MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020

The Periodic Table
Concept Summary

The Periodic Table

· The Periodic Table of the Elements organizes the elements according to their atomic numbers and reveals a pattern of similar chemical and physical properties among elements.

o Rows are called periods and are based on the same principal energy level, n.

o Columns are called groups. Elements in the same group have the same valence shell electron configuration.

Types of Elements

· The elements on the periodic table belong to one of three types.

o Metals are shiny (lustrous), conduct electricity well, and are malleable and ductile. Metals are found on left side and middle of the periodic table.

o Nonmetals are dull, poor conductors of electricity, and are brittle. Nonmetals are found on right side of the periodic table.

o Metalloids possess characteristics of both metals and nonmetals and are found in a stair-step pattern starting with boron (B).

Periodic Properties of the Elements

· Effective nuclear charge (Zeff) is the net positive charge experienced by electrons in the valence shell and forms the foundation for all periodic trends.

o Z_eff increases from left to right across a period, with little change in value from top to bottom in a group.

o Valence electrons become increasingly separated from the nucleus as the principal energy level, n, increases from top to bottom in a group.

· Atomic radius decreases from left to right across a period and increases from top to bottom in a group.

· Ionic radius is the size of a charged species. The largest nonmetallic ionic radii and the smallest metallic ionic radii exist at the metalloid boundary.

o Cations are generally smaller than their corresponding neutral atom.

o Anions are generally larger than their corresponding neutral atom.

· Ionization energy is the amount of energy necessary to remove an electron from the valence shell of a gaseous species; it increases from left to right across a period and decreases from top to bottom in a group.

· Electron affinity is the amount of energy released when a gaseous species gains an electron in its valence shell; it increases from left to right across a period and decreases from top to bottom in a group.

· Electronegativity is a measure of the attractive force of the nucleus for electrons within a bond; it increases from left to right across a period and decreases from top to bottom in a group.

The Chemistry of Groups

· Alkali metals typically take on an oxidation state of +1 and prefer to lose an electron to achieve a noble gas-like configuration; they and the alkaline earth metals are the most reactive of all metals.

· Alkaline earth metals take on an oxidation state of +2 and can lose two electrons to achieve noble gas-like configurations.

· Chalcogens take on oxidation states of −2 or +6 (depending on whether they are nonmetals or metals, respectively) in order to achieve noble gas configuration. They are very biologically important.

· Halogens typically take on an oxidation state of −1 and prefer to gain an electron to achieve noble gas-like configurations; these nonmetals have the highest electronegativities.

· Noble gases have a fully filled valence shell in their standard state and prefer not to give up or take on additional electrons; they have very high ionization energies and (for He, Ne, and Ar), virtually nonexistent electronegativities and electron affinities.

· Transition metals are unique because they take on multiple oxidation states, which explains their ability to form colorful complexes with nonmetals in solution and their utility in certain biological systems.