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Trends in the Periodic Table - HowStuffWorks
How the Periodic Table Works
by Craig Freudenrich, Ph.D.
Browse the article How the Periodic Table Works
Introduction to How the Periodic Table Works
It's human nature to organize things. Cooks painstakingly organize their spices into various
groupings, whether alphabetically or according to how often they're used. Kids dump out their
piggy banks and sort their riches into piles of pe​nnies, nickels, dimes and quarters. Even the
items in a grocery store are grouped a certain way. Head down the international aisle, and
you'll find packages of Chinese egg noodles sitting next to boxes of taco shells.
Chemists, as it turns out, are organizational junkies, too. They look for similar physical and
chemical properties among the elements, the basic forms of matter, and then ​try to fit them
into similar groups.
Scientists began attempting to organize the elements in the late 1800s when they knew of
about 60. Their efforts, however, were premature since they were missing a key piece of
information: the structure of the atom. While i​nitial efforts failed, one attempt by a Russian
chemist named Dmitry Mendeleyev showed much promise. Although Mendeleyev wasn't 100
The periodic table could represent one of our best attempts at trying to
organize the world.
© iStockphoto.com/Jacob H
percent correct, his approach laid the groundwork for what is now the modern periodic table
of the elements.
Today, the periodic table organizes 112 named elements and acknowledges several more
unnamed ones. It has become one of the most useful tools in chemistry, not only for students, but for working chemists as well. It classifies the elements according to
their atomic number (more on that soon), tells us about the nuclear composition of any given element, describes how electrons are arranged around a given element
and allows us to predict how one element will react with another.
So, exactly what is this feat of organization? Keep reading as we examine the history, organization and uses of this most handy chemical tool.
ATOMIC MASS, WEIGHT AND NUMBER
Atomic mass and atomic weight are often used
interchangeably and mean the same thing: the
amount of matter contained within an atom (usually
expressed in atomic mass units, or amus). The
atomic weight of an element indicates the average
mass of an atom of that element, taking into
account all its different isotopes. An isotope has
the same number of protons, but a different
number of neutrons. For example, hydrogen has
three different isotopes, all of which have one
electron and one proton but 0 (hydrogen/protium),
one (deuterium) or two (tritium) neutrons. Atomic
number, however, represents the number of
protons in an atom's nucleus. If you know the
number of protons, you also know the number of
electrons because all atoms contain equal numbers
of protons and electrons.
Getting Organized: Origins of the Periodic Table
In 1829, a German chemist by the name of J. W. Dobereiner noticed that certain groups of three elements had
similar properties. He called these groups triads and published a system of classification based on them. For
example, chlorine, bromine and iodine formed a triad, based on the fact that the atomic weight of bromine
(79.904) was close to the average of the atomic weights of chlorine (35.453) and iodine (126.904). Unfortunately
for Dobereiner and his scientific legacy, not all of the elements could be grouped into triads, so his efforts failed.
Another classification system unsuccessfully attempted to group the elements into octaves, like musical notes.
In 1869, Russian chemist Dmitry Mendeleyev published the first periodic table of elements, writing the chemical
properties and masses of each element on cards. He arranged the cards according to increasing atomic mass
and found that elements of similar properties appeared at regular intervals. But he took some liberties with his
table. In some cases, he violated his order of increasing atomic masses to keep elements with similar properties
together. For example, he placed tellurium (atomic weight 128) before iodine (atomic weight 127), so that iodine
could be grouped with chlorine, bromine and fluorine, all of which have properties similar to iodine. He also reasoned that if elements had to be reversed to preserve
the periodic pattern, then the atomic mass values must be wrong. Lastly, he left gaps in his table for elements that he reasoned should exist, but hadn't been
discovered.
Mendeleyev's periodic table predicted three elements of atomic weights 45, 68 and 70. He was proven right when these elements were later discovered and identified
as scandium, gallium and germanium, respectively. The atomic weights listed in modern periodic tables are slightly different than those in Mendeleyev's time because
methods for measuring atomic weights were improved during the 20th century. These discoveries demonstrated the usefulness of Mendeleyev's approach, even if it
wasn't without problems. Explanations would have to wait until the early 20th century, when the structure of the atom began to be revealed.
In 1911, English chemist Henry Moseley studied the frequencies of X­rays given off by various elements when high­energy electrons bombarded each. The X­rays
each element emitted had a unique frequency that increased with increasing atomic mass. Moseley arranged the elements in order of increasing frequency and
assigned each one a number called the atomic number (Z). He realized that the atomic number was equal to the number of protons or electrons. When the elements
were arranged by increasing atomic number, the periodic pattern was observed without having to switch some elements (as Mendeleyev did), and "holes" in the
periodic table led to the discovery of new elements. Moseley's discovery was summarized as the periodic law: When elements are arranged in order of increasing
atomic number, there's a periodic pattern in their chemical and physical properties. That law led to the modern periodic table.
Building the Periodic Table Block by Block
Each block of the periodic table houses an element, along with a few standard facts about that element:
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​Atomic number: integer equal to the number of protons or electrons in the element. Gold's atomic number is 79.
Element symbol: one or two letters. In the case of two letters, the first one is always capitalized. Hydrogen's symbol is just H,
while helium's is He. Symbols can be tricky because some are based on the first letter(s) of the element's common name, as
hydrogen's is, while other symbols are based on the Latin names of the element, such as Au for gold (or aurum in Latin).
Element name
Atomic weight: usually a decimal value, such as 196.966 569(4) for gold
Some periodic tables include the electron configuration (arrangement of electrons) in a corner of the block or below the name
of the element. In addition, some periodic tables thoughtfully include colored symbols to indicate whether the element is a solid,
liquid or gas at standard temperature (25 degrees C or 77 degrees F) and colored backgrounds to indicate the type of element
If you looked up gold on the
periodic table, this is the
information​ you'd likely find.
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(alkali metals, alkaline earth metals, nonmetals, noble gases and so on).
Within the table, ​the elements are arranged by increasing atomic number, as you'll recall. The elements stretch across seven
rows. Each row is called a period and indicates the energy levels or shells occupied by the electrons around the nucleus of that
element (see How Atoms Work). For example, the first energy level can only hold two electrons max,
so hydrogen and helium occupy period 1. In period 2, the second energy level begins to fill. The
pattern continues. The elements in period 7 have enough electrons to start filling the seventh energy
level. No known element yet has eight energy levels.
Each energy level above the first one has sublevels or orbitals. The orbitals are s (sharp), p
(principal), d (diffuse) and f (fundamental). But electrons don't fill directly in the order of s then p then
d then f. That would be too easy. There's some overlap between the orbitals of one energy level and
those of the one below it. For example, electrons in the fourth energy level fill in this order: 4s then
3d then 4p. (If you can't quite picture it, the American Chemical Society has a periodic table that
allows you to see how the various electron configurations work here.)
As the atomic number increases and one energy level fills, a new period begins. If you placed all of
the elements in order of increasing atomic number, the periodic table would span more than one tidy
sheet of standard paper. That's why chemist Glenn Seaborg suggested pulling out the lanthanoids
and actinoids and placing them below the table to make it more compact.
The electrons of ​the outermost energy levels are the restless ones that participate in chemical
reactions. So as each new period begins, there are elements that have similar chemical properties ­­
those with one outer electron, those with two, three and so on. Mendeleyev couldn't have predicted
Click here or on the image above to see a larger, more detailed
version of the periodic table. It will open in a separate window so you
can toggle between the article and the table.
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this periodic nature because he didn't know about atomic structure. But what about the columns?
The Columns Supporting the Periodic Table
The columns that comprise the periodic table are called groups ­­ 18 in total. Groups indicate
elements with similar chemical and physical properties. About 80 percent of the elements are
metals (shiny elements that conduct heat and electricity well), and 15 percent of the elements
are nonmetals (poor conductors of heat and electricity). The remaining elements are
metalloids, which share properties of both metals and nonmetals. Let's look at some of these
element cliques and remember, sometimes group members are spread around the table, not
necessarily in one neat column. For example, hydrogen looks like it should belong to group 1,
the alkali metals, but it actually prefers the company of nonmetals.
Alkali metals (group 1 or IA) such as lithium, sodium and potassium, are highly reactive and
aren't usually found freely in nature. They get their name from their chemical reactions with
water in which they produce highly alkaline substances such as sodium hydroxide or lye.
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Sodium, one of the elements that makes the Great Salt Lake so salty and
nice to float on, belongs to the alkali metal group.
George Frey/AFP/Getty Images
They have one valence electron (or outermost electron that's farthest from the nucleus),
which they give up in chemical reactions. Sodium gas fills streetlights, while sodium liquid is
used to transfer heat in certain types of nuclear reactors.
Alkaline earth metals (group 2 or IIA) include magnesium, calcium and barium among others. These elements have two valence electrons, which they yield in
chemical reactions. Although they're less reactive than alkali metals, they're not usually found alone in nature. For example, calcium combines with carbon to make
calcium carbonate, which makes up limestone, marble and seashells. Teeth and bones are also made of calcium compounds. Beryllium contributes to the bling found
in the gemstones aquamarine and emerald.
Lathanoids and actinoids (group 3 or IIIB) include shiny metals (lanthanide series or rare earth elements) and radioactive elements (actinide series). Lanthanoids
are abundant in the Earth's crust, but difficult to separate from their compounds. All of the actinoids are radioactive, but only actinium, thorium, protractinium and
uranium are found naturally. The other actinoids are made in nuclear reactors and particle accelerators.
Transition metals (groups 4­12 or IB, IIB and IVB­VIIIB) are all shiny metals that are found naturally, but are less reactive than groups 1 and 2. Electrons of the
outermost s orbital and the inner d orbital can participate in chemical reactions. They include elements that we usually think of as metals, like iron, nickel, chromium
and precious metals such as gold, copper, silver and platinum.
Metals are located mostly in group 13 (IIIA) and some in groups 14­16 (IVA ­ VIA). Metals include aluminum, tin, lead and bismuth. Metals are harder and denser than
those in groups 1 and 2, but softer and less dense than the transition metals. Most of them are found as compounds in nature, but can exist freely once refined, as
aluminum does.
Noble gases (group 18 or VIIIA) include helium, neon, argon, krypton, xenon and radon. Helium, of course, fills balloons and blimps. Neon, argon and xenon are used
in lights. Radon is a product of radioactive decay from the Earth and comes up through the soil into your home. The noble gases are also called inert gases because
they don't react chemically with other elements. Why not? The orbitals of their highest energy level are filled with electrons. Thus sated, they tend not to take or share
their valence electrons with other elements.
You're not quite done yet. Metalloids and nonmetals round out the groups. Nonmetals can form compounds by sharing valence electrons with each other or swiping
them from metals. One group of nonmetals (17 or VIIA) are highly reactive and called halogens (fluorine, chlorine, bromine, iodine and astatine).
How can all of this information help you detect some trends among Earth's elements?
Trends in the Periodic Table
It's handy to know about what group a particular element resides in and what its atomic
structure is like, but that's not all the periodic table has to tell you. If you're looking at it, you're
casually taking in work that scientists have spent lifetimes struggling with. And if you look at
the table as a whole, some big trends start to emerge that tell us how one element will react
with another.
Before we can see these trends, a quick chemistry recap might be good. First, metals react
with nonmetals to form ionic compounds. The nonmetal atom takes one or more valence
electrons from the metal atom. When an atom gains or loses a valence electron, it forms an
ion. An ion with more protons than electrons is positively charged and called a cation (comes
from the metal). An ion with more electrons than protons is negatively charged and is called
an anion (comes from the nonmetal). In the end, both ions have a full outer energy level.
Second, nonmetals tend to share electrons so that both atoms have full outer energy levels;
In case you're not satisfied with just a plain old printout of the periodic
table, you can also buy shirts, beach towels, mugs and tattoos of the
trusty chart.
AP Photo/Eric Risberg
they form covalent compounds. But how do you know which element will react with which to
produce an ionic or a covalent compound? That depends on a few factors:
Ionization energy: the amount of energy it takes to strip away the first valence electron
Electronegativity: a measure of how tightly an atom holds onto its valence electrons
Nuclear charge: the attractive force between the positive protons in the nucleus and the negative electrons in the energy levels. The more protons, the greater the
nuclear charge.
Shielding: inner electrons tend to shield the outer electrons from the attractive force of the nucleus. The more energy levels between the valence electrons and the
nucleus, the more shielding.
Let's see how these factors can help predict what type of chemical reactions any two elements will make.
If you look at the periodic table, ionization energy tends to decrease as you move down a column and increase as you move across a period from left to right. When
you compare elements in groups 1 and 2 (on the left) with those in 16 and 17 (on the right), you'll find that the elements in the first groups have lower ionization
energies, won't hold on to their valence electrons as tightly and will tend to form cations. So, elements in groups 1 and 2 will tend to form ionic compounds.
Like ionization energy, electronegativity decreases as you go down a column and increases as you go across a period from left to right. So, fluorine is more likely to
take electrons from another element than lithium. The difference in electronegativity between two elements will determine whether they exchange electrons (ionic
compounds) or share electrons (covalent compounds). You can use trends in ionization energy and electronegativity to predict whether two elements will form ionic or
covalent compounds.
Lastly, the nuclear charge increases as you go across and down the table, while the shielding stays constant across the periods, but increases as you go down the
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columns. These tendencies tell you about atom size. Atoms and ions get bigger as you go down the columns because the shielding effect outweighs the effects of the
nuclear charge, so the attraction between the nucleus and electrons is weaker and the atom expands in size. In contrast, atoms get smaller as you go across the
periods because the nuclear charge effect outweighs the shielding effect, so the attraction between the nucleus and the electron is greater and the atom shrinks in
size.
It's hard to believe that one measly sheet of paper can contain that much information.
IUPAC: The Gatekeeper of Elements
The International Union of Pure and Applied Chemistry (IUPAC) oversees the periodic table of elements, which, as of
November 2011, consisted of 112 officially named elements, like seaborgium and regular old potassium.
An official element is one that was claimed to have been discovered, the discovery was verified and the element was named. An
unofficial element is one that was claimed to have been discovered, but the claim has not been verified, so the element hasn't
been named. One of the more recent elements to claim fame on the periodic table was roentgenium, which was unearthed in
December 1994 and named after Wilhelm Roentgen, the scientist behind X­rays.
The last elements to be discovered had atomic numbers of 112, 114, 116 and 118. They are unofficially called ununbium (Uub),
ununquadium (Uuq), ununhexium (Uuh) and ununoctium (Uuo), respectively ­­ Greek for the atomic numbers of these elements.
There are spots in the periodic table for elements with atomic numbers 115 and 117, but these elements haven't been
discovered, much like Mendeleyev left gaps in his table for elements that hadn't turned up yet. Of course, nothing in science is
static, so it's always good to check with IUPAC if you're unsure about whether an element is official or not.
Dr. Glenn Seaborg, the physicist
who discovered plutonium and the
man behind the element
Seaborgium, holds a gift sculpted
by a fan. Who says science isn't
rewarding!
AP Photo/Susan Ragan
Ununquadium doesn't exactly roll off the tongue, so how does an element get a new title and achieve official status? And are
there any naming restrictions? Is christening an element after a beloved pet strictly frowned upon, but after a hometown or lab
location accepted?
Remember that the new elements are all radioactive ones that are made in particle accelerators and have short lives before they
decay into another element. In addition, any new element discovered must have a lifetime greater than 10­14 seconds. Two
difficulties exist in confirming these new elements: First, they're not produced in large quantities, and second, they don't last very
long. That means it's a long, difficult road to verify the claim that a new element has been discovered. But the procedure for naming an element is as follows:
The claim that a new element has been found must be published in the scientific literature.
IUPAC analyzes the claim as to who discovered it (often competing labs claim discoveries of new elements), whether the experiments were valid and whether it
meets the criteria for a new element. IUPAC publishes its analysis in its official journal Pure and Applied Chemistry in which it establishes who discovered the element
and how it was made.
The element gets a provisional Greek name and square in the periodic table.
IUPAC invites the credited discoverers to submit a name and symbol for the new element drawn from a mythological concept, mineral, country or place, property or
scientist.
The proposal is publicly reviewed, usually by neutral scientists.
IUPAC makes the final decision.
IUPAC publishes the name in Pure and Applied Chemistry and adds it to the periodic table.
Many people have made different representations of the periodic table, such as spiral forms, 3­D forms, even a humorous "Periodic Table of the Elephants" that
features a cartoon elephant with a slightly different take on the elements. For example, the helium block features an elephant balloon filled with helium and the
beryllium block features a cold elephant ­­ Brrr­illium. Get it? Still, none of these twists on the periodic table have yet proved as useful as the standard form that you
see in any chemistry textbook today.
If you're ravenous for more elemental reading, keep reading for links to articles on aluminum, lead and more next. Lots More Information
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More Great Links
American Chemical Society: Periodic Table of Elements
IUPAC: The Periodic Table of the Elements
New York Times: Periodic Table Image
Sources
Brown, T.L. et al. "Chemistry the Central Science," eighth edition. Prentice­Hall. 2002.
Chemistry Daily, History of the Periodic Table. Jan. 4, 2007.http://www.chemistrydaily.com/chemistry/History_of_the_periodic_table
IUPAC. "Naming of New Elements." Pure Appl. Chem. Vol. 74, No. 5, pp. 787­791 (2002).http://old.iupac.org/publications/pac/2002/7405/7405x0787.html
IUPAC. "Criteria that must be satisfied for the discovery of the new chemical element to be recognized." Pure Appl. Chem., Vol. 63, No. 6, pp. 879­886, 1991.
http://old.iupac.org/reports/1991/6306wapstra/index.html
Kaesz, H. "The synthesis and naming of elements 110 and beyond." Chem Int vol. 24 No. 2 Mar
2002.http://old.iupac.org/publications/ci/2002/2402/elements110.html
Tzimopoulos, N.D. et al. "Modern Chemistry," third edition. Holt Rinehart and Winston. 1990.
Wilbraham, A.C. et al. "Chemistry." Prentice­Hall. 2008.
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