The only group less willing to give up their valence electrons is the inert elements (noble gases). They already have a very stable electron configuration and are unwilling to disrupt that stability by losing an electron. Inert gases are among the elements with the highest ionization energies.
ELECTRON AFFINITY
The greedy halogens are among the worst of the bunch of elements that tend to hoard their electrons toward themselves. These elements also tend to be very anxious to gain the number of electrons necessary to complete their octets. Like nervous little squirrels frantically running around in search of nuts to pack into their accommodating cheek pouches, these elements go in search of other atoms that are willing to give up their electrons. When a gaseous atom of a particular elemental identity gains one or more electrons to complete its octet, it relaxes and breathes a sigh of relief. This “sigh of relief ” is a release of a quantity of energy called the electron affinity. Because energy is released when an atom gains an electron, we can describe this process as exothermic. By convention, the electron affinity is reported as a positive energy value, even though by the conventions of thermodynamics, exothermic processes have negative energy changes. Regardless of the sign, just remember that electron affinity is released energy. The stronger the electrostatic pull (that is, the Zeff) between the nucleus and the valence shell electrons, the greater the energy release will be when the atom gains the electron. Thus, electron affinity increases across a period from left to right. Because the valence shell is farther away from the nucleus as the principal quantum number increases, electron affinity decreases in a period from top to bottom. Groups IA and IIA (Group 1 and 2) have very low electron affinities, preferring rather to give up one or two electrons, respectively, to achieve the octet configuration of the prior noble gas. Group VIIA (Group 17) elements have very high electron affinities because they need to gain only one electron to achieve the octet configuration of the immediately following noble gases in Group VIIIA (Group 18). Although the noble gases are the group of elements farthest to the right and would be predicted to have the highest electron affinities according to the trend, they actually have electron affinities on the order of zero, since they already possess a stable octet and cannot readily accept an electron. Elements of other groups generally have low electron affinity values.
Mnemonic
To recall the various trends, remember this: Cesium, Cs, is the largest, most metallic, and least electronegative of all naturally occurring elements. It also has the smallest ionization energy and the least exothermic electron affinity.
Mnemonic
In contrast to cesium, fluorine (F ) is the smallest, most electronegative element. It also has the largest ionization energy and most exothermic electron affinity.
ELECTRONEGATIVITY
No, we are not referring to pessimistic electrons. Electronegativity is a measure of the attractive force that an atom will exert on an electron in a chemical bond. The greater the electronegativity of an atom, the greater is its attraction for bonding electrons. Electronegativity values are related to ionization energies: The lower the ionization energy, the lower the electronegativity; the higher the ionization energy, the higher the electronegativity. The electronegativity value for any element is not measured directly and there are different scales used to express it. The most common scale is the Pauling electronegativity scale, which ranges from 0.7 for cesium, the least electronegative (most electropositive) element, to 4 for fluorine, the most electronegative element. Electronegativity increases across a period from left to right and decreases in a period from top to bottom. Figure 2.1 Summarizes the behavior of atoms in terms of the periodic table.
MCAT Expertise
Electronegativity might better be called “nuclear positivity.” it is a result of the nucleus’ attraction for electrons; that is, the Zeff perceived by the electrons in a bond.
Figure 2.1
Types of Elements
It’s often been said that birds of a feather flock together, and this is no less true for the elements. When we consider the trends of chemical reactivity and physical properties taken together, we begin to identify whole clans, if you will, of elements that share sets of similarities. These larger collections of elements, which span groups and periods, are divided into three categories: metals, nonmetals, and metalloids (semimetals).
Key Concept
Left
Atomic radius
Ionization energy
Electron affinity
Electronegativity
Top
Atomic radius
Ionization energy
Electron affinity
Electronegativity
Note: Atomic radius is always opposite the other trends.
METALS
Metals, found both on the left side and in the middle of the periodic table, include the active metals, the transition metals, and the lanthanide and actinide series of elements. Metals are shiny solids, except for mercury, which is a liquid under standard state conditions. They generally have high melting points and densities, but there are exceptions, such as lithium, which has a density that is about half that of water. Metals have the ability to be deformed without breaking; the ability of metal to be hammered into shapes is called malleability, and its ability to be drawn into wires is called ductility. At the atomic level, low Zeff, low electronegativity (high electropositivity), large atomic radius, and low ionization energy define metals. These characteristics make it fairly easy for metals to give up one or more electrons. Many of the transition metals, for example, are known to have two oxidation states, and some have more than that. Because the valence electrons of all metals are only loosely held to their atoms, they are essentially free to move, which makes metals generally good conductors of heat and electricity (some are better than others). The valence electrons of the active metals are found in the s subshell, those of the transition metals are found in the d subshell, and those of the lanthanide and actinide series elements are found in the f subshell. Some transition metals—such as copper, nickel, silver, gold, palladium, and platinum—are relatively nonreactive, a property that makes them ideal substances for the production of coins and jewelry.