Answer:
Approximately [tex]0.0405[/tex] atomic mass units.
Explanation:
The actual mass of atoms like lithium-7 is less than the sum of the mass of the protons and neutrons that form the atom. The reason is that a significant amount of mass was released in the form of binding energy when the atom was initially formed. The mass defect of this atom refers to the energy difference between the mass of the atom and the sum of mass of the protons and neutrons in this atom.
The question provided the following information about this atom:
- Atomic number is [tex]3[/tex], which gives the number of protons in this atom, and
- Mass number is [tex]7[/tex], which gives the number of neutrons and protons, combined, in this atom.
The number of protons in this atom is the same as the atomic number. To find the number of neutrons in this atom, subtract the atomic number from the mass number:
- [tex](\text{Number of protons}) = (\text{Atomic Number}) = 3[/tex].
- [tex]\begin{aligned} & (\text{Number of neutrons}) \\ & = (\text{Mass Number}) - (\text{Atomic Number}) \\ &= 7 - 3 \end{aligned}[/tex].
The mass (atomic mass units) of protons and neutrons in this atom, combined, would be:
[tex]3 \times 1.007276 + (7 - 3) \times 1.008665[/tex].
To find the mass defect of this atom, subtract the atomic mass from the total mass of the constituents of this atom:
[tex](3 \times 1.007276 + (7 - 3) \times 1.008665) - 7.016003 \approx0.0405[/tex].
In other words, the mass defect of this lithium atom would be approximately [tex]0.0405[/tex] atomic mass units.
