From basic atomic structure, we know that an atom’s\u00a0mass comes from\u00a0its number of protons plus its number of neutrons.\u00a0 Take fluorine, for example:\u00a0 its mass is\u00a019.\u00a0 It’s got 9 protons (because its atomic number is 9),\u00a0so it must have ten neutrons.<\/p>\n
But what about chlorine, whose mass is listed at 35.5?\u00a0 (Possibly, it says 35.453 or something like\u00a0that, depending on which periodic table you’re using.\u00a0 By the way:\u00a0 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.)\u00a0 If chlorine’s mass comes from its protons plus its neutrons, how can its mass be 35.5?\u00a0 You can only have whole numbers of protons and neutrons.<\/p>\n
The answer comes from the concept of an isotope.\u00a0 Isotopes are like different versions of the same type of atom.\u00a0\u00a0The number of protons is what defines the type of atom you have.\u00a0 We see chlorine’s atomic number is 17, so it must always have 17 protons.\u00a0 However, its number of neutrons can vary:\u00a0\u00a0about 75% of chlorine atoms have 18 neutrons, while the other 25%\u00a0have twenty.\u00a0 Therefore,\u00a03 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.\u00a0 Let’s take the average:<\/p>\n
massave<\/sub>=(35+35+35+37)\/4<\/p>\n Leading to:<\/p>\n massave<\/sub>=35.5<\/p>\n So that’s how you get a mass of 35.5.\u00a0 No chlorine atom actually has a mass of 35.5; some have a mass of 35, while others have mass 37.\u00a0 More have mass 35, bringing the average<\/em> to 35.5.<\/p>\n When you’re calculating the number of neutrons, you should round the mass to the nearest whole number, then subtract the atomic number.\u00a0 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.<\/p>\n