Full Idea
The essential properties of uranium are its atomic number, and the common electron shell structure for all uranium atoms.
Clarification
The 'atomic number' is the number of protons in the nucleus of the atom
Gist of Idea
The essence of uranium is its atomic number and its electron shell
Source
Brian Ellis (The Philosophy of Nature: new essentialism [2002], Ch.3)
Book Reference
Ellis,Brian: 'The Philosophy of Nature: new essentialism' [Acumen 2002], p.54
A Reaction
For those who deny essences (e.g. Quineans) this is a nice challenge. You might have to add accounts of the essences of the various particles that make up the atoms. There is nothing arbitrary or conventional about what makes something uranium.