Which Group 2A element would you expect to be the most reactive? beryllium (Be)
calcium (Ca)
barium (Ba)
radium (Ra)

I'm not sure but i will try to find out

They can measure  the amount of energy gained or lost

Where is the following?

It is a mixture. simply put, a compound must be pure, that is, of only one chemical structure and composition. a mixture is made of different chemical substances (i.e. elements, molecules, compounds) that are not chemically bonded to each other, but instead physically grouped together, as in the case of gasoline (a homogeneous solution).