How would you rank the following elements in order of decreasing atomic size: Ba, Mg, Sr. And why would you choose that oder?
Atomic size decreases in the order:
Atomic size decrease across a Period (from left to right) BUT INCREASE DOWN A GROUP.
You have 3 alkaline earth metals: 3rd period magnesium; 5th period strontium; and 6th period barium. Clearly magnesium is the SMALLEST metal, and barium is the LARGEST.