1 to 18 (see Appendix). Horizontal
rows in the table are periods. The
Ürst three are called short periods;
the next four (which include transi-
tion elements) are long periods.
Within a period, the atoms of all the
elements have the same number of
shells, but with a steadily increasing
number of electrons in the outer
shell. The periodic table can also be
divided into four blocksdepending
on the type of shell beingÜlled: the
*s-block, the *p-block, the *d-block,
and the *f-block.
There are certain general features
of chemical behaviour shown in the
periodic table. In moving down a
group, there is an increase in metal-
lic character because of the increased
size of the atom. In going across a pe-
riod, there is a change from metallic
(electropositive) behaviour to non-
metallic (electronegative) because of
the increasing number of electrons
in the outer shell. Consequently,
metallic elements tend to be those
on the left and towards the bottom
of the table; nonmetallic elements
are towards the top and the right.
There is also a signiÜcant differ-
ence between the elements of the
second short period (lithium to
Ûuorine) and the other elements in
their respective groups. This is be-
cause the atoms in the second period
are smaller and their valence elec-
trons are shielded by a small 1s^2
inner shell. Atoms in the other peri-
ods have inner s- and p-electrons
shielding the outer electrons from
the nucleus. Moreover, those in the
second period only have s- and p-
orbitals available for bonding. Heav-
ier atoms can also promote electrons
to vacant d-orbitals in their outer
shell and use these for bonding. See
also diagonal relationship; inert-
pair effect.
A- The WebElements table produced by
Mark Winter at the University of
ShefÜeld- Over 50 different forms of the periodic
table in the Chemogenesis web book by
Mark R. Leach
periplanarSee torsion angle.Perkin, Sir William Henry
(1838–1907) British chemist, who
while still a student accidentally pro-
duced mauvine, theÜrst aniline dye
and theÜrst dyestuff to be synthe-
sized. Perkin built a factory to pro-
duce it, and made a fortune.PermalloysA group of alloys of
high magnetic permeability consist-
ing of iron and nickel (usually
40–80%) often with small amounts of
other elements (e.g. 3–5% molybde-
num, copper, chromium, or tung-
sten). They are used in thin foils in
electronic transformers, for magnetic
shielding, and in computer memo-
ries.
permanent gasA gas, such as oxy-
gen or nitrogen, that was formerly
thought to be impossible to liquefy.
A permanent gas is now regarded as
one that cannot be liqueÜed by pres-
sure alone at normal temperatures
(i.e. a gas that has a critical tempera-
ture below room temperature).permanent hardness See hard-
ness of water.permanganateSee manga-
nate(vii).
permonosulphuric(VI) acidSee
peroxosulphuric(vi) acid.PermutitTradename for a *zeolite
used for water softening.perovskite structure A type of
ionic crystal structure shown by com-
pounds of the type ABO 3. There is a
cubic arrangement with A atoms sur-
rounded by 12 O atoms and B atoms
surrounded by 6 O atoms. Examplesperiplanar 404p