Question

worksheet #2 – isotopes & average atomic mass answer the following questions regarding average atomic mass and isotopes. show all work for problems involving calculations! 1. define the word “isotope” 2. true or false: is the mass number of a given isotope the same as the average atomic mass on the periodic table for that element? explain!

Explanation:

Question 1

An isotope is a variant of a chemical element that has the same number of protons (atomic number) in its atomic nucleus but a different number of neutrons, resulting in a different mass number (sum of protons and neutrons). For example, carbon - 12, carbon - 13, and carbon - 14 are isotopes of carbon. Carbon - 12 has 6 protons and 6 neutrons, carbon - 13 has 6 protons and 7 neutrons, and carbon - 14 has 6 protons and 8 neutrons.

1. First, recall the definitions:

• The mass number of an isotope is the sum of the number of protons and neutrons in the nucleus of a single atom of that isotope. It is a whole number (e.g., for carbon - 12, the mass number is 12, which is 6 protons + 6 neutrons).
• The average atomic mass of an element is the weighted average of the masses of all the naturally occurring isotopes of that element. It takes into account the relative abundance of each isotope. For example, the average atomic mass of carbon is approximately 12.011 amu. This is because carbon - 12 is very abundant (about 98.93%), carbon - 13 is about 1.07% abundant, and carbon - 14 is present in trace amounts. The formula for calculating average atomic mass is $Average\ atomic\ mass=\sum (isotope\ mass\times relative\ abundance)$.

1. Then, compare the two:

• The mass number of a single isotope is a whole number representing the sum of protons and neutrons in one atom of that isotope. The average atomic mass is a weighted average of the masses of all isotopes of the element, considering their abundances. So, they are not the same. For example, chlorine has two main isotopes: chlorine - 35 (mass number 35, abundance about 75.77%) and chlorine - 37 (mass number 37, abundance about 24.23%). The average atomic mass of chlorine is $ (35\times0.7577)+(37\times0.2423)=35.45\ amu$, which is different from the mass numbers of its isotopes (35 and 37).

Answer:

An isotope is a variant of an element with the same atomic number (same number of protons) but a different number of neutrons (different mass number).

Question 2