Final answer:
The atomic number is the count of protons and is unique to each element, while the atomic weight is the weighted average of an element's isotopes based on their abundances and individual masses.
Step-by-step explanation:
The atomic number of an element is defined by the number of protons found in the nucleus of its atoms. It is unique to each element and serves as the identifying attribute that distinguishes one element from another. In contrast, the atomic weight, also known as atomic mass, is the weighted average of the masses of the isotopes of an element. This takes into account the different masses and relative percentages of each isotope.
To estimate an element’s atomic number, one simply identifies the number of protons in the nucleus of an atom, as this value is constant for all atoms of that element. On the other hand, the atomic weight is estimated by calculating the average mass based on the natural abundance and mass of each isotope. For instance, the periodic table will list the atomic number and the averaged atomic mass for each element.
For example, chlorine with an atomic mass of 35.45 indicates that it has isotopes with different mass numbers, primarily with atomic mass 35 and some with atomic mass 37, each having 17 protons but differing in the number of neutrons.