Kaplan MCAT General Chemistry Review

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 Z_eff 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 Right

Atomic radius

Ionization energy

Electron affinity

Electronegativity

Top Bottom

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 Z_eff, 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.

MCAT Expertise

The effective nuclear charge, Z_eff, can explain all periodic trends as well as chemical properties.

NONMETALS

Nonmetals are found on the upper right side of the periodic table. The metals claim that the nonmetals are jealous of them for their shiny hair and sparkly personalities. The nonmetals scoff back, yet quietly steal electrons from them. Nonmetals are generally brittle in the solid state and show little or no metallic luster. They have high ionization energies, electron affinities, and electronegativities; have small atomic radii; and are usually poor conductors of heat and electricity. Nonmetals are less unified in their chemical and physical properties than are the metals. They are separated from the metals by a diagonal band of elements called the metalloids.

METALLOIDS