Kaplan MCAT General Chemistry Review

The metalloids are also called the semimetals because they possess characteristics that are between those of metals and nonmetals. The electronegativities and ionization energies of the metalloids lie between those of metals and nonmetals. Their physical properties, such as densities, melting points, and boiling points, vary widely and can be combinations of metallic and nonmetallic characteristics. For example, silicon has a metallic luster but is brittle and a poor conductor. The particular reactivity of the metalloids is dependent upon the elements with which they are reacting. Boron (B), for example, behaves as a nonmetal when reacting with sodium (Na) and as a metal when reacting with fluorine (F). The elements classified as metalloids form a “staircase” on the periodic table and include boron, silicon, germanium, arsenic, antimony, tellurium, and polonium.

Real World

Metalloids share some properties with metals, and others with nonmetals. For instance, metalloids make good semiconductors due to their electrical conductivity.

The Chemistry of Groups

ALKALI METALS

The alkali metals, Group IA (Group 1), possess most of the classic physical properties of metals, except that their densities are lower than those of other metals (as is true of lithium). The alkali metals have only one loosely bound electron in their outermost shells, and their Z_eff values are very low, giving them the largest atomic radii of all the elements in their respective periods. The very low Z_eff values also result in low ionization energies, low electron affinities, and low electronegativities, and these atoms easily lose one electron to form univalent cations. They react very readily with nonmetals, especially halogens.

Key Concept

Alkali and alkaline earth metals are both metallic in nature because they both lose electrons easily from the s-orbital of their valence shells.

ALKALINE EARTH METALS

The alkaline earth metals, Group IIA (Group 2), also possess many properties characteristic of metals. They share most of the characteristics of the alkali metals, except that they have slightly higher effective nuclear charges and so have slightly smaller atomic radii. They have two electrons in their valence shell, both of which are easily removed to form divalent cations. Together, the alkali and alkaline earth metals are called the active metals because they are so reactive that they are not naturally found in their elemental (neutral) state.

HALOGENS

The halogens, Group VIIA (Group 17), are highly reactive nonmetals with seven valence electrons. They are rather “desperate” to complete their octets by each gaining an additional electron. The halogens are highly variable in their physical properties. For instance, the halogens range from gaseous (F₂ and Cl₂) to liquid (Br₂) to solid (I₂) at room temperature. Their chemical reactivity is more uniform, and due to their very high electronegativities and electron affinities, they are especially reactive toward the alkali and alkaline earth metals. Fluorine has the highest electronegativity of all the elements. The halogens are so reactive that they are not naturally found in their elemental state but rather as ions (called halides).

MCAT Expertise

Halogens are seen often on the MCAT. Remember that they only need one more electron to become “noble” (have that full valence shell).

NOBLE GASES

The noble gases, Group VIIIA (Group 18), are also known as the inert gases because they have very low chemical reactivities as a result of their filled valence shells. They have high ionization energies, little or no tendency to gain or lose electrons, and no real electronegativities. They are essentially snobby elements, as they refuse to mingle with the hoi polloi. After all, they already have everything they need. The noble gases have low boiling points, and all exist as gases at room temperature.

TRANSITION METALS