Tutoring high school chemistry, isotopes are an early topic.  Let’s have a look….

From basic atomic structure, we know that an atom’s mass comes from its number of protons plus its number of neutrons.  Take fluorine, for example:  its mass is 19.  It’s got 9 protons (because its atomic number is 9), so it must have ten neutrons.

But what about chlorine, whose mass is listed at 35.5?  (Possibly, it says 35.453 or something like that, depending on which periodic table you’re using.  By the way:  you can always tell the mass because it’s the one that can have a decimal, whereas the atomic number is always a whole number.)  If chlorine’s mass comes from its protons plus its neutrons, how can its mass be 35.5?  You can only have whole numbers of protons and neutrons.

The answer comes from the concept of an isotope.  Isotopes are like different versions of the same type of atom.  The number of protons is what defines the type of atom you have.  We see chlorine’s atomic number is 17, so it must always have 17 protons.  However, its number of neutrons can vary:  about 75% of chlorine atoms have 18 neutrons, while the other 25% have twenty.  Therefore, 3 out of 4 chlorine atoms (which is 75%) have a mass of 35, while the other 1 out of 4 has a mass of 37.  Let’s take the average:

mass_ave=(35+35+35+37)/4

Leading to:

mass_ave=35.5

So that’s how you get a mass of 35.5.  No chlorine atom actually has a mass of 35.5; some have a mass of 35, while others have mass 37.  More have mass 35, bringing the average to 35.5.

When you’re calculating the number of neutrons, you should round the mass to the nearest whole number, then subtract the atomic number.  For example, to calculate the neutrons in boron (atomic number 5, mass 10.82), we would round the mass to 11 and subtract 5, giving 6 neutrons.

Jack of Oracle Tutoring by Jack and Diane, Campbell River, BC