No, the mass number of an element is not always equal to the relative atomic mass rounded. The mass number represents the total number of protons and neutrons in the nucleus of an atom, while the relative atomic mass (also known as atomic weight) is the average mass of all the isotopes of an element, taking into account their relative abundances.
In most cases, the relative atomic mass is very close to a whole number because the atomic weight is determined by the abundances of the different isotopes of an element, and the isotopes with whole number mass values tend to be more abundant. However, there are some elements that have isotopes with significant fractional abundances, which can lead to a non-whole number relative atomic mass.
One example is chlorine (Cl), which has two stable isotopes: chlorine-35 and chlorine-37. Chlorine-35 is more abundant, with an abundance of about 75.77%, while chlorine-37 has an abundance of about 24.23%. The mass numbers of chlorine-35 and chlorine-37 are 35 and 37, respectively. When you calculate the relative atomic mass using the abundances and mass numbers of the isotopes, you get:
(0.7577 * 35) + (0.2423 * 37) = 35.453
So the relative atomic mass of chlorine is 35.453, which is not a whole number and differs from the mass number of either isotope.