The most reactive metal on the periodic table is Francium, However, Francium is an artificial element and only minimal quantities have been produced, so for all practical purposes, the most reactive metal available is Cesium, in the alkali metal family.
ANSWER In theory, Francium, in practice the Cesium
The number of valence electrons of an element can be determined by the periodic table group (vertical column) in which the element is categorized
I think you divide something
Acids react with calcium carbonate and more specifically carbonate to form carbon dioxide. An acid will give protons to the carbonate anion to produce carbonic acid which then decomposes into carbon dioxide and water. I hope this helps. Let me know if anything is unclear.