Final answer:
The most abundant isotope of argon in nature, based on the periodic table's atomic weight of 39.948 g/mol, is argon-40 (Ar-40).The correct option is A.
Step-by-step explanation:
Argon has three naturally occurring isotopes: argon-36 (Ar-36), argon-38 (Ar-38), and argon-40 (Ar-40). The atomic weight of argon on the periodic table is 39.948 g/mol.
This value is a weighted average of all the naturally occurring isotopes of argon and indicates which isotope is most abundant in nature.
Looking at the atomic weights of the isotopes individually, Ar-36 and Ar-38 are both lighter than the averaged atomic weight, while Ar-40 is the closest isotope to this weighted average. Thus, the most abundant isotope of argon in nature is argon-40 (Ar-40).
The concept behind this determination is similar to how we understand other elements like chlorine and neon, where the isotope closest in mass to the weighted average on the periodic table is the most abundant. For example, with chlorine having natural abundances of 75% chlorine-35 and 25% chlorine-37, the average atomic weight reflects the weighted natural abundance of these isotopes. The correct option is A.