This area chart displays atomic weight (average mass of all natural isotopes) across the periodic table. The smooth curve shows mass generally increases with atomic number.

While atomic weight mostly increases with atomic number, subtle variations exist. Argon (39.95) weighs more than potassium (39.10) despite having fewer protons, because argon's dominant isotope has more neutrons. Atomic weight is crucial for stoichiometry calculations and differs from mass number (protons + neutrons) due to isotope averaging and nuclear binding energy.