Atomic Structure:The Nucleus Atomic number (Z) Number of protons in the atom's nucleus All atoms of a given element have the same atomic number Mass number (A) Number of protons plus neutrons in the atom's nucleus Isotopes Atoms of the same element (same Z)that have different Numbers of neutrons and therefore different mass numbers(different A) Atomic mass (atomic weight) -Weighted average mass in atomic mass units (amu)of an element's naturally occurring isotopesAtomic number (Z) ▪ Number of protons in the atom's nucleus ▪ All atoms of a given element have the same atomic number Mass number (A) ▪ Number of protons plus neutrons in the atom’s nucleus Isotopes ▪ Atoms of the same element (same Z) that have different Numbers of neutrons and therefore different mass numbers (different A) Atomic mass (atomic weight) ▪ Weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes Atomic Structure: The Nucleus