The terms "atomic weight" and "actual mass" refer to different aspects of an element's characteristics.
Atomic Weight: The atomic weight of an element is a weighted average of the masses of its naturally occurring isotopes. Isotopes are variants of an element that have the same number of protons but different numbers of neutrons. Since different isotopes have different masses, the atomic weight takes into account the relative abundance of each isotope in nature. The atomic weight is listed on the periodic table of elements and is typically represented as a decimal number.
Actual Mass: The actual mass of an element refers to the specific mass of an individual atom of that element. It is usually expressed in atomic mass units (amu) or grams. The actual mass can vary from one atom to another because different isotopes of an element can have different masses. However, when discussing a specific isotope, its actual mass refers to the sum of the masses of its protons, neutrons, and electrons.
To illustrate the difference, let's take the element carbon as an example. Carbon has three naturally occurring isotopes: carbon-12, carbon-13, and carbon-14. Carbon-12 is the most abundant isotope, followed by carbon-13 and carbon-14. The atomic weight of carbon, as listed on the periodic table, is approximately 12.01 amu. This value takes into account the relative abundance of each isotope.
On the other hand, the actual mass of an individual carbon-12 atom is precisely 12 amu, since carbon-12 has 6 protons and 6 neutrons. Carbon-13 and carbon-14 atoms have actual masses of approximately 13 amu and 14 amu, respectively, due to their differing numbers of neutrons.
In summary, the atomic weight represents the average mass of all the naturally occurring isotopes of an element, while the actual mass refers to the specific mass of a particular isotope.