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The periodic table is one of those well-known classic images that you find in many science labs, classrooms and your science textbook. It is an image almost everyone has seen at some time in their life. You can also find the periodic table on various items such as t-shirts, mugs, beach towels, pillowcases and duvet covers, and plenty of other items. It even inspired a collection of short stories or rhymes.
No one can forget the periodic table put to music by the American Tom Lehrer, a Harvard mathematics professor who was also a singer or songwriter and satirist. His song, The Elements, includes all the elements that were known at the time of writing in 1959.
Since then, several new elements have been added to the periodic table, including the four that were formally approved last year by the International Union of Pure and Applied Chemistry (IUPAC).
But what exactly does the periodic table show?
In brief, it is an attempt to organise the collection of the elements – all of the known pure compounds made from a single type of atom. There are two ways to look at how the periodic table is constructed, based on either the observed properties of the elements contained within it, or on the subatomic construction of the atoms that form each element.
When scientists began collecting elements in the 1700s and 1800s, slowly identifying new ones over decades of research, soon they began to notice patterns and some similarities in the physical properties. Some were gases, some were shiny metals, and some reacted violently with water, and so on.
At the time when elements were first being discovered, the structure of atoms was not known. Scientists began to look at ways to arrange them systematically so that similar properties could be grouped together, just as someone collecting seashells might try to organise them by shape or colour. The task was made more difficult because not all of the elements were known. This left gaps, which made deciphering patterns a bit like trying to assemble a jigsaw puzzle with missing pieces.
Different scientists came up with different types of tables. The first version of the current table is generally attributed to Russian chemistry professor Dmitri Mendeleev in 1869, with an updated version in 1871.
Importantly, Mendeleev left gaps in the table where he thought missing elements should be placed. Over time, these gaps were filled in and the final version as we know it today emerged.
To really understand the final structure of the periodic table, we need to understand a bit about atoms and how they are constructed. Atoms have a central core (the nucleus) made up of smaller particles called protons and neutrons. It is the number of protons that gives an element its atomic number – the number generally found in the top left corner of each box in the periodic table.
The periodic table is arranged in order of increasing atomic number (left to right, top to bottom). It ranges from element 1 (hydrogen H) in the top left, to the newly approved element 118 (oganesson Og) in the bottom right. The number of neutrons in the nucleus can vary. This gives rise to different isotopes for every element.
For example, you may have heard of carbon-14 dating to determine the age of objects. This isotope is a radioactive version of normal carbon C (or carbon-12) that has two extra neutrons.
But why is there a separate box of elements below the main table, and why is the main table an odd shape, with a bite taken out of the top? That comes down to how the other component of the atom – the electrons – are arranged.
We tend to think of atoms as built a bit like onions, with seven layers of electrons called “shells”, labelled K, L, M, N, O, P, and Q, surrounding the core nucleus.
Each row in the periodic table sort of corresponds to filling up one of these shells with electrons. Each shell has subshells, and the order in which the shells/subshells get filled is based on the energy required, although it’s a complicated process. We’ll come back to these later.
In simple terms, the first element in each row starts a new shell containing one electron, while the last element in every row has two (or one for the first row) of the subshells in the outer shell fully occupied. These differences in electrons also account for some of the similarities in properties between elements. With one or two subshells in the outer layer full of electrons, the last elements of each row are quite unreactive, as there are no holes or gaps in the outer shell to interact with other atoms.
This is why elements in the last column, such as helium He, neon (Ne), argon (Ar) and so on, are called the noble gases (or inert gases). They are all gases and they are “noble” because they are not usually associated with other elements.
To summarize all of this, the periodic table is the chemist’s taxonomy of all the elements. It is the triumph that it is still highly relevant to scientists, while also being included in popular culture.
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