How did we find the number of electrons, protons, and neutrons in the past if we didn’t have the atomic number or weight?


How did we find the number of electrons, protons, and neutrons in the past if we didn’t have the atomic number or weight?

In: Chemistry

By analyzing the weight ratios of components in chemical reactions. Like H2 and O2 give you H2O, but it’s also possible to make H2O2 from those same components, given that you supply twice as much oxygen for the same amount of hydrogen. Why could that be? There aren’t that many possibilities to consider before you arrive at the corresponding chemical formulas for these compounds. And comparing the weight of H2 and O2 participating in the reaction you can figure out the ratio of the atomic weights of these elements. Piece by piece you’d be accumulating such information, until one day you figure out the periodic table. And after that it gets even more straightforward.

Early chemists used hydrogen as a base unit. Hydrogen was “1” and everything else was multiples of that. Since the natural state of atoms is electrical neutrality, they must have the same number of electrons as protons.

This use of hydrogen as a base unit didn’t always work out, but that was remedied when we discovered the neutron which accounted for those discrepancies.

If you measure the density of gases (at the same temperature and pressure) you’ll notice they have simple ratios. Helium is twice as dense as hydrogen, the lightest element, nitrogen is 14 times as dense, oxygen is 16 times as dense.

Similarly, you can notice that e.g. 1 gram of hydrogen reacts with 8 grams of oxygen to form 9 grams of water. They always react in that ratio. If you have more hydrogen then some hydrogen will be left over, if you have more oxygen then some oxygen will be left over. It’s not the same ratio of 16 from above because water is H2O, i.e. has twice as many hydrogen atoms as oxygen. Add many more reactions and you can assign relative masses to the atoms.

People sorted elements by mass and noticed that they form patterns. As an example, silicon (14th lightest element) reacts similarly to carbon (6), phosphorus (15) reacts similarly to nitrogen (7), sulfur (16) reacts similarly to oxygen (8), and so on. People started arranging the elements in what’s now known as periodic table. That’s a powerful tool, because now you can notice if you missed an element in between. If e.g. your “17th element” reacts like the 10th (i.e. not at all, because #10 is neon), then it’s probably the 18th element and you missed #17.

With better mass measurements people noticed that:

* sometimes the mass order is different from the order in terms of reactions
* some elements have atoms of different masses

This suggests two separate mechanisms. One that’s responsible for the chemistry (element number), and one that can influence the mass.

If you shoot particles at atoms you can notice that they have a very massive but tiny positively charged nucleus and then negative charges (electrons) around them. The electrons are responsible for the chemistry. An electrically neutral atom must have as many electrons as protons, so the element number is simply the number of protons. Regular hydrogen is simply a proton. Now you can put everything together:

Hydrogen has one proton, let’s call that mass “1” (a small fraction of atoms has one or even two neutrons). Helium as second element has 2 protons but mass 4, so it needs to have two neutrons (a small fraction of atoms has only one neutron). Carbon is element 6 and has mass 12 -> 6 protons and 6 neutrons.

Didn’t I say helium gas was twice as dense as hydrogen? This was indeed cause of some confusion early on. The problem is that hydrogen, oxygen, nitrogen and a few more gases form molecules of two atoms each, while helium does not (because it basically never reacts with anything).

Today you can simply put something in a mass spectrometer to measure the mass directly: You measure how much atoms are deflected by magnetic fields if you remove one of their electrons.

Once scientist knew that atoms were made up of a combination of protons neutrons and electrons, and figured out how that combination effected their properties [(Periodic law)](, they could say things like “I never had this element to test, but when I find it I know it’ll have these properties.

So whenever they *did* find an element with those properties, they had a good reason to believe they knew it’s number of electrons and protons.

Mostly by reaction ratios, like the stoichiometry (the thing where they reacted 2 ‘volumes’ of hydrogen to 1 volume of oxygen to make water) but there’s several ways to measure atomic mass.

I think we really started measuring it analytically with Millikan’s experiments with oil droplets though. He measured the charge of an electron with high precision.

From there you can calculate by deflecting oil droplets of a known substance. Different masses will deflect differently under the same charge. This is essentially how mass spectrometers work.

From there you can go from known masses to how many protons and neutrons it has by the amount it deflects.