1.Introduction
Keen science observers will have noted that this year, 2019, is an important celebratory milestone in the field of chemistry. In January, the United Nations launched the International Year of the Periodic Table of Chemical Elements celebrating the 150th anniversary since its creation by Russian scientist Dmitri Mendleleev. In keeping with these celebrations, Science Cafe welcomed Chick Wilson, Professor of Physical Chemistry at the University of Bath, to talk about those fundamental building blocks of matter, the elements.
2. The Nature of Elements
To get a feel for what an element is and its chemical characteristics, consideration must be given to the nature of the atom. The Neils Bohr model of the atom (for which he received the 1922 Nobel Prize for Physics) indicates a small dense nucleus surrounded by orbiting electrons (see Fig. 1). These orbit in discrete concentric shells around the nucleus, each shell being able to contain a maximum number of electrons. Inner shells fill completely before the next outer shell begins to accommodate additional electrons. The chemistry of the atom is determined by the number of electrons (valence electrons) in the outermost shell.
This model of the atom shows concentric electron shells around a central dense nucleus (red) which extend from K to Q. Electrons in the outer most shell (valence electrons) determine the chemical properties of the atom.
The number of protons within the nucleus of an atom is the Atomic Number (‘Z’) and an element is simply a substance where all atoms have the same number of protons. Elements have the distinction of being fundamental and cannot be broken down chemically into anything simpler. Often there will also be neutrons (of similar mass to protons) in the nucleus of the atom. These have little effect on the reactivity of an element but do add mass. Whilst the Z-number is the number of protons in the nucleus, the Atomic Weight is the sum of both the protons and neutrons. Nitrogen, for example, has seven protons and usually seven neutrons. Its Atomic Number will be 7 and its Atomic Weight, 14.
As one observer aptly remarked regarding atomic structure, ‘Protons give an atom identity whilst electrons determine its personality’.
3.Putting it together- Periodic Table
The Periodic Table (see Fig. 2) is one of the most resilient and memorable icons of science and has been amended and altered over the years without affecting its classical, graceful and powerful design. Few will have passed through school science labs without having the Periodic Table imprinted on their minds. In that respect, the Periodic Table bears a strong resemblance to Harry Beck’s London Underground map of 1931 which has been similarly expanded and overlain with new information without affecting its original, much loved, iconic format.
The seven horizontal rows are ‘periods’ (Z-numbers) and vertical columns ‘groups’ that contain elements with similar chemical and physical properties. Some groups have accepted names e.g. Halogens (group 17) and Noble Gases (group 18). Excluding transition elements, atoms in the same group have the same number of outer or valence electrons. See Fig. 4 for alternative versions of the Table
Scientists, keen to find new elements, classify their discoveries and draw connections between them started to place different elements together on tables, often based on Atomic Weight and element properties. One of the earliest was by John Newlands (1865), who placed elements in groups of eight (‘octaves’). It was perhaps ahead of its time and never quite caught on.
Dmitri Mendeleev (see Fig.3), a Siberian chemist, inspired by the game patience where cards are arranged horizontally (by suite) and vertically (by number) applied a similar process to the then 63 known elements.
His genius was in seeing similar patterns of element properties which he arranged in vertical columns. He was also brave enough to leave gaps for as yet undiscovered elements which allowed predictions of new elements to be made. Both Gallium (Ga-31) (‘chemical symbol’-‘atomic number’) and Scandium (Sg-21) were ‘discovered’ on paper by reference to unclaimed spaces on the Table.
However, the Periodic Table of Mendeleev was underpinned by false reasoning. The chemical reactivity of an atom depends on its electronic configuration; the numbers of electrons in stable atomic nuclei are the same as the Z-number. It is therefore Z-number, not Atomic Mass, which better indicated element position in the Table.
This underlying rationale was brilliantly demonstrated in 1913 by the relatively unknown Henry Moseley. He also predicted four new, as then undiscovered, elements ‘on paper’: Technetium (Tc-43), Promethium (Pm-61), Hafnium (Hf-72) and Rhenium (Re-75) as they are now known.
LHS The Spiral Periodic Table by Otto Theodor Benfrey (1946) included a branch for the as yet to be discovered superactinides from (Unbinium (Ubu-121) onwards). RHS the Curled Ribbon Periodic Table by James Franklin Hyde (1975); as an organo-silicon chemist he gave Silicon (Si-14) centre stage highlighting its connection with other elements in the Periodic Table.
Henry Moseley, a local boy, originally from Weymouth, joined the British army during the Great War, insisting on leaving his academic work and serving his country in the trenches. In 1915, he was killed in Gallipoli, ending a startling early career and depriving the scientific world of one of its most original researchers. The importance and brilliance of his work suggests that had he lived, he would have scooped a Nobel Prize (for physics or chemistry) in 1916. Unfortunately, a Nobel cannot be awarded posthumously which creates disquiet in some quarters.
4. Where elements came from
Hydrogen (H-1) and Helium (He-2) were formed in the Big Bang and Lithium (Li-3), Beryllium (Be-4) and Boron (B-5) by cosmic ray spallation. Elements with Z-numbers of between 6 and 26 (Carbon-Iron) were formed via Stella nucleosynthesis and those with Z-numbers greater than 26 (Cobalt onwards) from supernova nucleosynthesis.
Many elements are known from antiquity (e.g. gold, silver, copper). The first element to be discovered in modern times was phosphorous in 1669. German chemist, Henning Brandt, convinced that gold was to be found in urine (presumably the colour was the misleading clue), concentrated huge vats of the stuff. Instead of gold, he produced a white material which phosphoresced in the dark, thereby giving rise to the name of element no. 15.
Humphrey Davy, Britain’s most distinguished chemist, bagged a number of firsts including Sodium (Na-11), Potassium (K-19), Magnesium (Mg-12), Calcium (Ca-20) Strontium (Sr-38) and Aluminium (Al-13) between 1807 and 1812 by the then new cutting-edge technique of electrolysis. Modern day cutting-edge techniques are much grander in scale involving particle accelerators and require many years of planning.
As of 2019, there are 118 identified elements, 94 are naturally occurring and 24 are synthetic, produced, often only fleetingly and in very small quantities, in nuclear laboratories. Interestingly, only 4.6% of the universe consists of atomic elements (see Fig. 5).
5. Naming of the Elements
Element names are derived from a wide variety of sources including scientists, element properties (see phosphorous ante), countries, geographical areas, astronomical objects to name but a few.
Scientists
Many transuranic elements are named after Nobel Prize recipients. These include Curium (Cm-96) after Marie and Pierre Curie. Pierre died relatively young and Marie is the first person and only women to have received two Nobel prizes (in both physics and chemistry). Einsteinium (Es-99) is named after Albert Einstein, Fermium (Fm-100) after Enricho Fermi, originator of the atom bomb, Rutherfordium (Rf-104) after Ernest Rutherford, the father of nuclear physics and Lawrencium (Lr-103) after Ernest Lawrence, inventor of the cyclotron and known for his work in Uranium isotope separation during the Manhattan Project.
Scientists who did not receive a Nobel but are able to claim a connection with an element name include Alfred Nobel (Nobelium, No-102) of Nobel Prize fame, astronomer Nicolaus Copernicus (Copernicium, Cn-112), and most deservedly, Dmitri Mendeleev (Mendelevium Md-101).
Geography
Francium (Fr-87) and Gallium (Ga-31) are named after France, Germanium (Ge-32) after Germany and Niobium (Nb-41) after Japan. Americium (Am-95) and Europium (Eu-63) unsurprisingly commemorate continents.
States, towns, cities and individual labs are also celebrated. Dubnium (Db-105) is named after a science town in Russia, Californian(Cf-98) after North America’s most populous state and Berkelium (Bk-97) after the town of Berkeley, home to the University of California and its prestigious science labs.
Holmium (Ho-67) and Hafnium (Hf-72) are Latin for Stockholm (Sweden) and Copenhagen (Denmark) respectively and Copper (Cu-29) is old English for the island of Cyprus.
One of the most productive places for finding new elements has been the Swedish village of Ytterby on the island of Resaro which is rich in metallic ores and is commemorated in several element names. These include Yttrium (Y-39), Ytterbium (Yb-70), Erbium (Er-68), Terbium (Tb-65), Lanthanum (La-57) and Holmium (Ho-67).
Astronomical Objects
Extra-terrestrial objects have also been used as inspiration for the naming of elements. These include Plutonium (Pu-94) after Pluto, Uranium (U-92), after Uranus which, amongst naturally occurring elements, has the highest Atomic number. Neptunium (Np-93) is named after Neptune. It does not occur naturally but forms when Uranium breaks up. Mercury (Hg-80), unusually for a metal is a liquid at room temperature is named after our inner most planet and Cerium (Ce-58) after Ceres the largest object in the Asteroid Belt. Helium (He-2) the second most abundant element in the Universe (after Hydrogen, H-01) comes from the Greek name for the Sun and was one of first elements discovered from Earth by spectroscopy, in this case, of the Sun. It was discovered by astronomer, Norman Lockyer. On retirement, he established an observatory near to his home at Sidmouth, an observatory which can still be visited today1.
Extra-terrestrial discoveries have provided false alarms for element hunters. Spectral lines from Cat’s Eye Nebula suggested (1864) a new element, named Nebulium. This turned out to be oxygen with 2 electrons removed from its atomic structure. Likewise, the element Coronium was thought to have been discovered in the solar corona (1887) but was later determined to be the ion Fe13+, that is Iron minus 13 electrons. These two discoveries reflect the extreme stellar conditions in space and their effect on elements we are familiar with on Earth.
6. Future.
The number of elements now discovered has reached 118 and is confidently expected to go higher, although there is no consensus of the total number of possible elements. The first 94 elements occur naturally and elements 95 to 118 have only been synthesised in laboratories. New discoveries are expected to completely fill the 7th Period (row) and extend into a new 8th Period. Element 120, already under consideration, is likely to be highly unstable.
There is a trend for elements with higher atomic numbers to be increasingly unstable. However, research suggests there may be Islands of Stability where, as yet, undiscovered elements (e.g. element 126, possibly) will possess a ‘magic’ number of protons and neutrons which will provide those elements with a relatively long half-life, reversing the trend of decreasing stability with increasing Atomic Number.
Reference
1 Norman Lockyer Observatory, http://normanlockyer.com