Chapter 7

Handout 2 Analyzing Electron Configuration

The orbital blocks of the periodic table are shown in the diagram below.

Hund’s rule also applies to the transition metals. When filling the d orbitals, the electrons first fill separate orientations with parallel spins. Some examples are:

Vanadium [Ar] 4s23d3

Chromium [Ar] 4s13d5 Note that chromium is an exception to the general rule. The 4s orbital as well as each of the five 3d orbitals are singly occupied, for a total of 6 unpaired electrons.

Mn [Ar] 4s23d5

Fe [Ar] 4s23d6

Co

Cu

[Ar] 4s2 3d7

[Ar] 4s13d10

This is another exception, where the singly occupied orbital is in 4s, and the 3d shell is completely filled.

Zn

[Ar] 4s23d10

Further Analysis of Energy Levels Consider the sodium atom. Its ground state electron configuration is: [Ne]3s The outer shell is 3s, since 3s is lower in energy than 3p, and in turn, 3p is lower in energy than 3d. Why is 3s < 3p < 3d ? For hydrogen, the energy levels depend just on n, the principal quantum number:

A more general formula, for any one-electron species would be:

where Z = the atomic number (number of protons). For hydrogen, Z =1. This relatively simple formula applies to hydrogen and other one-electron species. But for other atoms, we can still say: Energy is proportional to – For sodium, Z = 11. However, the electron in the outer shell does not feel the full attraction of 11 protons. This is because it is shielded from the nuclear pull by the 10 inner electrons. A completely shielded electron would look approximately like this diagram:

And this electron would feel the effective pull of only 1 proton (11-10). The 10 electrons in the core would “cancel” 10 of the protons in the nucleus.

But although shielding is important, it is generally not complete. In particular, s electrons have significant probability near the nucleus. Even though their average distance is outside the core, they are said to “penetrate” the core and have a experience a higher nuclear charge. In general, s electrons have greater penetrating ability (less shielding) than p electrons, and p electrons have greater penetrating ability (less shielding) than d electrons. Since the s electrons experience a greater nuclear pull, their energy is lower. This ability of s electrons to penetrate the core also explains why 4s fills before 3d, as in the potassium atom. Potassium (element 19) has ground state [Ar]4s rather than [Ar]3d. The 3d orbital is almost completely shielded from the nuclear pull, but the 4s electron has considerably more penetrating ability (less shielded) and experiences a much greater nuclear pull. Even though it has a higher principal quantum number, in the potassium atom the 4s electron is lower in energy than 3d.

Electron Configurations of Ions Negative ions (anions) are formed by adding electrons to the atom. The electrons are added to the lowest-energy empty orbital available. This generally results in a configuration identical to that of a noble gas.

Note that in the above examples, the negative ion has the same electron configuration as the noble gas argon.

Note that in the above examples, the negative ion has the same electron configuration as the noble gas neon. Positive ions (cations) are formed by removing electrons from the highest energy orbital. This is generally the orbital with the highest principal quantum number. For main group elements, this will usually result in an ion with the same electron configuration as a noble gas. When two species, such as Na+ and Ne, have the same electron configuration, we say that they are isoelectronic. Ne, Na+, Mg2+, and Al3+, are all isoelectronic.

Note that in the above examples, the positive ion has the same electron configuration as the noble gas neon.

Note that in the above examples, the positive ion has the same electron configuration as the noble gas argon. Ar, K+, Ca2+, and Sc3+, are all isoelectronic. Transition metals will often form ions that do not have the same electron configuration as a noble gas. When transition metals form positive ions, the electrons are first removed from the orbital with the highest principal quantum number, which, for the first transition series, would be 4s, not 3d.

Note that in the above examples, the electrons are first removed from the 4s, not the 3d orbital. The resultant electron configurations are not the same as that of a noble gas. Tin and lead are main group elements (group 4A), which usually forms 2+ ions. The electrons being removed come from the highest-energy level, in this case 4p or 5p, with the resultant ion not being the same as that of a noble gas.