To calculate the atomic mass of element X, given that it has two isotopes with mass numbers 79 and 81, and the lighter isotope has a proton number of 35 and an abundance of 50.69%, we can follow these steps:
Determine the neutron numbers: Since the mass numbers of the isotopes are given as 79 and 81, subtract the proton number (atomic number) from the mass number to find the neutron numbers. For the lighter isotope with a proton number of 35, the neutron number is 79 - 35 = 44.
Calculate the weighted average atomic mass: Multiply the mass of each isotope by its abundance (expressed as a decimal), and then sum up these values.
For the lighter isotope (mass number 79, proton number 35): Mass = 79 atomic mass units (AMU) Abundance = 50.69% = 0.5069
For the heavier isotope (mass number 81, proton number unknown): Mass = 81 atomic mass units (AMU) Abundance = (100% - 50.69%) = 49.31% = 0.4931
To calculate the weighted average atomic mass: Atomic mass = (Mass of isotope 1 * Abundance of isotope 1) + (Mass of isotope 2 * Abundance of isotope 2) = (79 AMU * 0.5069) + (81 AMU * 0.4931)
Calculating this expression: Atomic mass = (40.0271) + (39.8011) = 79.8282 AMU
Therefore, the atomic mass of element X is approximately 79.8282 atomic mass units (AMU).