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The Periodic Table
The periodic table of the chemical elements (also periodic table of the elements or just the
periodic table) is a tabular display of the chemical elements. Although precursors to this table
exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who
intended the table to illustrate recurring ("periodic") trends in the properties of the elements.
The layout of the table has been refined and extended over time, as new elements have been
discovered, and new theoretical models have been developed to explain chemical behavior.
The periodic table is now ubiquitous within the academic discipline of chemistry, providing a
useful framework to classify, systematize, and compare all of the many different forms of
chemical behavior. The table has found many applications in chemistry, physics, biology, and
engineering, especially chemical engineering. The current standard table contains 118 elements
to date. (elements 1–118).
The layout of the periodic table demonstrates recurring ("periodic") chemical properties.
Elements are listed in order of increasing atomic number (i.e., the number of protons in the
atomic nucleus). Rows are arranged so that elements with similar properties fall into the same
columns (groups or families). According to quantum mechanical theories of electron
configuration within atoms, each row (period) in the table corresponded to the filling of a
quantum shell of electrons. There are progressively longer periods further down the table,
grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.
In printed tables, each element is usually listed with its element symbol and atomic number;
many versions of the table also list the element's atomic mass and other information, such as
its abbreviated electron configuration, electronegativity and most common valence numbers.
As of 2010, the table contains 118 chemical elements whose discoveries have been confirmed.
Ninety-four are found naturally on Earth, and the rest are synthetic elements that have been
produced artificially in particle accelerators. Elements 43 (technetium), 61 (promethium) and all
elements greater than 83 (bismuth), beginning with 84 (polonium) have no stable isotopes. The
atomic mass of each of these element's isotope having the longest half-life is typically reported
on periodic tables with parentheses. Isotopes of elements 43, 61, 93 (neptunium) and 94
(plutonium), first discovered synthetically, have since been discovered in trace amounts on
Earth as products of natural radioactive decay processes.
The primary determinant of an element's chemical properties is its electron configuration,
particularly the valence shell electrons. For instance, any atoms with four valence electrons
occupying p orbitals will exhibit some similarity. The type of orbital in which the atom's
outermost electrons reside determines the "block" to which it belongs. The number of valence
shell electrons determines the family, or group, to which the element belongs.
The total number of electron shells an atom has determines the period to which it belongs.
Each shell is divided into different subshells, which as atomic number increases are filled in
roughly this order (the Aufbau principle) (see table). Hence the structure of the table. Since the
outermost electrons determine chemical properties, those with the same number of valence
electrons are grouped together.
Progressing through a group from lightest element to heaviest element, the outer-shell
electrons (those most readily accessible for participation in chemical reactions) are all in the
same type of orbital, with a similar shape, but with increasingly higher energy and average
distance from the nucleus. For instance, the outer-shell (or "valence") electrons of the first
group, headed by hydrogen, all have one electron in an s orbital. In hydrogen, that s orbital is in
the lowest possible energy state of any atom, the first-shell orbital (and represented by
hydrogen's position in the first period of the table). In francium, the heaviest element of the
group, the outer-shell electron is in the seventh-shell orbital, significantly further out on
average from the nucleus than those electrons filling all the shells below it in energy. As
another example, both carbon and lead have four electrons in their outer shell orbitals.
Note that as atomic number (i.e., charge on the atomic nucleus) increases, this leads to greater
spin-orbit coupling between the nucleus and the electrons, reducing the validity of the
quantum mechanical orbital approximation model, which considers each atomic orbital as a
separate entity.
The elements ununtrium, ununquadium, ununpentium, etc. are elements that have been
discovered, but so far have not received a trivial name yet.
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At first glance, the periodic table -- that chart that appears on the walls of science classrooms everywhere -- appears
to be an oddly shaped collection of chemical information about the elements. A closer look, however, reveals the
source of the table's name: The elements are arranged "periodically;" that is, according to properties that repeat in
regular, predictable patterns. This periodic arrangement of the elements makes the table very useful, in that if you
know the location of an element in the table, you can predict its properties.
The more than 100 elements that make up the periodic table are organized in a series of 18 columns and 7 rows.
Each column is called a group, or family. Each row is called a period. Elements in the same group have similar
physical characteristics. For example, all of the elements in group 1 (at the far left) react easily with other elements.
Unlike the elements in a group, however, the elements in a period do not share properties. Rather, the properties of
the elements change as you move from left to right across the row. But to understand why the table is organized as it
is, it's helpful to understand the structure of atoms.
An atom is the smallest particle of an element. An atom of any given element is made up of a certain number of
protons, an equal number of electrons, and approximately the same number of neutrons. (The exception is hydrogen,
which can have zero neutrons.) Protons and neutrons form the nucleus of an atom, and electrons swarm around the
nucleus. This swarming isn't completely haphazard, though. Electrons inhabit various energy levels, or shells. The
electron configuration shown in the periodic table indicates how many electrons are found in each shell, from
innermost to outermost. For example, the electron configuration for calcium is 2, 8, 8, 2.
Electron configuration depends upon the energy state and magnetic spin of each electron, and these qualities place
electrons into particular subshells within each shell. The first shell, for example, includes only one subshell at the
lowest-level energy state and can hold no more than two electrons. The second shell, with two subshells that contain
four levels of energy states, can hold no more than eight electrons. The subshells containing the lowest energy states
fill first, and if a subshell is full, additional electrons are found in the next higher subshell -- which is generally in the
adjacent outer shell.
Beginning in the fourth shell, there are subshells that have lower energy states than those in the adjacent inner shell.
Since the electrons fill the levels in order of energy, electrons can start filling subshells in an outer shell before an
inner shell is completely full. This explains why, for example, the electron configuration for calcium can be 2,8,8,2
when the third shell can hold up to 18 electrons.
Elements are arranged in the periodic table according to atomic number, from left to right, top to bottom. The atomic
number of an element is equal to the number of protons found in an atom of that element. For example, an atom of
carbon has six protons in its nucleus; its atomic number is 6. The elements are also arranged according to atomic
mass. The mass of a single proton is equal to 1, while the mass of a neutron is very close to 1. An atom's atomic
mass, then, is close in number to the sum of its protons and neutrons. An atom of carbon, with six protons and, on
average, six neutrons in its nucleus, has an atomic mass of 12.0107. With the lighter elements, the atomic mass is
about double the element's atomic number. As you move up to the heavier elements, the number of neutrons relative
to protons increases, causing the mass to be increasingly more than double the atomic number.
When Dmitri Mendeleyev first devised the modern periodic table in 1869, he organized it such that elements with
similar characteristics fell into the same columns. Doing so naturally created rows within the table. What scientists
later found out was that these rows represented something very significant. They discovered that the elements in
each successive row contained an additional electron shell. For example, the atoms of hydrogen and helium in the
first row each had one electron shell; atoms of elements listed in the second row had two electron shells, and so on to
elements in the final row, whose atoms each have seven shells.
From this, scientists learned what caused elements to have different characteristics. Each element's physical
characteristics are determined, in large part, by the number of electrons in the outermost shell of its atoms. As with
the number of protons, the number of electrons increases by one as you move across the table from left to right, top
to bottom. Atoms of elements in the left-hand column have one electron in their outer shell, while atoms of elements
in the right-hand column have eight electrons in their outer shell. How does this determine an element's
characteristics? Single electrons in an outer shell can easily be taken away from the atom with the application of very
little energy. This makes atoms of elements in the left-hand column very reactive (and good conductors of heat and
electricity). It is very difficult, on the other hand, to add or remove electrons from an atom that has eight electrons in
its outer shell. The atoms of these elements, found in the column to the far right, are non-reactive.