Final answer:
The atomic mass on the periodic table is a weighted average of all isotopes of an element and their abundances, unlike the mass number which is specific to each isotope. The atomic number is unique to each element and equals the number of protons in its atoms.
Step-by-step explanation:
The atomic mass given on the periodic table is different from the mass number of any particular atom. The atomic mass is a weighted average that takes into account all of the naturally occurring isotopes of an element and their respective abundances. This value is usually a decimal number and represents the mass of an average atom of the element when compared to the unified atomic mass unit (amu). It can also be understood as molar mass in grams per mole. In contrast, the mass number of an atom is the sum of the protons and neutrons in a single atom, which is always a whole number and specific to that isotope of the element. For instance, carbon has an atomic mass of approximately 12.01 amu on the periodic table, which considers both the prevalent Carbon-12 isotope and other naturally occurring isotopes like Carbon-13 and Carbon-14. However, the mass number of Carbon-12 is exactly 12, representing its composition of 6 protons and 6 neutrons.
Every element has a unique atomic number, which is equal to the number of protons in the nuclei of its atoms, and it appears above the element's symbol on the periodic table. The atomic number defines the element itself, distinguishing one from another. For example, all carbon atoms have an atomic number of 6, while all oxygen atoms have an atomic number of 8.