Electrons in an atom can shield each other from the pull of the nucleus.
The closer particles are the stronger the attraction.
The electrostatic force holding the electrons to the nucleus is stronger for the second electron because it is closer to the nucleus. There is less electron shielding so those electrons in the second energy level feel more pull from the nucleus and are held tighter. 11 Na Sodium 23 1s 2 2s 2 2p 6 3s 1