Electrons in orbitals that experience more shielding are less stabilized and thus higher in energy.
![element with ns2 electron configuration element with ns2 electron configuration](https://cdn.britannica.com/w:400,h:300,c:crop/19/22319-050-9483B8D6/Thallium-Tl-square-periodic-table-some-properties.jpg)
This phenomenon is called shielding and will be discussed in more detail in the next section. Electrons that are closer to the nucleus slightly repel electrons that are farther out, offsetting the more dominant electron–nucleus attractions slightly (recall that all electrons have −1 charges, but nuclei have + Z charges). In any atom with two or more electrons, the repulsion between the electrons makes energies of subshells with different values of l differ so that the energy of the orbitals increases within a shell in the order s p > d > f. The energy of atomic orbitals increases as the principal quantum number, n, increases. The specific arrangement of electrons in orbitals of an atom determines many of the chemical properties of that atom. This allows us to determine which orbitals are occupied by electrons in each atom. Having introduced the basics of atomic structure and quantum mechanics, we can use our understanding of quantum numbers to determine how atomic orbitals relate to one another. Relate electron configurations to element classifications in the periodic table.Identify and explain exceptions to predicted electron configurations for atoms and ions.
![element with ns2 electron configuration element with ns2 electron configuration](https://s2.studylib.net/store/data/015310468_1-438a872d5b0a146d0219f33cedd4a3b9.png)
![element with ns2 electron configuration element with ns2 electron configuration](https://cdn.britannica.com/05/22305-050-66C49B13/Americium.jpg)
Derive the predicted ground-state electron configurations of atoms.By the end of this section, you will be able to: