You can determine the reactivity of a metal by looking at it's valence electrons. For example if the atom has one electron in it's valence shell it is more reactive because it wants to get rid of it and have a full valence shell.
Lithium is NOT the most active alkali metal. Francium is the most active but is available in such trace quantities and radioactive that for practical purposes it is Cesium that is considered most active.
chlorine
Francium
The most active metal in the calcium family is francium. It is the least stable alkali metal and can react violently with water, making it extremely rare and highly radioactive.
Largest.
Hydrogen...
carbon
The next is caesium.
No, a less active metal cannot replace a more active metal in a chemical reaction. In a single displacement reaction, a more active metal will displace a less active metal from its salt solution, but the reverse is not possible.
Potassium is the most active metal. It is highly reactive and can easily react with water to produce hydrogen gas and caustic potassium hydroxide.
Most metal musicians like, say Metallica use an active pickup. eg.( the EMG 81/85 )
The most active metal is francium (Fr). It is located farthest to the left and at the bottom. The most active nonmetal is fluorine (F). It is located at the top of Group 17 (the halogens).