Kaplan MCAT General Chemistry Review

Third, and finally, we can generally say that elements behave in such a way to gain or lose electrons so as to achieve the stable octet formation possessed by the inert (noble) gases (Group VIII or Group 18). We will soon learn (Chapter 3) that this so-called “octet rule” is hardly a rule at all, as a far greater number of elements can be exceptions to this rule than elements that always follow it. Nevertheless, for now, let’s just keep in mind that elements, especially the ones that have biological roles, tend to want to have eight electrons in their valence shell.

These three facts can be your guiding principles as you work toward an understanding of the more particular trends demonstrated among the elements organized as they are in the periodic table. In fact, all you really need to remember is the trend for effective nuclear charge across a period and the impact of increasing the number of inner shells down a group in order to correctly “derive” all the other trends.

ATOMIC RADIUS

If we imagine one atom of any element to essentially be a little cloud of electrons with a dense core of protons and neutrons, then the atomic radius of an element is equal to one-half the distance between the centers of two atoms of that element that are just touching each other. (We can’t measure atomic radius by examining a single atom because the electrons are constantly moving around and it becomes impossible to mark the outer boundary of the electron cloud.) As we move across a period from left to right, protons and electrons are added one at a time to the atoms. Because the electrons are being added only to the outermost shell and the number of inner-shell electrons (which act as insulation between the nucleus and the valence electrons) remains constant, the increasing positive charge of the nucleus holds the outer electrons more closely and more tightly. The Z_eff increases left to right across a period, and as a result, atomic radius decreases from left to right across a period.

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As an electron and a proton get farther away from one another, it becomes easier to pull them apart. This will help us understand all the trends with respect to the radius.

As we move down a group, the increasing principal quantum number implies that the valence electrons will be found farther away from the nucleus because the number of inner shells is increasing, separating the valence shell from the nucleus. Although the Z_eff remains essentially constant, atomic radius increases in a group from top to bottom. In summary, within each group, the largest atom will be at the bottom, and within each period, the largest atom will be within Group IA (Group 1).

IONIZATION ENERGY

Ionization energy (IE), also known as ionization potential, is the energy required to remove an electron completely from a gaseous atom or ion. Removing an electron from an atom always requires an input of energy (it’s always endothermic; Chapter 6). Anybody who has ever volunteered to make fundraising calls for a nonprofit organization knows that considerable energy must be invested in convincing a person to give a charitable donation, no matter how charitable the potential donor may be. Some are easier to convince than others are, but in the end, every donation is secured by a lot of hard work, effective communication, and skillful playing of the guilt and pity cards. The higher the atom’s Z_eff or the closer the valence electrons are to the nucleus, the more tightly they are bound to the atom. This makes it more difficult to remove one or more electrons, so the ionization energy increases. Thus, ionization energy increases from left to right across a period and decreases in a group from top to bottom. Furthermore, the subsequent removal of a second or third electron requires increasing amounts of energy, because the removal of more than one electron necessarily means that the electrons are being removed from an increasingly cationic species. The energy necessary to remove the first electron is called the first ionization energy; the energy necessary to remove the second electron from the univalent cation to form the divalent cation is called the second ionization energy, and so on. For example:

Mg(g) Mg⁺(g) + e^- First Ionization Energy = 7.646 eV

Mg⁺(g) Mg²⁺(g) + e^- Second Ionization Energy = 15.035 eV