What is the heaviest element?

Answer 1

What do you mean by "heaviest"? If you have a lot of feathers, it weighs more than a little bit of lead! There are two ways to answer this question. One is what is the element with the highest atomic weight, and what is the element with the highest density. The element with the highest atomic weight is the heaviest for the same number of atoms, and the most dense element is the heaviest element for the same volume of material.

Uranium (U) (atomic number 92) is the naturally occurring element with the highest atomic weight. Plutonium might be argued to be the heaviest naturally occurring element, but it many scientists disregard this. A few atoms of plutonium have been detected in naturally occurring uranium, but the trace ammounts were formed by neutron capture where some neutrons released in the natural decay (spontaneous fission) of uranium were captured by some other uranium atoms and transmuted into plutonium.

Ununoctium (Uuo) (atomic number 118) is the heaviest synthetic element, although only a couple of atoms have ever been made!

The most dense element is osmium (Os), which a density of 22.61 grams per cm3 (which is 22.61 times more dense than water!) That's almost twice the density of lead!

See the Related Question below for more information about the most dense elements.
Natural, known or possible?

• Natural 92 Uranium Heaviest element of all those in the earth when it formed (~6 billion years ago) that has not decayed to practically undetectable levels due to a short halflife.
• Known 112 Copernicium This changes periodically but as of February 24, 2010 element 112 was the heaviest known.
• Possible ∞ "Infinitium" I see no theoretical reason limiting the heaviest element, only practical.

Answer 2

The heaviest element is Oganesson, which has an atomic number of 118. It is a synthetic element and is highly unstable, with a very short half-life.