Why is strontium more reactive than calcium but less reactive than rubidium?

Sr is  below Ca and to the right of Rb in the Periodic Table (see the attached image).

Reactivity increases from top to bottom of a Group. Sr is below Ca, so it is more reactive than Ca.

Reactivity decreases from left to right in a Period. Sr is to the right of Rb, so it is less reactive than Rb.

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