1. Which contributes more to an atom’s mass, electrons or protons? Which contributes more to an atom’s volume (its size)?
2. Which is represented by a whole number—the mass number or the atomic mass?
3. Do two isotopes of iron have the same atomic number? The same atomic mass number?
1. Protons contribute more to an atom’s mass; electrons contribute more to its size.
2. The mass number is always given as a whole number, such as hydrogen-1 or carbon-12. Atomic mass, by contrast, is the average mass of the various isotopes of an element and is thus represented by a fractional number.
3. The two isotopes of iron have the same atomic number, 26, because they each have 26 protons in the nucleus. They have different atomic mass numbers if they have different numbers of neutrons in the nucleus.