The notation 3 d 8 (read "three–d–eight") indicates eight electrons in the d subshell (i.e., l = 2) of the principal shell for which n = 3.įigure 6.25 The diagram of an electron configuration specifies the subshell ( n and l value, with letter symbol) and superscript number of electrons. A superscript number that designates the number of electrons in that particular subshell.įor example, the notation 2 p 4 (read "two–p–four") indicates four electrons in a p subshell ( l = 1) with a principal quantum number ( n) of 2.The letter that designates the orbital type (the subshell, l), and.The number of the principal quantum shell, n,.We describe an electron configuration with a symbol that contains three pieces of information ( Figure 6.25): The arrangement of electrons in the orbitals of an atom is called the electron configuration of the atom. We will discuss methods for remembering the observed order. For small orbitals (1 s through 3 p), the increase in energy due to n is more significant than the increase due to l however, for larger orbitals the two trends are comparable and cannot be simply predicted. Electrons in orbitals that experience more shielding are less stabilized and thus higher in energy. 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). Within each shell, as the value of l increases, the electrons are less penetrating (meaning there is less electron density found close to the nucleus), in the order s > p > d > f. But this is not the only effect we have to take into account. Thus, the attraction to the nucleus is weaker and the energy associated with the orbital is higher (less stabilized). As the principal quantum number, n, increases, the size of the orbital increases and the electrons spend more time farther from the nucleus. The filling order is based on observed experimental results, and has been confirmed by theoretical calculations. Thus, many students find it confusing that, for example, the 5 p orbitals fill immediately after the 4 d, and immediately before the 6 s. Such overlaps continue to occur frequently as we move up the chart.įigure 6.24 Generalized energy-level diagram for atomic orbitals in an atom with two or more electrons (not to scale).Įlectrons in successive atoms on the periodic table tend to fill low-energy orbitals first. The 3 d orbital is higher in energy than the 4 s orbital. However, this pattern does not hold for larger atoms. The energy increases as we move up to the 2 s and then 2 p, 3 s, and 3 p orbitals, showing that the increasing n value has more influence on energy than the increasing l value for small atoms. The 1 s orbital at the bottom of the diagram is the orbital with electrons of lowest energy.
Figure 6.24 depicts how these two trends in increasing energy relate. 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.
Identify and explain exceptions to predicted electron configurations for atoms and ions.Derive the predicted ground-state electron configurations of atoms.By the end of this section, you will be able to:
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