Look at strontium (Sr) in the periodic table. Would you expect it to be more or less reactive than calcium? Explain?

It is expected that strontium is more reactive than calcium.

Both of strontium and calcium are Group 2 (Group IIA) metals. When they react, the outermost electrons are lost. Strontium atom is larger in size than calcium because strontium atom has a greater number of shells. The electrons in the outermost shell of strontium would be lost more readily because strontium atom has a greater distance between the outermost shell and the nucleus resulting in a weaker nuclear attraction towards the electrons in the outermost shell. This leads to a higher reactivity of strontium than calcium.

Strontium vs calcium....

In terms of the standard reduction potential as an indicator of "reactivity" it hardly seems that there is a difference. Calcium has a standard reduction potential of -2.87V while the potential for strontium is -2.89V. Does that seem like a big difference to you? (Table below has some other potentials, for comparison.)

Na+(aq) + e- --> Na(s).............. E = -2.714V
Ca2+(aq) + 2 e- --> Ca(s)......... E = -2.87V
Sr2+(aq) + 2 e- --> Sr(s)........... E = -2.89V
Ba2+(aq) + 2 e- --> Ba(s)......... E = -2.9V
Rb+(aq) + e- --> Rb(s).............. E = -2.925V
K+(aq) + e- --> K(s).................. E = -2.925V
Li+(aq) + e- --> Li(s) ................ E = -3.045V

Also be sure of what you are talking about. The reduction potential doesn't always match the "vigorousness" of the reaction. Lithium has a greater reduction potential than does sodium, but the reaction of sodium with water is more vigorous than that of lithium. Lithium is higher in the activity series, but is not as "reactive" as some elements below it, particularly if "reactive" is referring the the kinetics of the reaction.

More reactive!!!
Reason There is less electronic attraction to the protons in the nucleus , because there is an extra shell of electrons shielding the outer pair of electrons.

NB
Similar to Group 1 , Group 2 becomes more reactive as you go down the Group .