MCAT General Chemistry Review - Alexander Stone Macnow, MD 2019-2020
The Periodic Table
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 Zeff 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.