Kaplan MCAT General Chemistry Review

Half-filled and fully filled orbitals have lower energies than intermediate states. We will see this again with transition metals.

The presence of paired or unpaired electrons affects the chemical and magnetic properties of an atom or molecule. The magnetic field of materials made of atoms with unpaired electrons will cause the unpaired electrons to orient their spins in alignment with the magnetic field, and the material will be weakly attracted to the magnetic field. These materials are considered paramagnetic. Materials consisting of atoms that have all paired electrons will be slightly repelled by a magnetic field and are said to be diamagnetic.

VALENCE ELECTRONS

The valence electrons of an atom are those electrons that are in its outermost energy shell, most easily removed, and available for bonding. As with the unruly children sitting farthest from the checkers game who are most likely to get themselves into trouble, the valence electrons are the “active” electrons of an atom and to a large extent dominate the chemical behavior of the atom. For elements in Groups IA and IIA (Groups 1 and 2), only the outermost s subshell electrons are valence electrons. For elements in Groups IIIA through VIIIA (Groups 13 through 18), the outermost s and p subshell electrons in the highest principal energy level are valence electrons. For transition elements, the valence electrons are those in the outermost s subshell and in the d subshell of the next-to-outermost energy shell. For the inner transition elements (that is, those in the lanthanide and actinide series), the valence electrons include those in the s subshell of the outermost energy level, the d subshell of the next-to-outermost energy shell, and the f subshell of the energy shell two levels below the outermost shell. All elements in period 3 and below may accept electrons into their d subshell, which allows them to hold more than eight electrons in their valence shell, in apparent “violation” of the octet rule (see Chapters 2 and 3). We’ll learn that this is perfectly acceptable for these elements, however, and is occasionally preferred.

Mnemonic

Remember that paramagnetic means that a magnetic field will cause parallel spins in unpaired electrons and therefore cause an attraction.

MCAT Expertise

The valence electron configuration of an atom helps us understand its properties and is ascertainable from the periodic table (the only “cheat sheet” available on the MCAT!). The “EXHIBIT” button on the bottom of the screen on Test Day will bring up a window with the periodic table. Use it often!

Example: Which are the valence electrons of elemental iron, elemental selenium, and the sulfur atom in a sulfate ion?

Solution: Iron has 8 valence electrons: 2 in its 4s subshell and 6 in its 3d subshell.

Selenium has 6 valence electrons: 2 in its 4s subshell and 4 in its 4p subshell. Selenium’s 3d electrons are not part of its valence shell.

Sulfur in a sulfate ion has 12 valence electrons: its original 6 plus 6 more from the oxygens to which it is bonded. Sulfur’s 3s and 3p subshells can contain only 8 of these 12 electrons; the other 4 electrons have entered the sulfur atom’s 3d subshell, which in elemental sulfur is empty.

Conclusion

Congratulations! You’ve made it through the first chapter, and as far as we can tell, you’re still alive and there seems to have been minimal shedding of tears and blood. Good! Now that we have covered topics related to the most fundamental unit of matter, the atom, you’re set to advance your understanding of the physical world in more complex ways. This chapter described the characteristics and behavior of the three subatomic particles—the proton, the neutron, and the electron. In addition, it compared and contrasted the two most recent models of the atom. The Bohr model is adequate for describing the structure of one-electron systems, such as the hydrogen atom or the helium ion, but fails to describe adequately the structure of more complex atoms. The quantum mechanical model theorizes that electrons are found, not in discrete-pathway orbits, but in “clouds of probability,” or orbitals, by which we can predict the likelihood of finding electrons within given regions of space surrounding the nucleus. Both theories tell us that the energy levels available to electrons are not infinite but discrete and that the energy between levels is a precise amount called a quantum. The four quantum numbers completely describe the position and energy of any electron within a given atom. Finally, we learned two simple recall methods for the order in which electrons fill the shells and subshells of an atom and that the valence electrons are the troublemakers in an atom—and the ones we need to keep our eyes on.

CONCEPTS TO REMEMBER