Question

the element sodium has an atomic number of 11 and an average atomic mass of 22.99 amu. what does amu mean? based on this information, which isotope of sodium is likely the most abundant?

Explanation:

• amu: amu stands for atomic mass unit. It is a unit used to express the mass of atoms and molecules. One atomic mass unit is defined as \( \frac{1}{12} \) of the mass of a carbon - 12 atom.
• Isotope of sodium: The average atomic mass of an element is a weighted average of the masses of its isotopes. Sodium has an average atomic mass of \( 22.99\ amu \). The isotope with a mass closest to the average atomic mass is likely to be the most abundant. Sodium - 23 (\( ^{23}\text{Na} \)) has a mass of \( 23\ amu \), which is very close to the average atomic mass of \( 22.99\ amu \). Sodium - 11 is not a valid isotope as the mass number (sum of protons and neutrons) for sodium (atomic number \( Z = 11 \)) cannot be 11 (since the number of neutrons \( N=A - Z \), if \( A = 11\) and \( Z = 11\), \( N=0\), which is not stable). Sodium - 22 has a mass of \( 22\ amu \), which is further from \( 22.99\ amu \) compared to sodium - 23.

Answer:

• amu: atomic mass unit.
• Isotope: sodium - 23.