So the statement is false.

22.

• a. In nature, most elements occur as a mixture of two or more isotopes. This is true. For example, oxygen has \(O - 16\), \(O - 17\), and \(O - 18\) isotopes.
• b. Isotopes of an element have a specific natural percent abundance. For example, the natural abundance of \(C - 12\) is about \(98.93\%\) and \(C - 13\) is about \(1.07\%\). So this statement is false.
• c. The average atomic mass of an element is calculated as \(\sum_{i}(m_i\times x_i)\) (where \(m_i\) is the mass of isotope \(i\) and \(x_i\) is its abundance). The isotope with the highest natural abundance will have the largest contribution to the sum. So the average atomic mass of an element is usually closest to that of the isotope with the highest natural abundance. This statement is true.
• d. The average atomic mass of hydrogen is calculated as \((1\times0.999885)+(2\times0.000115)+(3\times\text{trace amounts})\approx1.00794\) amu (not \(2\) amu). So this statement is false.

1. Let the abundance of \(Cl - 35\) be \(x = 0.75\) (since for every \(3\) atoms of \(Cl - 35\) and \(1\) atom of \(Cl - 37\), the fraction of \(Cl - 35\) is \(\frac{3}{3 + 1}=0.75\)) and the abundance of \(Cl - 37\) be \(y=0.25\). The average atomic mass of chlorine, \(M\), is given by \(M=(35\times0.75)+(37\times0.25)=26.25 + 9.25=35.5\) amu. Since \(35.5\) is closer to \(35\) amu than to \(37\) amu.

Answer:

1. Atoms are extremely small, and amu is a more convenient unit for atomic - scale masses.
2. Carbon - 12; \(12\) amu.
3. False.
4. a. True; c. True.
5. a. \(35\) amu.