Different categories of elements in the periodic table
The periodic
table arranges all of the known elements in order of increasing atomic number.
Order generally coincides with increasing atomic mass. The different rows of
elements are called periods. The period number of an element signifies the
highest energy level an electron in that element occupies (in the unexcited
state). The number of electrons in a period increases as one moves down the
periodic table; therefore, as the energy level of the atom increases, the number
of energy sub-levels per energy level increases.
Elements that
lie in the same column on the periodic table (called a "group") have
identical valance electron configurations and consequently behave in a similar
fashion chemically. For instance, all the group 18 elements are inert, or noble
gases.
Element groups
are either nonmetals or various subsets of metals, but there is no distinct
line between the two types of elements. Metal elements are usually good
conductors of both electricity and heat. Subsets are based on similar
characteristics and chemical properties. Our version of the periodic table uses
the most commonly accepted demarcations between the elements.
Alkali
metals: The
alkali metals make up Group 1 of the table, and comprise lithium (Li) through
francium (Fr). These elements have very similar behavior and characteristics.
Hydrogen is Group 1, but it exhibits few characteristics of a metal and is
often categorized with the nonmetals.
Alkaline
earth metals: The
alkaline earth metals make up Group 2 of the periodic table, from beryllium
(Be) through radium (Ra). The alkaline earth metals have very high melting
points and oxides that have basic alkaline solutions.
Lanthanides: The lanthanides comprise elements 57 — lanthanum
(La), hence the name of the set — through 71, lutetium (Lu). They, along with
the actinides, are often called "the f-elements" because they have
valence electrons in the f shell.
Actinides: The actinides comprise elements 89, actinium
(Ac), through 103, lawrencium (Lr). They, along with the lanthanides, are often
called "the f-elements" because they have valence electrons in the f shell.
Only thorium (Th) and uranium (U) occur naturally with significant abundance.
They are all radioactive.
Transition
metals: The
transition elements are metals that have a partially filled d subshell
and comprise Groups 3 through 12 and the lanthanides and actinides.
Post-transition
metals: The
post-transition elements are aluminum (Al), gallium (Ga), indium (In), thallium
(Tl), Tin (Sn), lead (Pb) and bismuth (Bi). As the name implies, these elements
have some of the characteristics of the transition metals, but they tend to be
softer and conduct more poorly than the transition metals.
Metalloid: The metalloids are boron (B), silicon (Si),
germanium (Ge), arsenic (As), antimony (Sb), tellurium (Te) and polonium (Po).
They sometimes behave as semiconductors (B, Si, Ge) rather than as conductors.
Metalloids are also called "semi-metals" or "poor metals."
Nonmetals: The term "nonmetals" is used to
classify hydrogen (H),
carbon (C), nitrogen (N), phosphorus (P), oxygen (O), sulfur (S) and selenium
(Se).
Halogens: The halogen elements are a subset of the
nonmetals. They comprise Group 17 of the periodic table, from fluorine (F)
through astatine (At). They are generally very chemically reactive and are
present in the environment as compounds rather than as pure elements.
Noble
gases: The
inert, or noble, gases comprise Group 18. They are generally very stable
chemically and exhibit similar properties of being colorless and odorless
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