The mass number of an atom represents the total number of protons and neutrons in its nucleus. Since the number of protons, also known as the atomic number, determines the element's identity, you can typically find it on the periodic table or by looking up the element. However, if you don't know the number of neutrons, you can still determine the mass number if you know the atomic number.
To find the mass number, you can use the following equation:
Mass number = Atomic number + Number of neutrons
In a neutral atom, the number of protons is equal to the number of electrons, so the atomic number also gives you the number of electrons. By subtracting the atomic number from the mass number, you can determine the number of neutrons.
For example, let's consider an atom with an atomic number of 6. If you don't know the number of neutrons, you can still determine the mass number.
Let's say the mass number is 'X':
X = 6 (Atomic number) + Number of neutrons
If you have additional information about the atom, such as its charge or isotope, you may be able to deduce the number of neutrons based on that information. Otherwise, without any further information, you cannot directly determine the number of neutrons or the exact mass number of the atom.