The Periodic Table of the Chemical Elements is a cultural icon and an extraordinary object in science space.
This page explores what the periodic table is in terms of basic & simple elemental substance and the philosophy of science.
Two classic periodic tables:
WebElements employs the most common, of many possible formulations that can be explored using the INTERNET Database of Periodic Tables and Periodic Table Formulations.
The vast majority of Periodic Tables are used to arrange physical, chemical, technological & historical data/information about the chemical elements in a systematic way.
The chemogenesis web book explores how chemical reactivity emerges from the periodic table of the elements using a root-trunk-branch chemistry-tree metaphor, with the periodic table at the base of the trunk:
The periodic schema is used in many ways and so it is interesting to ask what the periodic table actually is, and what it is showing?
It transpires that matters are a little more involved than they may at first appear...
Philosophy, Chemistry & The Periodic Table
Philosophers of chemistry consider the chemical elements in two distinct ways:
Firstly, there is the chemical element as the basic element, that is the abstract or transcendental element, the essence of the element, a bearer of properties but not having any actual properties, except for [historically, atomic mass, but now] atomic number Z. Chemical symbols, such as H and Cu, are assigned to the basic element.
Secondly, there is the element as simple substance, for example, a real piece of copper metal placed on a table has numerous, measurable, intrinsic properties such as: purity, density, conductivity, colour, melting point, molar volume, etc.
Crucially, only the basic element survives in a compound. Sodium's metallic properties and chlorine, the green gas, do not exist in the ionic salt, sodium chloride.
- There is a metaphysical view about the nature of the elements as basic substances and bearers of properties that goes back to the ancient Greeks, long before the discovery of atoms.
- Mendeleev insisted that his periodic classification system concerned the elements as basic substances possessing only one attribute, atomic weight.
- Paneth – one of the founders of modern radiochemistry – took Mendeleev's view about the nature of basic and simple substance, but changed the basic/transcendental/abstract property of an element from atomic weight/mass to atomic number, Z.
- Elements as basic substance represent natural kinds, a well understood philosophical position concerning the nature of classification. Elements as simple substances fail the natural kind test, due to the existence of isotopes and allotropes, etc.
Eric Scerri points out that the periodic table has, at times, been characterised as a:
- ordered domain
This author agrees with Scerri that the periodic table is an ordered domain. But it is also a schema and it is a map, that that can be used to organise information, data & knowledge concerning the chemical elements.
Morphing Multi-Periodic Tables
Three initial periodic tables can be identified:
- The periodic table of basic (abstract/transcendental) substances.
- The periodic table of gas phase atoms with their associated spectra. This is the simplest form that the element as the simple (real) substance can take.
- Then there is the periodic table of chemicals in bottles: these are the elemental materials under standard conditions: 25°C and 1.0 atmosphers of pressure (298K and 100kPa).
These three periodic tables morph into each other to give the compound object commonly presented as The Periodic Table. When moving across these various PTs the system complexity increases. Different periodic tables emphasise different aspects.
There is a real problem that needs to be understood. A periodic table may state that oxygen has a an atomic number (Z) of 8, a mass of 15.999 au and a boiling point of –183°C. However, the boiling point is actually referring to the material substance oxygen, which exists as the diatomic molecule O2.
There are many other periodic tables:
- Periodic tables showing phase changes: melting points, boiling points, etc., that show the real materials behave at 1.0 atm but not at a standard temperature
- Dates of discovery
- NMR properties
- Abundance in the Earth's crust and abundance in the solar system
- Methods of extraction & purification
Periodic Table of Basic (Abstract, TRANSCENDENTAL) Elements
Periodic tables generally show the chemical elements as the basic substance:
The usual periodic table schema simply shows the element symbols in their respective periods, groups & blocks. This is the periodic table as Mendeleev would have intended it: a schema showing the elements as basic substances with their positions in the schema emphasising the periodic law:
"The periodic law is the principle that certain properties of elements occur periodically when arranged by atomic number. These similarities can be reflected best by a table, so that commonalities between elements appear both in rows and in columns of the table." Wikipedia
The basic element has only one property, atomic number, Z. The element name & symbol are assigned [mapped to] Z.
Periodic Table of Gas Phase Atoms
The periodic table of ground state gas phase atoms is known; this is the periodic table of the very simplest of simple (real) substances. In this state the chemical elements are physically normalised with respect to each other.
Ground-state, monoatomic gas phase atoms of the material substance
- For some elements this state is trivial. The group 18 rare gases: He, Ne, Ar, Kr, Xe & Rn are naturally mono-atomic, gas phase entities under standard conditions 25°C and 100 kPa.
- Oxygen is a diatomic molecular gas, O2, at room temperature, so it must be converted (atomised) into an atomic gas. Likewise with the diatomic molecules of hydrogen, H2, nitrogen, N2, fluorine, F2, chlorine, Cl2, bromine, Br2 & iodine, I2.
- Some metals, such as mercury (Hg, boiling point 356.7°C) are easy to atomise, others such as tungsten (W, boiling point 5555°C) are more difficult.
- Carbon boils at 4027°C and it difficult to obtain a vapour of ground-state carbon atoms, carbon gas, but it is possible.
Apart from the title, the graphic is exactly the same as the periodic table of basic substances, but this periodic table represents real chemical entities with actual, measurable, physical properties, including:
- Atomic radii
A brief word should be said about atomic radii. There are several ways of considering (defining) atomic radii, including: van der Waals, ionic, covalent, metallic & Bohr. Here we are considering the radii of the gas phase atoms. However, gas phase atoms to not have a sharply defined edge, and so the radii must be calculated with a pre-determined cut-off. The calculated radii (above) are usually taken from Enrico Clementi's work.
Periodic Table Chemical of Chemicals in Bottles
Under standard conditions, 25°C & 1.0 atm, the chemical elements as simple substances – real chemicals in bottles – present as:
- Gases, liquids & solids
- Metals, metalloids & non-metals:
- Metallic materials, network covalent materials & molecular materials
The chemical elements as material substances have many, many properties, including:
- Properties at standard conditions 25°C and 1.0 atm, including: crystal structure, molar volume, hardness, etc.
- Phase changes under non-standard conditions: melting point, boiling point, critical point, etc.
- Abundance, history, biology: