Final answer:
Argon's isotopes, with mass numbers 36, 38, and 40, differ by the number of neutrons in their nuclei. They all have the same atomic number (18) but varying neutron counts: 18, 20, and 22 neutrons respectively, which gives them different mass numbers but identical chemical properties.
Step-by-step explanation:
The naturally occurring isotopes of argon with mass numbers 36, 38, and 40 differ only in the number of neutrons they contain in their nuclei. Each argon isotope has the same number of protons, which is 18, because the atomic number of argon is constant; however, Argon-36 has 18 neutrons, Argon-38 has 20 neutrons, and Argon-40 has 22 neutrons. This variation in neutron number leads to different mass numbers while maintaining identical chemical properties.
Isotopes are written as the element symbol followed by the mass number (e.g., Ar-36, Ar-38, Ar-40). The prefix 'Ar' represents argon, and the respective numbers indicate the total mass of protons and neutrons in the nucleus of the isotope. Since the chemical properties are dictated by the electron configuration and number of protons, which are the same for all isotopes of a given element, all argon isotopes exhibit the same chemical behavior despite having different mass numbers.
The atomic weight of an element is a weighted average of the mass numbers of its isotopes, based on their natural abundance on Earth. For argon, although Argon-40 is the most abundant, the contribution of Argon-36 and Argon-38 is also factored into its average atomic mass. This value is typically not a whole number due to the presence of these multiple isotopes in varying ratios.