Isotopes of an element differ in neutron count but behave the same chemically. Relative atomic mass is the weighted mean of an element's isotopes, which is why most table values are not whole numbers.
Last week you turned two numbers into three counts, and one number on the chart was left alone: the one with a decimal in it. Copper reads 63.5. Chlorine reads 35.5. No nucleus can hold half a particle, so those figures cannot be counts of anything inside a single atom. The club has walked into the reason. Atoms of one element do not all carry the same number of neutrons, and the chart is reporting a mixture.
Isotopes are atoms of the same element that hold different numbers of neutrons. The proton count never moves, because the proton count is what makes the element. Change it and you have changed elements. Add or take away a neutron and you still have the same element, only a heavier or a lighter atom of it. Chlorine turns up as two common isotopes. Chlorine-35 holds 17 protons and 18 neutrons. Chlorine-37 holds 17 protons and 20 neutrons. The number after the name is the mass number, so subtracting still gives the neutrons.
A neutral atom carries as many electrons as protons, and both chlorine isotopes carry 17 of each. Reactions are electron business, and the nucleus takes no part. So chlorine-35 and chlorine-37 behave the same way in every test tube on the bench. Nothing they do in a beaker will separate them. An instrument that sorts atoms by mass will. That is the claim the rest of this term rests on. Chemistry is decided by electrons.
Weigh a real sample and you are weighing the mixture. Out of every 100 chlorine atoms, about 75 are chlorine-35 and about 25 are chlorine-37. Each isotope's share is called its abundance, and the shares fill the hundred between them, so one gives you the other. Find the mass of those 100 atoms first. 75 lots of 35 comes to 2625. 25 lots of 37 comes to 925. Together that is 3550 for 100 atoms. Divide by 100 and one average atom comes to 35.5. That is the relative atomic mass, and it is why the chart prints 35.5 when no chlorine atom weighs that.
A relative atomic mass always lands between the lightest and the heaviest isotope, and it sits closer to whichever one is more common. Copper's 63.5 tells you most copper atoms are copper-63 rather than copper-65. That gives you a free check on any answer. If your working puts the average outside the range of the isotopes, something has gone wrong. Relative atomic mass is a weighted average over a mixture, not a count of particles, which is why it is the one figure in the box that need not be a whole number.
Isotopes are atoms of one element carrying different numbers of neutrons. They react identically, because their electron counts match. The decimal figure on the chart is a weighted average over the mixture, which is why chlorine reads 35.5 when its atoms weigh 35 and 37.
The notebook is still open at the sodium page, where two numbers turned into three counts. Today the club opens a cylinder of chlorine gas, and the wall chart prints 35.5 beside it.
How many particles does one chlorine nucleus actually hold?
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24 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.