difficult to separate. However, they are not rare and are now
more appropriately called the lanthanides. Technically the lan-
thanides should be placed between elements 57 (lanthanum)
and 72 (hafnium). Since this would nearly double the width of
the periodic table, they are usually placed below all the other
elements. Keeping in mind that size decreases left to right
across a period, hafnium (72) is much smaller than its neighbor
lanthanum (57). In fact, hafnium is essentially the same size as
the element above it, zirconium (40). With comparable chemi-
cal reactivity and size, zirconium and hafnium are difficult to
separate. This also suggests that the second and third rows of
the transition metals will possess many common chemical fea-
tures, as they do. The decrease in size due to the 14 elements
between lanthanum and hafnium is called the lanthanide con-
traction. Below the lanthanides are 14 more metallic elements
(90–103) called the actinides.
Elements
Each box in the periodic table contains a one- or two-letter
symbol representing a different element such as C for carbon
(6) or Sg for Seaborgium (106) [see illustration]. The number
in the upper left corner is the atomic number indicating how
many protons are in the atom's nucleus. The atomic mass gen-
erally appears below the symbol indicating the average mass
observed for that element. For example, most carbon (99%)
contains 6 protons and 6 neutrons, leading to a mass of 12.
However, since about 1% of carbon has an extra neutron, the
average mass of carbon as given in the periodic table is 12.011.
If an element has no stable isotopes, then the mass of the
longest-lived isotope is given in parentheses. More complex
periodic tables often include information on density, melting
points, and boiling points. Separate tables are available, indicat-
ing crystal structures, magnetic properties, radioactive decay
patterns, and other properties.
All of the elements in the periodic table have been officially
ratified by the International Union of Pure and Applied Chem-
istry (IUPAC). When a newly discovered element has been
independently verified, the original discoverer (often a team)
earns the right to propose a name to IUPAC. The elements
beyond 92 (uranium) do not occur naturally and are produced
using nuclear reactions. Elements beyond 100 are not particu-
larly useful, since they generally undergo rapid nuclear decay
by emitting radiation.
Other properties
Many periodic tables include a stair-step line separating metals
from the metalloids and nonmetals. Most elements are metals
and generally have physical properties that include luster (high
reflectivity), good conductivity for both electricity and heat,
high density, usually high melting points, ductility (the ability
to be drawn into a wire), and malleability (the ability to be ham-
mered into thin sheets). The chemical properties of most met-
als include corrosivity such as iron rusting and silver tarnishing,
as well as the ability to give up electrons. Nonmetals are found
to the right of the metals and their characteristics are the
inverse. This means most have no luster (appearing dull), are
poor conductors of electricity and heat, have low density, low
melting points, and are brittle. Chemically, nonmetals like to
gain electrons and often react with metals to produce salts. For
example, combining an alkali metal with one valence electron
with a halogen that needs one electron to complete its valence
shell produces an alkali halide salt such as sodium chloride or
common table salt. Metalloids straddle the stair-step line and
often have properties in between metals and nonmetals. With
intermediate conductivities, elements such as silicon form im-
portant semiconductors used in computer chips and solar cells.
Periodic Table (continued)
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