Isotopes

Objective: Calculate neutron number and separate isotopes from ions.

Mass number A is the count of protons plus neutrons in a specific nuclide; neutron number is A − Z. Isotopes of one element have the same proton number but different neutron counts, whereas ions differ in electron count. Carbon-12 has six protons and six neutrons; carbon-14 has six protons and eight neutrons. The periodic-table atomic mass is often a weighted average of naturally occurring isotope masses, not the mass number of every atom. Isotopes can differ in stability and physical properties while sharing many chemical tendencies because their electron structures can be similar. A change in neutron count does not make a new element.

Objective: Apply isotopes to calculations, evidence, and specified situations.

Isotopes have the same atomic number but different neutron counts. A weighted atomic mass is the sum of isotope masses multiplied by fractional abundances; abundance fractions must sum to one. An integer mass number is a nucleon count, not the measured isotopic mass in unified atomic mass units. Problems here explicitly give simplified isotope masses for arithmetic; real atomic masses require measured isotopic masses. A sample’s isotope proportions can differ from a listed natural average.