Did you know 2019 is the Year of the Periodic Table and its 150th birthday? Me neither! It has to be one of the least publicised “Year Of’s” and yet one of the most important. Dmitri Mendeleev’s creation attracts me in two respects, first for the science and second for the use of the highly visual illustration to simply explain it. The Table has evolved in to the colourful all-in-one-page presentation of data with shapes or pictures that I like – think Infographic, think London Tube map.
The Periodic Table is wonderful in that it answers so many questions about physical science, and if all that you know about Chemistry is the Periodic Table and the answers to the questions below, then you are well on the way to a GCSE Chemistry pass. As an adult you are welcome to this understanding but please feel free to skip to the “fascinating facts” towards the end and find Mendeleev’s position in the history and philosophy of science.
What is an element?
An element is a substance that contains only one type of atom, such as hydrogen; in contrast to a compound which contains more than one type of atom, such as H2O. A molecule contains more than one atom – of the same type such as O2, or different types such as H2O).
Some elements have an obvious single letter and some don’t; why is that?
Hydrogen and oxygen simply are called H and O whereas Magnesium is Mg and Potassium even more strangely is K (from the Latin Kalium). So there are many reasons, for instance Beryllium, Boron and Bromine couldn’t all be B.
What’s the difference between a Group and a Period?
The Groups are the downward columns and the Periods run across. Groups generally have elements of similar properties like Group 1 metals and Group 7 Halogen gases. But the properties from left to right of a Period are completely different e.g. from metallic to gaseous. The common factors in Periods is the electronic shell, so the second Period 2 is the second electronic shell.
What’s the difference between the top number and the bottom number of an element?
The top number is the atomic mass (A.M.) while the lower one is atomic number (A.N.). The atomic mass is the number of protons and neutrons while the atomic number is just the number of protons (and also electrons). So sodium has 11 protons and 11 electrons (A.N. 11) and adding in the 12 neutrons makes A.M. 23. The modern Periodic Table is in the order of atomic number; Mendeleev ordered elements by atomic weight (became mass) which in the end is very similar.
What is the order of elements?
As you go across the table left to right, the atomic number increases by 1 each element, going from Hydrogen (A.N. 1) to element 118, Oganesson (Og), formerly Ununoctium (UUO, A.N. 118). Atomic mass also increases, albeit sometimes by more than 1. Along with many elements towards the end of the table, UUO is unstable and in fact only 3 atoms of it have been produced since 2002. When Mendeleev first published his Table in 1869, he left some gaps, but made predictions of properties which in due course did fit new elements such as Group 3 Gallium.
Can we use the Periodic table to identify metals and non-metals?
Broadly the metals are on the left and in the centre while the non-metals are on the right.
The transition metals in the middle don’t seem to follow the group number pattern. Why?
At GCSE level we just mainly consider the first four periods and so for example period 2 group 1 e.g. Lithium and group 2 e.g, Beryllium then skip over the transition metals to group 3 e.g Boron and on through groups 4,5,6,7 to the final column for Nobel gases.
Why is the final group called group 8 sometimes, but also group 0 ?
This gets to the heart of the electronic structure of periodic table. The common factor of the final columns is that all the elements have stable outer electronic shell configurations which at GCSE level generally means 8 electrons in the outer shell, and so zero in electrons in the next shell.
So what other parts of the periodic table relate to electronic structure?
Sodium’s electronic shell structure
The group number determines the number of electrons in the outer shell (and vice versa). So group 1 metals have 1 electron in the outer electronic shell, and for instance sodium is A.N. 11; so its 11 electrons have configuration 2,8,1 Then group 2 elements have 2 electrons in the outer shell, and so on through group 4 with 4, and group 7 halogens with 7 in the outer shell.
Does group number determine the type of reactions elements have?
Absolutely! Group 1 elements are keen to release their single outer shell electron to go back to a stable outer shell of 8 and so react strongly to, for instance, water and acids to form ionic compounds in which the metal ion has a charge of 1+. Meanwhile group 7 halogens are adept at gaining the one electron for a stable outer shell. So Na+ and Cl- from an ionic bonded compound whereas chlorine bonds covalently with hydrogen or itself by sharing rather than exchanging an electron. Group 0 (or 8) Noble gases like argon are inert (they barely react) because they are already content with their full outer shell.
Can we predict which elements will form multiple bonds from the position in the Periodic Table?
Yes, oxygen in group 6 has 6 outer shell electrons and so needs 2 more and forms a double bond with itself or two bonds with hydrogen (which needs 1 electron) to form H2O (water). So the very familiar formula of water owes its existence to the position of hydrogen and oxygen in the Periodic Table. Nitrogen in Group 5 needs 3 more electrons so shares them with three hydrogen atoms to from the very familiar ammonia NH3. At the other end of the 2nd period, Beryllium in Group 2 cannot be bothered to gain 6 to make 8, rather it loses 2 to from the Be2+ ion, That is why group 2 metals like magnesium form 2+ ions.
Group 1 metals get more reactive as you go down the group whereas Group 7 halogens get less reactive. How does the periodic table explain this?
As you go down a group the atom gets bigger so the outer electron shell is further away from the positive nucleus. For metals such as potassium this means it is easier to prize away an electron from the claws of the nucleus than it is with the smaller lithium. On the other hand the larger iodine is less willing to accept an additional electron than chlorine, because for iodine the positive nucleus is further way from the incoming electron.
Some elements like chlorine have a decimal place in the atomic mass whereas as carbon does not. Why?
Isotopes is the answer. Chlorine has a 35 A.M. isotope and an 37 A.M. isotope in the ratio 75% : 25% and so the weighted average is 37.5. Carbon has several isotopes such as the carbon dating isotope C14 but they are in tiny proportions so the base isotope of C12 is used in the Table.
Why is Period 1 only 2 elements?
Hydrogen and Helium have respectively 1 and 2 electrons, after which the first shell is full and we move to the second shell which has the more familiar 8. Hydrogen and Helium are the main constituents of the Sun and indeed the Universe, which begs the question, are there any elements in space not in the Periodic Table? You will find the answer in the final section, Strange Facts (about the Periodic Table)
So which Periods and Groups are important for GCSE?
For the first three periods i.e from elements H to Ar you should know each element in detail and arguably be able to recite and know their properties, reactions and electronic structure. Equally Groups 1 (alkali metals), 7 (Halogens) and 8 (0) Nobel Gases (and to a lesser extent Group 2 Metals) are important to understand in detail, and for these Groups extend your knowledge to Period 4 as well, for instance down to potassium and bromine.
For transition metals in the middle you don’t have to know their groups, periods or electronic configurations, but should be aware of their names and properties. For example copper, which conducts electricity and has highly coloured compounds like its sulphate, and which features in core experiments. You do not need to know the details of radioactive elements but should understand the principles of radioactive decay.
Strange facts concerning the Periodic Table
Mendeleev’s elements – the Donald Rumsfeld of his day?
The Table has the look of a Patience card game. This is not a coincidence because Mendelev was a card player and initially sorted the elements by atomic mass, wrote them on cards, and placed them in columns of similar properties and increasing weight.
Eek! He initially missed out around a third of the elements because they had not been discovered but he was able to predict some of the missing element properties. When Mendeleev proposed his periodic table, he noted gaps in the table and predicted that then-unknown elements existed with properties appropriate to fill those gaps. He called them eka-boron, eka-aluminium, eka-silicon, and eka-manganese. Eka aluminium for instance correctly foretold the discovery later of Group 3 Gallium, with the atomic mass of 69 and density 6 times that of water – very close to what he had predicted. Eka-silicon correctly became Germanium (atomic mass 72).
On the other hand he made no space for Group 8 Nobel gases – in a sense an omission but, being inert, they hadn’t fully been discovered yet because often elements were discovered by their reactions. Another reason for omissions may have been that he was running out of time to publish, especially since other versions and lists were beginning to be circulated.
Iodine (127) has a lower atomic mass than tellurium (128). So iodine should be placed before tellurium in Mendeleev’s tables. However, since iodine has similar chemical properties to the halogens chlorine and bromine, Mendeleev swapped the positions of iodine and tellurium and made Iodine follow Tellurium, to be positioned in the right place below its halogen friends.. And in fact that’s how they appear in the modern table because of their atomic numbers (Te 52, followed by I53). Mendeleev didn’t yet know about the significance of proton-based atomic number but in a sense he was predicting it. This is one of the very few examples where the order of atomic mass is not the same as atomic number.
The 92nd element is uranium but it can transform itself into other elements like lead through radioactive decay. Elements above A.N. 92 do not actually exist – not naturally anyway – they have to be artificially created and they also radioactively decay. .
Some scientists believe that although we have reached 118 now, we could go as high as 137 – but no higher because energy levels would not permit it.
The original classical elements as proposed by among others Aristotle were earth, fire, air and water, with aether the heavenly element soon added. The alchemists began to identify more conventional elements like sulphur and mercury, so that by Mendelev’s time the modern, full set was in reach.
So one might imagine Mendeleev as Donald Rumsfeld, who famously was mocked yet admired for his “known unknowns” description of military strategy. Mendeleev’s “known unknowns” were elements like gallium, germanium and scandium whose properties and existence he predicted before discovery. His “unknown knowns” were the iodine-tellurium pair which he placed in the wrong order because he was unaware of isotopes; and his “unknown unknowns” were arguably Group 0 inert gases because, being inert, they formed no compounds; and the high atomic number elements including lanthanides, actinides and radioactive elements.
Although in GCSE exams you are given the Periodic table, so it doesn’t need to be memorised, but just in case, you may wish to consult Google and the 120,000 ways of memorising it. Including songs like this one.
Each element has a story
There are only 2 liquid elements at room temperature – bromine, and mercury the “liquid metal”. Most of the rest are solid except for around 10 gases.
The lower elements are often named after famous people (yes, there is an Einsteinium, and a Curium) and also planets (Uranium, Plutonium, Neptunium). Note that Mercury the element and planet are both named after the god.
The country Argentina is named after the element Silver(Ag). Meanwhile Gold (after the Latin word Aurum) is so precious because it does not tarnish, being so unreactive, and it is a metal, again related to its position in the Periodic Table.
Carbon is the building block of life and forms many millions of organic compounds. Because of its position in Group 4 it requires 4 electrons for stability so typically forms 4 single bonds, or 2 singles and a double, with itself or other elements like Hydrogen in Methane (CH4, Natural Gas). Yet it is also the single element in strong and precious diamond. It also forms graphite which has several layers of hexagonally arranged carbon – the graphite pencil works by a layer peeling away on to the page. Graphene is a new material – it is only one layer of graphite – so only one atom thick – yet is between 10 and 200 times as strong as steel (depending on the steel type)
A good reference for the details, pictures and uses of every element can be found in this link
Electronic structure and Heisenberg’s role in the War (possibly)
As you go across a Period, more protons and electrons are added, but the atomic radius, strangely, gets smaller. This is because the additional electrostatic attraction of more protons outweighs that of negative electrons. But when you jump to a new Period and new electronic shell comes into play. Hence as you go down Group 1 alkali metals the elements get bigger and more reactive since the outer shell electrons gets further way from the nucleus.
Mendeleev had no knowledge in 1869 of the astonishing advances in the early 1900’s of the knowledge of atomic structure, but he was on the right track with his view that the properties of elements depended on their atomic weight and hence position in the periodic table. One of these pioneers was Neils Bohr – who developed the theory of electron shells and the quantum theory in the early 1900’s and which matches the Periodic table so well – was from a footballing family – a good player himself, his brother was a Danish international. Neils, of Jewish descent, also stood out against the Nazis, whereas Heisenberg (of Uncertainty fame) was more accommodating and the two had a mysterious and fractious meeting in 1941 concerning the development of the German atomic programme. Heisenberg showed a drawing, but there was disagreement over whether it was for a bomb or reactor. There is even uncertainty (of course!) about whether Heisenberg advanced Germany’s nuclear programme after the iconic meeting, or held it in check.
The book “Periodic Table” by author Primo Levi is a collection of short stories which links his love of science with his experiences in fascist Italy and in Auschwitz.
Space exploration, the Big Bang and the rest is history
We have not so far found elements in space that are not listed not in the Periodic Table. In fact all elements are thought to have been produced from Hydrogen and Helium after the Big Bang through various processes of fusion, fission, collisions, disintegrations involving for instance supernovas and neutron stars. Many of the processes involving the protons, neutrons, and electrons of He and H began in the enormous temperatures in the first fraction of a second of the Universe, following which traces of Lithium and Beryllium, the next elements by A.N., emerged. Carbon soon followed (OK after a few million years!) and the rest is history (literally!)
Although the earth is principally solid, less than 1% of matter in the Solar System is solid. Exceptions include iron which is thought to be at the centre of all planets in our solar system.
And finally…the man himself
…more about Dmitri Ivanovich Mendeleev himself. A Russian scientist, from Siberia, one of 17 siblings. The world may have been a different place if his second fiancé had not agreed to marry him (he threatened suicide otherwise). She did marry him, a month before his divorce from his first wife (interesting timeline!). A Chemistry teacher who had just written the definitive textbook of the era, he claims to have envisaged the Periodic Table in a dream and upon awakening reproduced it.
He incorporated the periodicity of the properties of elements, and although he focused on atomic weight not number, his work seemed to hint at the future through his use of “valence” which would later evolve to reflect atomic number and electron shells. The repeating patterns had been observed a few years before by scientists like Newlands and Meyer, but as is often the case timing is everything. So it was Mendeleev that is principally remembered; not just due to luck but also because his Paper, presented to the Russian Society of Chemists, included a coherent “pull it all together” theory which included predictions of new elements.
Mendelev was all set to receive the coveted Nobel prize in 1906 but at the very last the committee changed its mind -ostensibly because of the 37 year gap, but probably because of a trivial tiff (to which scientists are not immune!) The influential scientist Ahrrenius objected to a previous criticism of one of his theories!
One of Mendeleev’s originals (notice the gaps at 68 and 72)
Mendeleev studied at St Petersberg, and helped to create the first Russian oil refinery. One of his first tables is shown above from 1871,
Also below – what is now recognised as the oldest classroom chart version, dated 1885, found in St Andrews University; and in true Antiques Roadshow style, it was found in a dusty clear-out.
Credit : ST Andrews Periodic Table
There is a crater on the moon named after him, and, as you would expect, one of the elements, mendelevium, A.N. 101.
When Mendeleev died in 1907 his Periodic Table was well on the way to international acceptance but his last words were to his Physician. “Doctor, you have science, I have faith”.
For an amusing insight to the Periodic Table here is a Podcast by Professor Brian Cox