# How did they figure out the periodic table of elements before they had electron microscopes?

A STAFF REPORT FROM THE STRAIGHT DOPE SCIENCE ADVISORY BOARD

Dear Straight Dope:

I am wondering how in the world the person who developed the periodic table of elements was able to determine how many electrons and protons and such were in each elemental atom. This table was started in the days before electron microscopes and computers, so how did they know? I have asked science teachers and librarians but no one can answer this question. Surely you can.

CKDextHavn replies:

You just have to know who to ask. The following was contributed by my son, currently working on a master’s degree in biochemistry:

The short answer is, when they first developed the periodic table, they had NO IDEA how many protons or electrons were in each element. The original periodic table wasn’t based on the number of protons in each elemental atom, as it is today. The original periodic table, developed in 1871 by Dmitri Mendeleev, was based on the periodic law, which claimed at the time that repeating characteristics (hence, “periodicity”) are seen among the elements when they are listed according to atomic mass. For example, lithium, sodium, and potassium have similar sorts of reactions, as do chlorine, bromine, and iodine.

Atomic mass was defined as the mass of the atom, which was determined by chemical experiments I won’t describe here. Scientists at the time thought the atom was sort of like a billiard ball, one solid unbreakable chunk of matter. Different atoms had different weights (mass) ’cause they were thought to be different types of billiard balls. We now know atomic mass is the total of all the subatomic particles that make up an atom–protons, neutrons, and electrons. As it turns out, electrons have an insignificant amount of mass compared to the protons and neutrons, so the atomic mass is really just the combined masses of protons and neutrons.

It was later determined that the number of protons in an atom is its atomic number. Atomic mass and atomic number are related, but not the same thing. The atomic mass is the atomic number plus the number of neutrons in the nucleus. The number of neutrons has very little relevance to the chemical properties of an element and has much more room for variance. Thus, atomic mass is not always a perfect indicator of how many protons are in an element. Because it was based on this faulty premise, there were a few problems with the original periodic table.

The most illustrative of these was the problem with argon and potassium. The atomic mass of argon is higher than that of potassium. However, when ordered by atomic mass in the original periodic table, potassium was listed as a noble gas (at one time called inert gases), which was clearly not correct. It didn’t behave like one at all. Similarly, the table predicted that argon would have similar properties to lithium and sodium, with which it had very little in common. So they switched the ordering of those two to maintain the periodicity. However, they had no theoretical basis for this anomaly whatsoever.

The problem wasn’t solved until 1913, when H. G. J. Moseley discovered atomic numbers. At that point, the solution became clear: Argon had a lower atomic number (fewer protons), even though it had the higher mass. (It turned out that argon had more neutrons than potassium, but they didn’t discover neutrons until 1932.) This led to a new definition of the periodic law, specifically, that periodicity among the elements is seen when they are listed according to atomic number.

Thus it was over 40 years after the creation of the periodic table before they actually started ordering it by the number of protons in the elemental atom. (And even then, they didn’t quite know what it meant. Protons weren’t specifically discovered until 1919!)

Hope that clears it up for you.

–Son of

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