Ba is more reactive than Ca.
Ca is more reactive than Mg.
Reactivity increases going down the
group of group 2 metals.
Similarly, since
sodium reacts with cold water but magnesium requires steam,
sodium is more reactive than magnesium.
In general then, going
across the periodic table metals get less reactive.
–Potassium
is more reactive than calcium for instance.
–Aluminum,
another metal in the same period as sodium and magnesium is
even less reactive than magnesium.
–(Can
you tell why aluminum would be less reactive than magnesium?)
–So,
going across a period, the reactivity of metals in the period decreases.