Michelle Echenique, age 10, of Costa Mesa, California, for her question:

What is the true number of natural elements?

The natural elements are the different atoms found in the world of nature on our home planet. Michelle is confused becaus various reference books seem to disagree about the number of them. Some list 92, others list 91, or 90 and one limits the number to 89. The correct figure is exactly 92 and it goes by the atomic numbers of the chemical elements. It starts with hydrogen, atomic number 1, and advances one by one to uranium, atomic number 92.

Perhaps we can excuse the confusion on account of different percentages. In our world of nature, about 90 per cent of all the solids, liquids and gases are made from only five of the most plentiful elements. Others, such as gold and helium, are very scarce. Actinium, astatine and several others occur in tiny traces, almost too small to be measured. But a few atoms of the rare ones have been found and they rightly belong on the list of 92. It is confusing and unfair to omit them.