The student is expected to explain the use of chemical and physical properties in the historical development of the Periodic Table AND use the Periodic Table to identify and explain the properties of chemical families, including alkali metals, alkaline earth metals, halogens, noble gases, and transition metals.
Early scientists, such as Dmitri Mendeleev and Lothar Meyer, observed that the chemical properties of the known elements repeated in a predictive manner when the elements were placed in order of increasing mass (a physical property). These patterns eventually led to the development of the modern Periodic Table. Group A elements (the representative elements) within the same Group (column) have similar chemical properties.
Alkali metals are found in Group 1A. The elements within this chemical family have 1 valence electron that is easily lost, forming a cation with a 1+ charge. All elements within this family will combine with other elements in the same way, giving these elements similar chemical properties. These elements also have similar physical properties related to their metallic appearance.
Alkaline earth metals are found in Group 2A. The elements within this chemical family have 2 valence electrons that can be readily lost, forming a cation with a 2+ charge. All elements within this family will combine with other elements in the same way, giving these elements similar chemical properties. These are also metallic elements with similar physical properties.
The halogens are non-metals found in Group 7A of the Periodic Table. The elements within this chemical family are highly reactive as they have 7 valence electrons, forming an anion with a 1- charge. All elements within this family will combine with other elements in the same way, giving these elements similar chemical properties.
The noble gases are found in Group 8A. The noble gases have the maximum number of valence electrons (2 for helium and an octet or 8 valence electrons for all other noble gases), so they have little tendency to gain or lose electrons and are non-reactive.
The Group B elements are the transition metals found in the middle of the Periodic Table. They have a variable number of outer electrons, leading to a variety of possible charges for each element. Even though these elements have a variable number of valence electrons, they share many of their chemical and physical properties. The Lanthanide series and the Actinide series, located at the bottom of the periodic table, are the inner transition metals (the rare earth metals).
Historical Development of the Periodic Table
Dimitri Mendeleev: Early scientists used different characteristics of the elements to develop a basic elemental table. In the 1860s, Russian chemist Dmitri Mendeleev proposed an arrangement of elements that formed the basis for what we know today as the periodic table of the elements. Mendeleev based his table on the atomic mass of each element. Atomic mass is the average of the total mass of all the naturally occurring isotopes of an element. The mass of an atom comes from the number of the protons, neutrons, and electrons in the atom. Mendeleev also ordered the elements into groups based on similar chemical properties. At the time of Mendeleev’s work, only 63 elements were known. Mendeleev’s table, however, included empty spaces for additional elements. Although these elements had not yet been discovered, the patterns established by Mendeleev’s table predicted their existence.
Julius Lothar Meyer: As new elements were discovered, they were added to the periodic table. At nearly the same time as Mendeleev, Julius Lothar Meyer wrote about similar trends in the elements. Meyer was able to show how certain elements behaved similarly based on the volume and weight of their atoms. Almost fifty years later, a British scientist named Henry Moseley used X-rays to order the elements based on the number of protons in each atom’s nucleus. Moseley’s method is how our current periodic table of the elements is arranged.
The Modern Periodic Table of Elements
These patterns eventually led to the development of the modern Periodic Table as a tabular arrangement of the chemical elements, organized on the basis of their atomic numbers, electron configurations, and recurring chemical properties. Elements are presented in order of increasing atomic number, or number of protons. The standard form of the table comprises an 18-column by 7-row main grid of elements. There is also a double row of elements below the main grid consisting of elements that have similar chemical properties. The rows of the table are called periods; the columns are called groups or families. The groups are numbered numerically from 1 to 18 from the leftmost column (the alkali metals) to the rightmost column (the noble gases).
Groups of Metals, Nonmetals, and Metalloids
Metals are located to the left of the Periodic Table and have few valence electrons. Most elements are metals. They are usually shiny, very dense, and melt only at high temperatures. Their shape can be easily changed into thin wires (ductility) or sheets (malleability) without breaking. Similar to rusting iron, metals will corrode and gradually wear away. Heat and electricity travel easily through metals, making most good conductors.
Nonmetals are located to the right of the Periodic Table, have a full shell of valence electrons, and are very different from metals. Their surface is dull and they are poor conductors of heat and electricity. In comparison to the metals, they have low density and will melt at low temperatures. Nonmetals cannot change their shape easily as they are brittle and can break.
Metalloids are elements that have properties of both metals and nonmetals. The metalloids have an intermediate number of valence electrons. They can be shiny or dull and their shape is easily changed. Electricity and heat can travel through metalloids, but not as easily as they travel through metals.
A-B Group Notation
Groups can also be divided into eight groups with an A-B notation.
Group A Notation: The tall group columns on the table (Group A) are called Representative Elements or Main Group Elements (Groups 1, 2 and 13-18). There are correlations among the representative elements. These correlations include the number of valence electrons in an atom and the tendency of the element to act as a metal, nonmetal, or metalloid.
Group B Notation: The short group columns (Groups 3-12) on the table represent the Transition Elements located in the center region of the table. They consist of dozens of metallic elements (Groups 3-12), including the lanthanide and actinide series.
Group A Characteristics
Elements in the same group or family generally have the same electron configurations in their outer valence electron shells. As a result, element groups share similar chemical and physical properties.
Group 1A Alkali Metals: Alkali metals are found in Group 1A on the left side of the Periodic Table. The elements within this chemical family have one valence electron that is easily lost, forming a cation with a 1+ charge. All of the elements within this family are shiny, soft, highly reactive metals that will combine with other elements in the same way. This is what gives these elements similar chemical properties. Alkali metals must be stored under oil to prevent reaction with air or water. Group 1A elements are found naturally only in salts, but never as a free element. These elements also have similar physical properties related to their metallic appearance.
Group 2A Alkaline Earth Metals: Alkaline earth metals are found in Group 2A. The elements within this chemical family have two valence electrons that can be readily lost, forming a cation with a 2+ charge. All of the elements within this family are shiny, silvery-white, somewhat reactive metals that will combine with other elements in the same way, which gives these elements similar chemical and physical properties. Some examples are magnesium, calcium, and radium.
Group 7A Halogens (Non-Metals): Valence electrons play a vital role with regards to elements. The halogens found in Group 7A are highly reactive as they have seven valence electrons that form anions with a 1- charge. All elements within this family will combine with other elements in the same way, giving these elements similar chemical properties. Halogens are the only Periodic Table group that contains elements in all three familiar states of matter at standard temperature and pressure. When bonded to hydrogen, all of the halogens form acids. Most halogens are typically produced from minerals of salts and are all toxic. Examples are fluorine, chlorine, and iodine.
Group 8A Noble Gases: The noble gases, which are odorless, colorless, and monatomic, are found in Group 8A. The noble gases have the maximum number of valence electrons. There are two for helium, and a full octet (8 valence electrons) for other noble gases. Therefore, they have little tendency to gain or lose electrons and are non-reactive. The noble gases have many applications. At night, lighted displays on buildings and advertisement boards discharge colors that come from noble gases. They release different colors and spectra when exposed to electric discharge. Examples are helium, neon, argon, krypton, and xenon.
Other Representative Element Groups: 3A (Boron Family), 4A (Carbon Family), 5A (Nitrogen Family), and 6A (Oxygen or Chalcogen Family).
Group B Characteristics
Transition Metals: Part of the Group B elements are the transition metals found in the short middle columns of the Periodic Table. They have a variable number of outer electrons present in more than one shell, leading to a variety of possible charges for each element. Even though these elements have a variable number of valence electrons, they share many chemical and physical properties. As with all metals, the transition elements are both ductile and malleable, conduct electricity and heat, have high density, and high melting/boiling points. Examples include common metals such as iron, nickel, copper, gold, and silver. Compounds of transition metals are usually colored.
Inner Transition Metals (Rare Earth Metals): The other part of the Group B elements are the Lanthanide and Actinide series. They are located at the bottom of the Periodic Table and are called the inner transition metals, or rare earth metals. The rare earth metals are rarely found in pure form and are costly to extract. TV and computer monitors, the glass polishing industry, and permanent magnets are a few of the large number of items whose production depends on rare earth metals. All of the Actinides are radioactive, and most do not occur naturally on Earth. Instead, they are synthetic, that is, chemists make them in laboratories.