Final answer:
The order of elements in terms of decreasing atomic size from largest to smallest is Sr > Ca > Se, based on their positions in the periodic table and the general trends in atomic radii.
Step-by-step explanation:
The elements Sr, Ca, and Se should be ordered in terms of decreasing atomic size as follows: Sr > Ca > Se. This order can be determined by looking at their positions on the periodic table. As we move down a group in the periodic table, the atomic size increases because each successive element has an additional electron shell compared to the one above it. Strontium (Sr) is below calcium (Ca) in the same group, so it has a larger atomic size.
Selenium (Se) is in a different group and period and generally, nonmetals have smaller atomic sizes compared to metals in the same period due to their increased nuclear charge pulling electrons closer.