This paper offers a clear framework for understanding basic atomic concepts.Throughout the manuscript, mass expressions are described as scalar quantitiesthatare characterized by a symbol, a numerical value, and a unit of measurement.It defines the role of the IUPAC ratio,explains the meaning of absolute and relative atomic mass, and determines the origin and relationship among molar mass, Avogadro’s number, and the amount of substance. The approach moves from isolated isotopes to poly-isotopic elements, and from absolute atomic mass to relative atomic mass when applicable.The atomic mass ratio, , plays an important role in this work. Absolute atomic mass, a magnitude expressed in kilograms, differs from relative atomic mass, denoted by the numerical value with the unified atomic mass unit . The relationship between the ratio and the periodic table of elements is highlighted, emphasizing factors, besides the controversy between atomic mass and atomic weight, that may confuse users of the table.The concept of any mass quantity,,is introduced to clarify the meaning of the atomic mass-weighted average. Furthermore, the ideain conjunction with the ratio..is used to demonstrate the origin and relationship among molar mass, Avogadro’s number, and the amount of substance.

scalar quantity, IUPAC ratio A_r, absolute and relative atomic mass, molar mass, Avogadro´s Number, amount of substance