* Atomic Size: Strontium is a large atom. The outermost electron is further from the nucleus, meaning the attraction between the nucleus and the incoming electron is weaker.

* Shielding: The inner electrons shield the outer electron from the positive charge of the nucleus, further reducing the attraction.

* Energy Level: Strontium's outermost electron is in a higher energy level, making it easier to remove.

Electron Affinity:

Electron affinity is the energy change that occurs when an atom gains an electron. A positive electron affinity means energy is released when an atom gains an electron, while a negative electron affinity means energy is required.

Since Sr has a low electron affinity, it does not readily accept an extra electron. This is typical for alkaline earth metals.