Core electrons are inner-shell electrons. They have the electron configuration of the nearest noble gas element, which makes up the noble gas core. The core electrons are not involved in chemical bonding.
Work Step by Step
For example, $$Li: 1s^22s^1$$ $1s^2$ refer to 2 core electrons. And $1s^2$ is also the electron configuration of the noble gas helium, we can also write $$Li: [He]2s^1$$