Early Organization of the Elements
In the 18th and 19th centuries, the field of chemistry saw numerous advancements. Chemistry had only recently emerged as a scientific discipline, being definitively differentiated from alchemy, the quest to turn materials into other materials. Elements were defined and discovered, their properties measured, and their abilities to react with one another demonstrated and recorded. By the 19th century, many elements had been identified and isolated, and scientists began to notice similarities among some of them. In 1817 German chemist Johannes Döbereiner first proposed arranging elements by their properties. Döbereiner grouped elements according to similarities in triads, or groups of three, such as lithium, sodium, and potassium, which all have violent reactions with water. He further noted that, when arranged from smallest to largest mass, the element in the middle of the triad had a mass of about the average of the other two elements and had properties in between them. For example, sodium has a mass of about 23 amu (atomic mass units)—with 1 amu approximately being equal to the mass of one proton or one neutron—which is near the average of the masses of lithium (7 amu) and potassium (40 amu). When lithium is exposed to water, heat is generated, and a gas is released. When potassium is exposed to water, the reaction produces a violent explosion. Sodium, located between the two, produces a gas that can spontaneously ignite when exposed to water.
In 1865 English chemist John Newlands built on Döbereiner's ideas and created the first table of elements. He arranged the elements in order of mass. He noticed that each element had properties similar to an element eight places away from it. Thus, he divided his table into eight columns, which he called octaves.
Newlands' Table of Octaves
|Br||Rb||Sr||Ce, La||Zr||Di, Mo||Ro, Ru|
However, Newlands' octaves did not work for many heavier elements, so his table was not given much credence by other scientists.
Newlands was on the right track, despite the shortcomings of his table. Around 1869 German chemist Lothar Meyer and Russian chemist Dmitri Mendeleev both independently proposed that properties of elements recur periodically when the elements are arranged in order of atomic weight. Today the concept is slightly revised as the periodic law, which states that certain chemical and physical properties of the elements recur periodically when the elements are arranged in the order of their atomic numbers. Additionally, Meyer and Mendeleev each created tables that arranged elements according to this periodicity. In these tables, elements are arranged in order of increasing atomic weight and grouped according to shared properties. An important difference between the tables was that Mendeleev left blank spaces for elements that he assumed existed but had not yet been discovered.
The Modern Periodic Table
The periodic table can be further divided into blocks based on the orbital in which the valence electrons of elements are found. The s-block elements are found in groups 1 and 2 and are highly reactive metals (except hydrogen, which is a nonmetal and a gas). They are called s-block elements because the electrons in their valence shell are found in the s orbital. For this reason, helium is sometimes considered an s-block element, although its properties make it more similar to the other noble gases in group 18.
The p-block elements are those found in groups 13 through 18. Their valence electrons are found in the p orbital. P-block elements have varied physical and chemical properties. Some p-block elements are solids, some are gases, and a few are liquids at room temperature.
A main group element is an element in group 1, 2, or 13 to 18 of the periodic table. The s-block and p-block elements together make up the main group elements, also known as the representative elements. These elements form bonds with each other according to the octet rule. This rule states that, when atoms form bonds, they do so in a way that ensures each atom has eight electrons in its valence shell, which is the outermost shell of electrons. Main group elements are the most common elements in the universe.The d-block elements are found in groups 3 through 12 of the periodic table. The valence electrons of d-block elements are in the d orbital. The f-block elements are found in periods 6 and 7 and between groups 3 and 4. The f-block elements are often shown as an appendage, although some representations of the periodic table nest them within the d-block. The valence electrons of f-block elements are in the f orbital. The elements can be divided according to common properties. The three main divisions are the metals, the nonmetals, and the metalloids. A metal is one of a class of elements that tend to have a metallic luster, be good conductors of electricity and thermal energy, and be malleable (can be bent or pounded into specific shapes) and ductile (can be drawn into thin wire). Metals also tend to be fusible (can be melted and fused together). They tend to have high densities and be shiny solids. Metals also tend to form cations and react with atmospheric oxygen to form oxides, such as rust (oxidized iron). A nonmetal is one of a class of elements that tend to not have metallic luster, be poor conductors of electricity and thermal energy, and be neither malleable nor ductile. Nonmetals tend to be volatile (easily vaporized) and are good insulators of heat and electricity. Nonmetals usually form anions and have high ionization energy, the energy required to remove an electron from a gaseous atom or ion. At room temperature, most nonmetals are either gases or brittle solids. Bromine is an exception, being a liquid at room temperature. A metalloid is one of a class of elements that tend to have properties of both metals and nonmetals. Metalloids tend to be brittle solids at room temperature and can form alloys with metals. Many metalloids are semiconductors, meaning they are insulators at room temperature but good conductors at higher temperatures. Because metalloids have properties of metals and nonmetals, specific classification can be difficult. For example, selenium is sometimes considered to be a metalloid but is usually considered a nonmetal. Within the broader categories of metals and nonmetals, further classifications can be made. The metals are categorized into the alkali metals, alkaline earth metals, transition metals, and inner transition metals. An alkali metal is an element (except hydrogen) in group 1 of the periodic table. Alkali metals have only one electron in their outer shell, which makes them very reactive. They react violently with water. An alkaline earth metal is an element in group 2 of the periodic table. The outer s shell of an alkaline earth metal is full, having two electrons. Alkaline earth metals are fairly reactive at standard temperature and pressure (0°C and 1 atm). A transition metal is an element in groups 3 through 12 of the periodic table, which form bonds in a different way than other elements. They form coordination compounds in which the metal acts as a Lewis acid, accepting electrons donated from an atom or larger molecule acting as a Lewis base. Any element in the f-block of the periodic table is called an inner transition metal. Inner transition metals can further be classified as a lanthanide or an actinide. A lanthanide is an f-block element in period 6 of the periodic table, which consists of the elements lanthanum (La) to lutetium (Lu). An actinide is an f-block element in period 7 of periodic table, which consists of the elements actinium (Ac) through lawrencium (Lr).
The nonmetals are similarly divided into smaller classifications. Group 14 starts with carbon and is sometimes called the carbon family. Carbon is the only nonmetal in this group. Carbon can form long chains and can make up to four bonds, making it a very versatile molecule. Carbon is the basis of all life on Earth.
Group 15 starts with nitrogen. In group 15, nitrogen and phosphorus are nonmetals. Nitrogen is an inert gas and is one of the main components of air. Pure phosphorus can appear in different forms because phosphorus is able to bond with other phosphorus atoms in different configurations. Phosphorus is generally reactive, and some forms of it ignite when exposed to air.
Group 16 starts with oxygen. This group is sometimes called the chalcogens, from the Greek chalcos, which means "copper ore forming." Chalcogens are commonly found as part of copper ores. Oxygen, sulfur, and selenium are the nonmetals in this group. Oxygen is a reactive element and forms compounds with a wide variety of other elements.
A halogen is an element in group 17 of the periodic table. The word halogen means "salt-producing"; halogens form salts when they react with metals. Halogens are highly reactive because they are highly electronegative, which means they have a tendency to gain electrons in chemical bonds. Halogens form acids when bonded to hydrogen. Halogens are also unique in that this is the only group in which elements in their pure states exist in three states of matter (solid, liquid, and gas) at standard temperature and pressure: Fluorine and chlorine are gases, bromine is a liquid, and iodine and astatine are solids.
An element in group 18 of the periodic table is a noble gas, also called an inert gas. Group 18 elements are gases at standard temperature and pressure and do not react with other elements, because their valence shells are full. Noble gases are odorless and colorless.