Why is UuX used as a symbol for unnamed elements on the periodic table?

Question

Until recently, the last few elements on the periodic table had the chemical symbols (now they have formal chemical symbols!):

Uub, Uut, Uuq, Uup, Uuh, Uus, and Uuo

Why did they have these temporary symbols? Why the letter U? Is there any reasoning behind the third letter? Why not:

Uua, Uub, Uuc, Uud, Uue, etc...

I was just wondering some of the history of nomenclature and the thought process behind this specific nomenclature.

Answer 1

In 1978, the IUPAC Commission on the Nomenclature of Inorganic Chemistry decided that it would be necessary to have a systematic naming for the elements with atomic number greater than 100, even for those which had not been discovered. The recommendations are as follows:

1. The name is derived directly from the atomic number of the element using the following numerical roots:

$$\require{enclose} \small \begin{array}{lcc} \hline \text{Number} & \text{Latin Word} & \text{Greek Word} & \text{IUPAC Nomenclature} \\ \hline \text{0} & \enclose{circle}{\text{nihil}} & \text{mēdén} & \text{nil} \\ \text{1} & \enclose{circle}{\text{ūnus}} & \text{en} & \text{un} \\ \text{2} & \enclose{circle}{\text{duo}} & \enclose{circle}{\text{dúō}} & \text{bi } \\ \text{3} & \text{trēs} & \enclose{circle}{\text{tria}} & \text{tri} \\ \text{4} & \enclose{circle}{\text{quattuor}} & \text{téttara} & \text{quad} \\ \text{5} & \text{quīnque} & \enclose{circle}{\text{pénte}} & \text{pent} \\ \text{6} & \text{sex} & \enclose{circle}{\text{héx}} & \text{hex} \\ \text{7} & \enclose{circle}{\text{septem}} & \text{heptá} & \text{sept} \\ \text{8} & \enclose{circle}{\text{octō}} & \enclose{circle}{\text{oktṓ}} & \text{oct} \\ \text{9} & \text{novem} & \enclose{circle}{\text{ennéa}} & \text{enn} \\ \hline \end{array} $$

2. The roots are put together in the order of the digits which make up the atomic number and terminated by 'ium' to spell out the name. The final 'n' of 'enn' is elided when it occurs before 'nil', and the final 'i' of 'bi' and of 'tn' when it occurs before 'ium'.

3. The symbol of the element is composed of the initial letters of the numerical roots which make up the name.

4. The root 'un' is pronounced with a long 'u', to rhyme with 'moon'. In the element names each root is to be pronounced separately.$^{[1]}$

Though not in the original text, I went ahead and added the words from which the IUPAC names were derived. Though not obvious, bi- comes directly from di-, which is a result of duo, found in both languages.

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_{$^{[1]}$ Recommendations for the Naming of Elements of Atomic Numbers Greater than 100, Pure Appl. Chem., Vol. 51, No. 2, (1979) 381-384.}