Atomic Mass Units, Subatomic Mass, and Isotopes - Quick Notes

Atomic Mass Unit (amu)

• AMU is the standard unit for atomic masses.
• 1 amu = mass of a proton or a neutron.
• Electron mass is negligible for atomic mass calculations; electrons do not contribute significantly to the mass.

Subatomic Particles and Mass

• Protons and neutrons have mass ~1 amu each.
• Electrons have negligible mass in the context of atomic mass calculations.

Atomic Number and Mass Number

• Atomic Number, $Z$ = number of protons.
• Mass Number, $A$ = number of protons + number of neutrons, so $A = Z + N$ where $N$ = number of neutrons.
• Isotopes: same $Z$, different $A$ (different numbers of neutrons).

Nuclide Notation

• Nuclide symbol convention:
  • Mass number $A$ in the upper left, element symbol in the middle, atomic details (like charge) included as appropriate.
  • General notation (common form): $^{A}_{Z}X$, where $X$ is the chemical symbol.
  • For ions, charge can be shown as a superscript, e.g., $X^{q+}$ or as $^{A}_{Z}X^{q+}$.
• Charge relation: $q = Z - E$ where $E$ is the number of electrons. For neutral atoms, $q = 0$.

Isotope Example: Oxygen-15

• Element: Oxygen, symbol $O$.
• Isotope name: Oxygen-15 (written as $^{15}O$ or $^{15}_{8}O$ since oxygen has atomic number $Z = 8$).
• For this nuclide: $A = 15, Z = 8, N = A - Z = 7$.
• Note: Isotopes of the same element have the same $Z$ but different $A$ and $N$.

Quick Takeaways

• Protons and neutrons determine mass; electrons contribute negligible mass.
• Atomic number $Z$ = protons; Mass number $A = Z + N$.
• Nuclide notation encodes mass number, atomic number, and element; isotopes differ in mass number.
• Oxygen-15 is an example of an isotope with $Z = 8$ and $A = 15$.