The alchemists thought about elements very differently from us. They believed that there were four elements: Fire, Water, Earth and Air, but unlike us, they saw the element more as a property associated with some stuff that it was imposed on. Often the empty stuff was called “first matter” and the quest of many alchemists was to dissociate the elements from matter in order to get something pure. They believed that matter had some balance of the elements in it, and in this way viewed matter the same way we might today view charge, or color.
One of the reasons why chemistry didn’t become an organized science for so long, despite being one of the most ancient practices of humanity had a great deal to do with wrongness of their approach (instead of studying and identifying different elements, they were constantly trying to examine it in terms of it’s fire/earth ratio, and to separate out it’s earthly properties). Another was that the ancients viewed weight as just another property, like color. When wood burns, the fire is released and the ashes get lighter. When copper calcifies (copper gets heated until it turns green) or iron rusts, they both get slightly heavier. As a result, it wasn’t until the 1700’s that the idea of conservation of matter started to gain traction.
We know elements now as distinct substances held in common by having a similar number of protons and electrons, and whose properties are largely caused by the shape and relative fullness of it’s outermost electron shell. Most of us were taught it that way from the beginning. What we may not have known was the first periodic table was created by Dimitri Mendeleev in the 1850’s, and people did not know much about atoms until the 1900’s. So suppose you were transported back in time 500 years, and were called upon to rediscover the elements and rebuild the periodic table: what might you do?
As it turns out, the one special piece of equipment you need to rediscover and rebuild the periodic table is a piece of equipment readily available in the 1500’s, one that a descent merchant (particularly in gold) should always posses: a good set of scales. All of the discoveries you need to make depend on your ability to weigh all of the reactants of your chemical processes and their products. You may have difficulty investigating gases and combustion, and in fact, this is one of the reasons the law of conservation of mass wasn’t discovered until the 1700’s. When items burned, they would appear to get lighter, but once they were burned in a large vessel (such as a sealed bottle), and the bottle was weighed before and after combustion, it appeared not to have lost any mass (the gases were still contained inside).
Next we need to consider how we are going to define an element. I will use the one that was used in the 1800’s. Any item that can’t be decomposed into two lighter components, and cannot be created from two lighter components by any chemical reaction will be tentatively held as an element. In this way, you can go outside and take a log from a tree and burn it. Since burning it produces ashes and smoke, “tree” is not an element. If you take dirt, you can pull water out of it, and separate metals from the soil using various chemical processes, so “dirt” is not an element. Once you are dealing with homogeneous substances, your task may get harder. For instance, water was considered an element until after hydrogen was discovered. It was only after hydrogen and oxygen were discovered that someone produced water by combining them, and removed hydrogen from the list of known elements.
So you’ve got your glassware, your scales and your chemical lab. Your list of tentative elements is growing, You may even think your job is done, but you’re not! Now you have to get the molecular weights of the elements and arrange them into a table, much like Mendeleev. Well, now that you’ve been weighing your reactants and products, and you’ve established some elements (like hydrogen), you have what it takes to start your periodic table.
You will notice when setting up chemical reactions, that for a given reaction with two or more ingredients, the ingredients consistently react in the same proportions. For instance, when for every gram of hydrogen, eight grams of oxygen react to make water. For every gram of hydrogen reacted to Fluorine to create hydrofluoric acid, nineteen grams of Fluorine react. When reacting hydrogen to oxygen to produce hydrogen peroxide instead of water, the ratio goes from 1:8 to 1:16. If you use hydrogen (the lightest known element) as your base, then you will have discovered the molecular weight of Oxygen. If you react oxygen with iron to produce Iron (II) Oxide (which is FeO), you will find that for every 18 grams of oxygen that react, 55.8 grams of Iron react, suggesting that Fe has a molecular weight of ~55-56. If you repeat this process, you will eventually rebuild the periodic table with the correct molecular weights.
The biggest thing stifling your experiments into molecular weight is the fact that atoms often react in uneven ratios. For instance rust consists largely of hydrated (possessing water) iron (III) oxides, which has a ratio of 2 Iron atoms to every 3 oxygen atoms. John Dalton, the man who proposed atomic theory based on the “Law of Multiple Proportions” (which is the law we are currently exploiting to build the periodic table) believed that oxygen had a molecular weight of 8, not 16, because he insisted that 1:1 ratios were how atoms bonded. Collecting and correlating larger amounts of data about reactions is the best way to avoid this error.
To conclude, many of the things we take for granted today had to be earned at great cost and struggle by our scientific ancestors. Ideas like the conservation of mass that seem so self-apparent now, were not intuitive then. Having the right presuppositions to our ideas makes discovery vastly easier, as evidenced by the short time that passed between the adoption of the law of conservation of mass and the modern world of physics and chemistry. After all, who would have thought that all we needed to build periodic table and to discover all the elements that could be chemically isolated, was a good set of scales?
