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The Periodic Table
As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such groupin…
The periodic table of elements is an important reference in chemistry for organizing the elements. The table has seven rows, or periods, numbered top to bottom, and eighteen columns called groups or families.
While the modern numbering system recommended by the International Union of Pure and Applied Chemistry or IUPAC, numbers the groups from 1 to 18, the Chemical Abstracts Services uses roman numerals and letters to designate the groups. Here, the main group elements are designated with letter A and the transition elements are designated with letter B.
The elements in a periodic table are ordered from left to right and top to bottom by increasing atomic number. Those with similar properties fall in the same group.
Some of the groups have common names. The elements in group 1, except for hydrogen, are known as alkali metals and the elements in group 2 as alkaline earth metals. All of these metals are very reactive and are found in nature combined with other elements.
Highly reactive, nonmetallic elements in group 17 are known as halogens. The nonreactive, nonmetallic elements of group 18 are referred to as noble or inert gases.
The elements can be divided into three broad classes: metals, nonmetals, and metalloids.
Metals are grouped toward the left and middle of the periodic table and are the vast majority of the elements. They are good conductors of electricity and heat. All are solid at room temperature, except for mercury, which is liquid.
Metals are shiny, and they can be hammered into sheets and drawn into wires. Metals, like potassium, tend to lose electrons to form cations, positively charged ions, when they react with other elements.
Nonmetals are grouped on the right side of the periodic table and usually poor conductors of heat and electricity. At room temperature, some are solid, while others are liquids or gases. Nonmetals tend to gain electrons and form anions, negatively charged ions, when they react with other elements.
Metalloids are elements that are found along the divide between the metals and non-metals. They are the smallest class of elements and exhibit mixed properties.
All metalloids exist as solids at room temperature. Some of them, for example, silicon and germanium, act as semiconductors, meaning that unlike metals, they get better at conducting electricity as the temperature increases.
Elements in the periodic table are also divided into three categories according to their location: the main-group elements, the transition metals, and the inner transition metals also known as the lanthanides and the actinides for the first element in the series in each row.
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Q1: How is the periodic table organized by atomic number and groups?
The periodic table arranges elements left to right and top to bottom by increasing atomic number. Elements with similar properties are grouped in vertical columns called groups or families. The table has seven rows (periods) and eighteen columns (groups), with elements ordered so that those sharing chemical behavior fall in the same group.
Q2: What are the main differences between metals, nonmetals, and metalloids?
Metals, grouped left and middle, conduct electricity and heat well and are shiny and malleable. Nonmetals, on the right side, are poor conductors and often dull. Metalloids, along the metal-nonmetal divide, exhibit mixed properties and include semiconductors like silicon that conduct better at higher temperatures.
Q3: Why do alkali metals and alkaline earth metals have different chemical properties?
Alkali metals in group 1 are much more reactive than alkaline earth metals in group 2. Alkali metals form compounds with oxygen in a 2:1 ratio, while alkaline earth metals form 1:1 compounds. These differences arise from their distinct electron configurations and tendencies to form cations and anions.
Q4: What are halogens and noble gases, and how do they differ?
Halogens in group 17 are highly reactive nonmetallic elements that readily gain electrons to form anions. Noble gases in group 18 are nonreactive nonmetallic elements. This dramatic difference in reactivity reflects their opposite tendencies: halogens need one electron to complete their outer shell, while noble gases already have complete outer shells.
Q5: How did Mendeleev's predictions support the development of the periodic table?
Mendeleev used his periodic table to predict undiscovered elements with properties similar to aluminum and silicon. The later discoveries of gallium in 1875 and germanium in 1886 confirmed his predictions, providing strong evidence that elements follow a periodic relationship based on atomic number rather than atomic mass.
Q6: What are main-group elements, transition metals, and inner transition metals?
Main-group elements occupy columns 1, 2, and 13-18 and include alkali metals, alkaline earth metals, and halogens. Transition metals fill columns 3-12. Inner transition metals, called lanthanides and actinides, occupy two rows at the table's bottom and include many radioactive elements.
Q7: Why are some elements shown with atomic mass in square brackets?
Elements like technetium, promethium, polonium, and heavier elements have atomic masses in square brackets because they consist entirely of unstable radioactive isotopes. An average atomic weight cannot be reliably determined since their radioisotopes vary significantly in abundance or may not exist naturally.