U is for Ununoctium and the Island of Stability

Quick, without looking it up: how many elements are there on the periodic table?

If I had asked that question before the first hydrogen bomb exploded in 1952, the answer would have been 98.   In that year, humans succeeded in synthesizing the first element that the crucibles of stars and supernovae hadn’t supplied to Earth: Einsteinium.

Boom.  Image public domain
Boom. Image public domain

Since then, we’ve been busy bees, building bigger atoms by smashing protons and neutrons together.  63 years after the first atoms of Einsteinium were made off the coast of an atoll in the Pacific and according to the International Union of Pure and Applied Chemistry (IUPAC), there are 114 official, named elements.  Those 16 additional elements were not easy to make, but we’re far from done.

I want to tell you the story of the outer reaches of the periodic table.  The tale involves magic (no, seriously… there’s an important concept called magic numbers) and a legendary island in the midst of a terribly unstable sea (again, not just metaphors here… Chemists have theorized of an island of stability that lies in the midst of a sea of instability), but the edge of the chemical world is dark and full of terrors.  Before making our way to the brink, I need to prepare you with the tools you need to wade out into the sea of instability to find the island of stability.

Always have to be careful in the world of chemistry.  Image by Eliot Phillips
Always have to be careful in the world of chemistry. Image by Eliot Phillips

So what is an “element” anyways?

Atoms, the basic building blocks of matter, are made up of electrons whizzing around in clouds around central nuclei.  A nucleus is made of positively charged protons and neutrons without a charge. An atom is said to be a particular element because of the number of protons it has.  If an atom has one proton, it will be called hydrogen.  A hydrogen with extra neutrons or electrons will still be hydrogen, but as soon as another proton is introduced, you’ve gone and made yourself a helium.

A cartoon of an atom with electrons in black, neutrons in blue, and protons in red.
A cartoon of a Lithium atom with electrons in black, neutrons in blue, and protons in red.

In that sense, atoms are just like me with breakfast: change up the cereal or the fruit but touch the coffee and I turn into a whole different person.

At this point you might be asking yourself what the point of neutrons or electrons is if they have no effect on the name of an atom.  The utility of electrons is pretty obvious: the tiny, whizzing balls of negative charge allow atoms to bind together and, because atoms are mostly empty space, their mutual repulsion is what gives matter the illusion of being solid.

Neutrons and Why We Need Them

The role of neutrons is a little bit less obvious.  They have almost all the same properties as protons (same mass, same size, made up of quarks) but lack a charge.  This similarity but lack of charge keeps them subject to the strong nuclear force, just like protons, but avoids the electromagnetic force.  The strong force acts only at very short distances and, like its name suggests, is very strong.  The electromagnetic force, like Paula Abdul suggested in the 80s, acts to keep like charges apart and opposite charges together.

That means protons have a problem if they want to live together in a nucleus.  Protons are by definition positively charged and would be repelled by each other if it were up to the electromagnetic force alone.  This is where neutrons come in.

Neutrons act like nuclear glue: they exert extra strong nuclear force pressure to keep protons together without any electromagnetic effects.  Small nuclei don’t need much glue: Helium has 2 protons, 2 neutrons; Lithium has 3 protons and 4 neutrons.  Bigger nuclei need a lot more glue (e.g. gold – 79 protons, 118 neutrons, lead – 82 protons, 126 neutrons).

Charting the Nuclear Waters

The question soon became: how big can we go?

It has long been known that any element with more than 82 protons (anything past lead on the periodic table) will be inherently unstable.  It will decay radioactively by shedding protons and neutrons until a stable configuration is reached.

Radioactive elements are still elements, though.  They just don’t stick around for as long.  Typically, heavier atoms are less stable.  Just ask Livermorium, whose atoms have a half-life of only 60 milliseconds.

If you start to graph the stability of atoms according to their number of protons and neutrons, it quickly becomes apparent that larger nuclei need proportionally more neutrons to be stable.

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The black line at 45 degrees shows when proton numbers = neutron numbers. The black dots are stable nuclides. Past 82 protons (lead), there are no more permanently stable nuclides.  Image by Sjlegg

Some scientists think there could be as many as 7000 different nuclides (combinations of protons and neutrons) that would be stable enough to observe, if only for a fraction of a second.  We currently know of 3000.

Another trend that scientists noticed is that there appears to be particular numbers of neutrons or protons that make for unusually stable atoms.  Those numbers, as of 2007, are 2, 8, 20, 28, 50, 82, and 126.  They have been dubbed “magic numbers”.  Atoms with a magic number of protons and a magic number of neutrons, like Helium (2 and 2) or Calcium (20 and 20) are said to be “double magic”.

The Island of Stability

In the 60s, it was suggested that beyond the current range of the periodic table lies a set of theoretical atoms that could be very large and very stable.  With a “magic” number of protons and neutrons, the atomic components could be arranged in just such a way as to maximize the glueyness of neutrons and spread out the repulsion of protons.

The fabled island itself.
The fabled island itself.  Image by InvaderXan

The metaphor was so vivid that it soon became adopted and has been used ever since.  Scientists even talk of landing on the shores of the island, but its oases still lie undiscovered and unspoilt.

Ununoctium

Before they were confirmed to be synthesized in the lab, the edges of the periodic table were given temporary names according to a set of naming conventions.  These rules are a strange hybrid of greek and latin roots for the element’s number.

The most recent transition from lati-greek to English and official additions to the periodic table were Flerovium (element 114, previously ununquadium – quad is latin, tetra would be greek) and Livermorium (element 116, previously ununhexium – hex is greek, sex would be latin).

The synthesis of element 117, Ununheptium, was announced in 2014, but IUPAC is still reviewing the findings.  The chemistry world continues its search for Ununoctium and speculation about its properties varies from unusually stable to unusually reactive.

One thing is for certain: when it is synthesized, it won’t last long.

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