Atomic weight is defined as which of the following?

Atomic weight is defined as which of the following?

• A. The simple average of isotopes
• B. The mass number of the most abundant isotope
• C. The weighted average of the isotopes based on natural abundance
• D. The number of protons

Answer: (C)

Atomic weight is the weighted average mass of an element’s naturally occurring isotopes, taking into account how common each isotope is. Isotopes differ in the number of neutrons, so their masses aren’t all the same. To get a single value for the element, you multiply each isotope’s mass by its natural abundance and add them up. This is why the atomic weight isn’t a whole number or just the mass number of the most abundant isotope. It’s also not the number of protons—that number is the atomic number and defines the element, not its weight. For example, chlorine has two main isotopes with different masses and abundances, which together give an atomic weight around 35.45 amu. If you did a simple average assuming equal amounts, you’d get a different number, which wouldn’t reflect nature’s actual isotopic mix.

Atomic weight is the weighted average mass of an element’s naturally occurring isotopes, taking into account how common each isotope is. Isotopes differ in the number of neutrons, so their masses aren’t all the same. To get a single value for the element, you multiply each isotope’s mass by its natural abundance and add them up. This is why the atomic weight isn’t a whole number or just the mass number of the most abundant isotope. It’s also not the number of protons—that number is the atomic number and defines the element, not its weight. For example, chlorine has two main isotopes with different masses and abundances, which together give an atomic weight around 35.45 amu. If you did a simple average assuming equal amounts, you’d get a different number, which wouldn’t reflect nature’s actual isotopic mix.