In the periodic table of the elements, elements are arranged in a series of
rows so that those with similar properties appear in a column. Elements
of the same period have the same number of electron shells; with each group
across a period, the elements have one more proton and electron and become less
metallic. This arrangement reflects the periodic recurrence of similar
properties as the atomic number increases. For example, the alkaline
metals lie in one group and share similar properties, such as high
reactivity and the tendency to lose one electron to arrive at a noble-gas
electronic configuration. The periodic table of elements has a total of 118
elements. Modern quantum mechanics explains these
periodic trends in properties in terms of electron shells. As atomic number
increases, shells fill with electrons in approximately the order shown at right.
The filling of each shell corresponds to a row in the table.
In the s-block and p-block of the periodic table, elements within the same
period generally do not exhibit trends and similarities in properties. However
in the d-block, trends across periods become significant, and in the f-block
elements show a high degree of similarity across periods.
Periods Seven periods of elements occur
naturally on Earth. For period 8, which includes elements which may be
synthesized after 2013, see the extended periodic table.
A group in chemistry means a family of objects with similarities like different
families. There are 7 periods, going horizontally across the periodic table.
= Period 1= The first period contains fewer elements
than any other, with only two, hydrogen and helium. They therefore do not follow
the octet rule. Chemically, helium behaves as a noble gas, and thus is
taken to be part of the group 18 elements. However, in terms of its
nuclear structure it belongs to the s block, and is therefore sometimes
classified as a group 2 element, or simultaneously both 2 and 18. Hydrogen
readily loses and gains an electron, and so behaves chemically as both a group 1
and a group 17 element. Hydrogen is the most abundant of the
chemical elements, constituting roughly 75% of the universe's elemental mass.
Ionized hydrogen is just a proton. Stars in the main sequence are mainly composed
of hydrogen in its plasma state. Elemental hydrogen is relatively rare on
Earth, and is industrially produced from hydrocarbons such as methane. Hydrogen
can form compounds with most elements and is present in water and most organic
compounds. Helium exists only as a gas except in
extreme conditions. It is the second lightest element and is the second most
abundant in the universe. Most helium was formed during the Big Bang, but new
helium is created through nuclear fusion of hydrogen in stars. On Earth, helium
is relatively rare, only occurring as a byproduct of the natural decay of some
radioactive elements. Such 'radiogenic' helium is trapped within natural gas in
concentrations of up to seven percent by volume.
= Period 2= Period 2 elements involve the 2s and 2p
orbitals. They include the biologically most essential elements besides
hydrogen: carbon, nitrogen, and oxygen. Lithium is the lightest metal and the
least dense solid element. In its non-ionized state it is one of the most
reactive elements, and so is only ever found naturally in compounds. It is the
heaviest primordial element forged in large quantities during the Big Bang.
Beryllium has one of the highest melting points of all the light metals. Small
amounts of beryllium were synthesised during the Big Bang, although most of it
decayed or reacted further within stars to create larger nucleii, like carbon,
nitrogen or oxygen. Beryllium is classified by the International Agency
for Research on Cancer as a group 1 carcinogen. Between 1% and 15% of people
are sensitive to beryllium and may develop an inflammatory reaction in
their respiratory system and skin, called chronic beryllium disease.
Boron does not occur naturally as a free element, but in compounds such as
borates. It is an essential plant micronutrient, required for cell wall
strength and development, cell division, seed and fruit development, sugar
transport and hormone development, though high levels are toxic.
Carbon is the fourth most abundant element in the universe by mass after
hydrogen, helium and oxygen and is the second most abundant element in the
human body by mass after oxygen, the third most abundant by number of atoms.
There are an almost infinite number of compounds that contain carbon due to
carbon's ability to form long stable chains of C—C bonds. All organic
compounds, those essential for life, contain at least one atom of carbon;
combined with hydrogen, oxygen, nitrogen, sulfur, and phosphorus, carbon
is the basis of every important biological compound.
Nitrogen is found mainly as mostly inert diatomic gas, N2, which makes up 78% of
the Earth's atmosphere. It is an essential component of proteins and
therefore of life. Oxygen comprising 21% of the atmosphere
and is required for respiration by all animals, as well as being the principal
component of water. Oxygen is the third most abundant element in the universe,
and oxygen compounds dominate the Earth's crust.
Fluorine is the most reactive element in its non-ionized state, and so is never
found that way in nature. Neon is a noble gas used in neon
lighting. = Period 3=
All period three elements occur in nature and have at least one stable
isotope. All but the noble gas argon are essential to basic geology and biology.
Sodium is an alkali metal. It is present in Earth's oceans in large quantities in
the form of sodium chloride. Magnesium is an alkaline earth metal.
Magnesium ions are found in chlorophyll. Aluminium is a post-transition metal. It
is the most abundant metal in the Earth's crust.
Silicon is a metalloid. It is a semiconductor, making it the principal
component in many integrated circuits. Silicon dioxide is the principal
constituent of sand. As Carbon is to Biology, Silicon is to Geology.
Phosphorus is a nonmetal essential to DNA. It is highly reactive, and as such
is never found in nature as a free element.
Sulfur is a nonmetal. It is found in two amino acids: cysteine and methionine.
Chlorine is a halogen. It is used as a disinfectant, especially in swimming
pools. Argon is a noble gas, making it almost
entirely nonreactive. Incandescent lamps are often filled with noble gases such
as argon in order to preserve the filaments at high temperatures.
= Period 4= Period 4 includes the biologically
essential elements potassium and calcium, and is the first period in the
d-block with the lighter transition metals. These include iron, the heaviest
element forged in main-sequence stars and a principal component of the earth,
as well as other important metals such as cobalt, nickel, copper, and zinc.
Almost all have biological roles. = Period 5=
Period 5 contains the heaviest few elements that have biological roles,
molybdenum and iodine. It includes technetium, the lightest exclusively
radioactive element. = Period 6=
Period 6 is the first period to include the f-block, with the lanthanides, and
includes the heaviest stable elements. Many of these heavy metals are toxic and
some are radioactive, but platinum and gold are largely inert.
= Period 7= All elements of period 7 are
radioactive. This period contains the heaviest element which occurs naturally
on earth, californium. All of the subsequent elements in the period have
been synthesized artificially. Whilst one of these is now available in
macroscopic quantities, most are extremely rare, having only been
prepared in microgram amounts or less. Some of the later elements have only
ever been identified in laboratories in quantities of a few atoms at a time.
Although the rarity of many of these elements means that experimental results
are not very extensive, periodic and group trends in behaviour appear to be
less well defined for period 7 than for other periods. Whilst francium and
radium do show typical properties of Groups 1 and 2 respectively, the
actinides display a much greater variety of behaviour and oxidation states than
the lanthanides. These peculiarities of period 7 may be due to a variety of
factors, including a large degree of spin-orbit coupling and relativistic
effects, ultimately caused by the very high positive electrical charge from
their massive atomic nuclei. = Period 8=
No element of the eighth period has yet been synthesized. A g-block is
predicted. It is not clear if all elements predicted for the eighth period
are in fact physically possible. There may therefore be no ninth period.
References