The mass number of an atom is determined by the sum of its protons and neutrons. It is typically denoted by the letter "A." On the periodic table, the mass number is often listed as a decimal number below the element's symbol.
Atomic mass, also known as atomic weight, is the weighted average mass of all the naturally occurring isotopes of an element. It takes into account the relative abundance of each isotope and is expressed in atomic mass units (u). Atomic mass is represented by the symbol "M" or "Má´€" on the periodic table.
The key difference between atomic mass and mass number is that the atomic mass considers the average mass of all isotopes of an element, while the mass number specifically refers to a particular isotope.
Isotopes are atoms of the same element that have different numbers of neutrons, and they are written with the mass number to distinguish them. The mass number is written as a superscript to the left of the element's symbol or as a prefix followed by a hyphen and then the atomic symbol. For example, carbon-12 is written as ^12C, where 12 is the mass number and C is the symbol for carbon.