definitions

• The definition of shielding: The reduction of the experienced nuclear charge by an electron from the nucleus's true charge due to the repulsive forces induced by the negative charges of the electrons.
• definition of penetration in multi-electron atoms?

  Penetration is the potential for an electron to be found close to the nucleus. If an electron in an orbital is more penetrating than another electron in a separate orbital, the first electron will experience less shielding. The concept of penetration allows for the predictions of which orbitals are filled up first (orbitals with greater penetration will have lower energies), and what coulombic effects each electron will experience.
• definition of pauli exclusion principle?

  No two electrons in the same atom can have the same 4 quantum numbers