Final answer:
Isotopes are atoms with the same atomic number but different mass numbers due to varying neutrons. The mass number equals the sum of protons and neutrons. The atomic mass accounts for the weighted average of an element's isotopes and can be a fraction.
Step-by-step explanation:
What Are Isotopes?
Isotopes are atoms of the same element that have the same atomic number but different mass numbers. The atomic number (Z) corresponds to the number of protons in the nucleus, which defines the type of element. However, the number of neutrons can vary among atoms of the same element, leading to different isotopes. The mass number (A) is the sum total of protons and neutrons in the nucleus. By subtracting the atomic number from the mass number, one can determine the number of neutrons in an isotope.
Mass Number and Atomic Number
The mass number of an atom indicates the sum of the number of protons and neutrons. Although electrons are part of an atom, their mass is negligible when calculating the mass number. The atomic number helps scientists identify distinct elements. For example, all chlorine atoms have 17 protons, but isotopes of chlorine can have different numbers of neutrons, like 18 or 20. Hence, chlorine with 17 protons and 18 neutrons has a mass number of 35, while chlorine with 17 protons and 20 neutrons has a mass number of 37.
Understanding Atomic Mass
Evaluation of an element's atomic mass considers the average of the mass numbers for its naturally occurring isotopes, weighted by their relative abundances. For elements with multiple isotopes, their atomic mass will often be a fractional number, reflecting this weighted average.