For non-transition elements such as rubidium, cesium, and strontium, the most chemically similar element to any other element is always one in the same column of the periodic table but in an adjacent row. Since strontium is not in the same column as rubidium but cesium is in the same column and an adjacent row, the answer follows. (This rule is sometimes true also for transition elements, but not always: For example, for elements in columns 8, 9, and 10, the most nearly chemically similar element is one in the same row and an adjacent column within this group, rather than one in the same column but an adjacent row.)
The quest to put Americans on the moon before the Soviets do. During the Space Race, a couple of astronauts went up into space only to be tragically killed in an explosion.
