Atomic Number and Atomic Mass The atomic number(Z)is the number of protons in the atom's nucleus The mass number (A)is the number of protons plus neutrons All the atoms of a given element have the same atomic number ■ Isotopes are atoms of the same element that have different numbers of neutrons and therefore different mass numbers The atomic mass of an element is the weighted average mass in atomic mass units (amu)of an element's naturally occurring isotopes Atomic Number and Atomic Mass „ The atomic number ( Z) is the number of protons in the atom's nucleus „ The mass number ( A) is the number of protons plus neutrons „ All the atoms of a given element have the same atomic number „ Isotopes are atoms of the same element that have different numbers of neutrons and therefore different mass numbers „ The atomic mass of an element is the weighted average mass in atomic mass units (amu) of an element’s naturally occurring isotopes