The terms "atomic weight" and "atomic mass" are often used interchangeably in chemistry, but there can be a subtle distinction between them depending on the context.
Atomic Weight: Atomic weight is the average mass of the atoms of an element, taking into account the natural abundance of its isotopes. It is expressed in atomic mass units (u) or grams per mole (g/mol). The atomic weights listed on the periodic table represent these average values.
Atomic Mass: Atomic mass refers to the mass of a single atom of an element. It is also expressed in atomic mass units (u). Atomic mass values are typically obtained by mass spectrometry or other experimental methods.
The atomic weight can be seen as a weighted average of the atomic masses of the isotopes of an element, considering their natural abundance. Since different isotopes have different masses due to the varying number of neutrons, the atomic weight accounts for the relative occurrence of each isotope in nature.
Now, regarding the difference of four units between atomic weight and atomic mass for most elements on Earth, this is primarily due to the presence of carbon-12 (^12C) as the reference isotope.
The atomic mass unit (u) was initially defined based on assigning a mass of exactly 12 atomic mass units to the ^12C isotope. Therefore, the atomic mass of carbon-12 is exactly 12 u by definition. However, the atomic weight of carbon, which considers the natural abundance of isotopes, is approximately 12.01 u.
The difference of approximately 0.01 u arises from the existence of other carbon isotopes, primarily carbon-13 (^13C) and a small amount of carbon-14 (^14C), which have slightly different masses. Since the atomic weight takes into account the abundance of each isotope, the contribution of the slightly heavier isotopes causes the atomic weight of carbon to be slightly higher than the atomic mass of carbon-12.
This discrepancy of four units (approximately) between atomic weight and atomic mass is not universal for all elements. It depends on the specific isotopic composition of each element and how it is measured. For some elements, the difference between atomic weight and atomic mass may be negligible, while for others, it can be more significant.
It's important to note that atomic weights can vary slightly depending on the isotopic composition of an element in different locations or sources. However, the periodic table typically lists average atomic weights that are generally applicable and representative for most purposes in chemistry.