They have the same number of electrons in their outer shell, so similar chemical properties. Sample exam questions - atomic structure and the periodic table - AQA Understanding how to approach ...
Einsteinium-255, produced through irradiation in ILL’s high-flux reactor, provided a continuous supply of fermium-255, ...
The ions formed have a full outer electron shell, so are very stable. See the study guide on groups in the periodic table for more information.