Question

how many neutrons does the isotope used to treat thyroid cancer contain? 73.9 neutrons
the isotope used to study blood circulation is sodium - 24, but on the periodic table the mass shown for sodium is 22.99. which one is the atomic mass, which one is the mass number, and what is the difference between the two?

Explanation:

• Atomic mass: The atomic mass of an element is the weighted average of the masses of all its isotopes, considering their natural abundances. For sodium, the value \(22.99\) on the periodic table is the atomic mass.
• Mass number: The mass number (\(A\)) of an isotope is the sum of the number of protons (\(p\)) and neutrons (\(n\)) in the nucleus of an atom of that isotope. For Sodium - 24 (\(^{24}\text{Na}\)), the mass number is \(24\). Sodium has \(11\) protons (\(Z = 11\), from the periodic table).
• Difference: The mass number is for a specific isotope (\(A=p + n\)), while the atomic mass is a weighted - average for all isotopes. The difference between the mass number (\(A = 24\)) and the atomic mass (\(22.99\)): \(24-22.99 = 1.01\). This difference arises because the atomic mass accounts for the fact that most sodium atoms are not the \(^{24}\text{Na}\) isotope (natural sodium has a higher abundance of \(^{23}\text{Na}\) isotope).

Answer:

• Atomic mass: \(22.99\)
• Mass number: \(24\)
• Difference: \(24 - 22.99=1.01\)