Electron Configurations Help (page 2)

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By — McGraw-Hill Professional
Updated on Aug 28, 2011


A simple example is that of oxygen (O). The atomic number of oxygen is 8. The electron configuration of oxygen is:

1s 2 2s 2 p 4

2 electrons in the 1s subshell, 2 electrons in the 2s subshell, and 4 electrons in the p subshell give a total of 8.

An orbital diagram (shown as circles) is the notation used to show the number of electrons in each subshell. Each subshell is labeled with its subshell notation, s, p, d, or f. An orbital diagram also makes it easy to see the sequence of how subshells are filled. If you use small circles to stand for a subshell, then the orbital diagram can be used to find the orbital configuration of nearly every element. Figure 6.2 shows the order of an orbital filling sequence.

Electron Configurations Electron Configuration

Fig. 6.2. Orbitals fill in a regular sequenced order.

Ground State

Atoms contain an infinite number of possible electron configurations. The configuration associated with the lowest energy level of the atoms is called the ground state . When an atom’s energy levels go from a lower energy to a higher energy level, the change is sometimes seen as a flash of color or sudden heat.

The Aufbau principle , also called the building-up principle, is used to show an atom’s ground state.

Atoms fill up subshells like eggs in a carton. An atom’s ground state adds electrons in a particular building order.

The standard building order for the description of an atom’s orbital configuration is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, and 5f. An example of an atom’s orbital configuration is illustrated in Figure 6.3 for the element magnesium.

Electron Configurations Ground State

Fig. 6.3. The orbital configuration of magnesium shows how it can combine with other elements.

The Aufbau principle shows how energy increases in subshells. By filling the lowest energy subshells first, the ground state is built up. As the energy of the atom increases, the number of subshells filled increases.

When the number of electrons in an atom equals the atomic number (the number of protons), then the atom is neutral. Electron configurations are seen in the Periodic Table under the atomic number and element symbol.

The noble gases are in the ground state with the 1s 2 (He) and p orbitals filled (Ne, Ar, and Kr) so they usually have no interest in reacting with anything else. In Figure 6.4 the Periodic Table is shown with the location of the element orbitals.

Electron Configurations Ground State

Fig. 6.4. Periodic Table with electron orbitals.

Practice problems for these concepts can be found at – Electron Configuration Practice Test

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