Elements Beyond Uranium

Introduction

For most of history, the heaviest element known was uranium, with atomic number 92. Today the periodic table has 118 elements, and every one after uranium was first made by scientists rather than dug out of the ground. These transuranium elements show that the periodic table is not a finished list but a growing map, and that the rules of the table still hold even for atoms that exist for only a fraction of a second.

Core explanation

What makes an element new. An element is defined by its atomic number — the number of protons in the nucleus. To make a new element, scientists must create a nucleus with more protons than any known before. No chemical reaction can do this, because chemical reactions only rearrange electrons. It needs a nuclear process that changes the nucleus itself.

The first steps beyond uranium. In 1940, scientists in the USA bombarded uranium with neutrons. Some uranium nuclei absorbed a neutron and then underwent radioactive decay in which a neutron changed into a proton, raising the atomic number by one. This produced neptunium (93). Soon afterwards plutonium (94) was made in a similar way. Both are named, like uranium, after planets.

Heavier elements. Beyond about element 100, adding neutrons one at a time no longer works well. Instead, scientists use particle accelerators to fire a beam of lighter nuclei at a target of heavier nuclei. Very occasionally, two nuclei fuse, and the atomic number of the new nucleus is the sum of the two. For example, a calcium nucleus (20 protons) fusing with a berkelium nucleus (97 protons) gives a nucleus with 117 protons, element tennessine.

Tiny amounts, short lives. For the heaviest elements, experiments running for weeks or months may produce only a handful of atoms. Most exist for milliseconds or less before they decay. Scientists identify them by detecting the chain of radioactive decays that follows, each step leading to a known, lighter nucleus.

Confirming and naming. A claim for a new element must be checked and confirmed by an independent panel of IUPAC. The discoverers then propose a name and symbol. In 2016 the last four elements of Period 7 were named: nihonium (Nh, 113), moscovium (Mc, 115), tennessine (Ts, 117) and oganesson (Og, 118). With these, Period 7 is complete.

Where they fit. Elements 93 to 103 are actinides, shown in the separate row below the main table. Elements 104 to 118 continue across Period 7 in the main body, falling into the same groups as lighter elements. Oganesson therefore sits in Group 0 below radon.

Step-by-step reasoning

To work out the atomic number of a new element made by fusing two nuclei:

1. Find the atomic number (proton number) of the beam nucleus. 2. Find the atomic number of the target nucleus. 3. Add them together to get the number of protons in the fused nucleus. 4. Use the atomic number to locate the element in the periodic table.

Visual explanation

Picture the periodic table with everything up to uranium shaded as "found in nature" and everything after it shaded in a second colour as "made in the laboratory". The second colour covers the end of the actinide row and all of the lower right part of Period 7, ending at oganesson in the bottom right corner.

Real-world analogy

Making a superheavy element is like trying to make two marbles stick together by firing one at the other across a room. Most shots miss or bounce off. Only after trillions of attempts does one pair stick for an instant, and even then the combined marble soon falls apart.

Real-world example

Some synthetic elements are useful. Americium-241 is used in many household smoke detectors: it emits a tiny, sealed stream of particles that ionise the air in a small chamber, and smoke entering the chamber interrupts the current and sets off the alarm. Plutonium-238 has powered space probes such as Voyager, whose electrical supply comes from the heat of its slow radioactive decay.

Why?

Why are the heaviest elements so unstable? Every proton in the nucleus repels every other proton. With more than 100 protons, this repulsion becomes so large that the strong nuclear force holding the nucleus together can barely resist it, so the nucleus quickly breaks apart or decays.

Common misconception

"Elements made in a laboratory are not real elements." A synthetic atom with 117 protons is just as much an element as an atom of carbon; the definition depends only on proton number. Many synthetic elements, such as plutonium, have been made in large enough amounts to study their chemistry.

Worked example

Question: Scientists fuse a nickel nucleus (28 protons) with a lead nucleus (82 protons). What is the atomic number of the new nucleus, and which period is it in?

Reasoning: Atomic number = 28 + 82 = 110. Period 7 runs from element 87 to element 118, so element 110 is in Period 7.

Answer: Atomic number 110 (darmstadtium), in Period 7.

Quick check

1. What is the atomic number of the heaviest element currently in the periodic table, and what is it called? Answer: 118, oganesson.

Exam focus

You may be asked what a transuranium element is, why new elements must be made by nuclear rather than chemical reactions, and how to find the atomic number of a fused nucleus by adding proton numbers. Remember that new elements still fit into existing groups and periods.

Advanced insight

Theory suggests an "island of stability" near about 114 protons and 184 neutrons, where nuclei might last much longer than their neighbours. Reaching it is difficult because current methods produce nuclei with too few neutrons. Making element 119 or 120 would begin Period 8, and chemists are keen to test whether it behaves like the Group 1 and Group 2 metals above it.

Summary

Transuranium elements have atomic numbers above 92 and were first made artificially, starting with neptunium and plutonium in 1940. Lighter ones were made by neutron capture followed by decay; heavier ones by fusing nuclei in particle accelerators. They are made in tiny amounts, are highly radioactive and often very short-lived. New elements are confirmed and named by IUPAC, and with oganesson (118) Period 7 is complete.

Practice questions

1. Why can a chemical reaction never make a new element? Answer: Chemical reactions only rearrange electrons; the number of protons in the nucleus, which defines the element, stays the same. 2. A calcium nucleus (20 protons) fuses with a californium nucleus (98 protons). State the atomic number of the product and name the element. Answer: 118; oganesson. 3. Explain why only a few atoms of the heaviest elements have ever been made. Answer: Fusion of two nuclei is extremely rare, and the nuclei that do form decay within fractions of a second. 4. In which group would you expect oganesson to be placed, and why? Answer: Group 0, because it is element 118 at the end of Period 7, directly below radon.