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Name: Adam
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Question:
What is the largest possible atom?



Replies:
Adam,

By 'large', do you mean atomic number or do you mean atomic radius?

Some large atomic mass atoms can be created by scientists that are very unstable and decay in a fraction of a second. Researchers attempt to build very large atoms by smashing other atoms together, but these atoms are very unstable. For instance, the current largest atomic number element, with atomic number 118, survived for less than a thousandth of a second. Usually it takes years of testing and retesting and arguing for scientists to be convinced a new atom was in fact made -- and some will still disagree. Then, it takes years more to come up with a name for the atom.

Scientists will continue to try to create larger atoms -- even though they will decay nearly instantly. Is there an upper limit to how large these atoms can get? We do not know. Right now the technical hurdles are preventing us from making infinitely larger atoms.

If you mean atomic radius instead of atomic number, then the answer is different. The newly created atoms are not directly observed -- we 'conclude' they existed by observing the products of their decay. Therefore, we do not have data on properties like atomic radius, but even if we did, they likely would not be the largest. The arrangement of electrons and protons makes a big difference on size. Atoms at the far left of the periodic table tend to have the largest atomic radii because they 'let' their electrons stray further away, and the new super-heavy elements are not on the far left. Cesium, at the bottom of the 1st group, has the largest radius, around 0.3nanometers. Keep in mind that the actual radius changes a lot -- it depends on if the atom is an ion or elemental, if it's a part of a chemical compound, etc.

The heaviest naturally occurring element that is commonly found is Uranium, with an atomic mass of 92, although tiny traces of plutonium (94) can be found in nature as well.

Hope this helps,

Burr Zimmerman


Largest atom is Uranium with 92 protons and 92 electrons.

Prof. Przekop, Physicist


Hello Adam,

That is a fun question and one that depends upon a couple of different things. There are a few different answers depending upon how we interpret what we mean by largest atom possible.

First, let us start with the largest stable atom. Typically when scientists speak of the size of an atom it is, in one way or another, a measure of the number of protons, neutrons, and electrons in a single atom. As such, the heaviest stable atom we have found is a Bismuth atom. It has 83 protons, 83 electrons, and a whopping 126 neutrons. If we make such an atom heavier by adding protons or neutrons, it will no longer be stable. There are heavier elements found in nature, however once they are created (typically in the violent death of very massive stars), they will eventually decay into lighter (and eventually more stable) elements.

Now... let us assume we do not care about the lifetime of the atom, just so long as we can find it regardless of how short lived it might be. In such cases, nuclear physicists and chemists start with already heavy atoms and attempt to make even heavier elements and isotopes through collisions and interactions. The heaviest atom created in such a fashion is an isotope of element 118 : It has 118 protons and 176 neutrons and is tentatively called Ununoctium! This element is so difficult to create that the group of scientists that discovered it only managed to make 3 entire atoms of the substance. The atoms were extremely short lived and found to possess a half life of less than one millisecond.

The actual history of its discovery is interesting, though somewhat embarrassing, as the "initial" discovery of the element in 1999 was found to be based on forged and fraudulent data.

Now... There are two exotic possibilities for even larger atoms! First, in the conventional sense of an atom, nuclear scientists have long speculated that there are very stable nuclei possible with more protons and neutrons. The problem is getting there in the first place. These super stable nuclei should be possible when enough protons and neutrons are present to fill all the available states for the nucleons within the atom. In such cases, the atom is expected to then be quite stable (at least compared to other heavy atoms) as it would cost a great deal of energy to pluck any of the neutrons or protons out of it. No one has succeeded in creating or observing such atoms.

Lastly, in some respects neutron stars seem like extremely large atoms. They are not atoms in the conventional sense, but the extreme density of the star means that all the nuclear material is compressed together. I would not interpret that literally as a neutron star actually being a giant atom, just that you can draw interesting comparisons (perhaps with a little poetic license).

best wishes,

Michael S. Pierce
Materials Science Division
Argonne National Laboratory



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