The substances with which the chemist has to deal admit of
classification into elements and compounds. Of the former about eighty
may be regarded as well characterized, although many more have been
described.
_Elements._--The following table gives the names, symbols and atomic
weights of the perfectly characterized elements:--
_International Atomic Weights_, 1910.
Atomic Atomic
Name. Symbol. Weights. Name. Symbol. Weights.
O=16. O=16.
Aluminium Al 27.1 Mercury Hg 200.0
Antimony Sb 120.2 Molybdenum Mo 96.0
Argon A 39.9 Neodymium Nd 144.3
Arsenic As 74.96 Neon Ne 20
Barium Ba 137.37 Nickel Ni 58.68
Beryllium or Be} 9.1 Nitrogen N 14.01
Glucinum Gl} Osmium Os 190.9
Bismuth Bi 208.0 Oxygen O 16.00
Boron B 11.0 Palladium Pd 106.7
Bromine Br 79.92 Phosphorus P 31.0
Cadmium Cd 112.40 Platinum Pt 195.0
Caesium Cs 132.81 Potassium K 39.10
Calcium Ca 40.09 Praseodymium Pr 140.6
Carbon C 12.0 Radium Ra 226.4
Cerium Ce 140.25 Rhodium Rh 102.9
Chlorine Cl 35.46 Rubidium Rb 85.45
Chromium Cr 52.0 Ruthenium Ru 101.7
Cobalt Co 58.97 Samarium Sa 150.4
Columbium Cb} 93.5 Scandium Sc 44.1
or Niobium Nb} Selenium Se 79.2
Copper Cu 63.57 Silicon Si 28.3
Dysprosium Dy 162.5 Silver Ag 107.88
Erbium Er 167.4 Sodium Na 23.0
Europium Eu 152.0 Strontium Sr 87.62
Fluorine F 19.0 Sulphur S 32.07
Gadolinium Gd 157.3 Tantalum Ta 181.0
Gallium Ga 69.9 Tellurium Te 127.5
Germanium Ge 72.5 Terbium Tb 159.2
Gold Au 197.2 Thallium Tl 204.0
Helium He 4.0 Thorium Th 232.42
Hydrogen H 1.008 Thulium Tm 168.5
Indium In 114.8 Tin Sn 119.0
Iodine I 126.92 Titanium Ti 48.1
Iridium Ir 193.1 Tungsten W 184.0
Iron Fe 55.85 Uranium U 238.5
Krypton Kr 83.0 Vanadium V 51.2
Lanthanum La 139.0 Xenon Xe 130.7
Lead Pb 207.10 Ytterbium Yb 172
Lithium Li 7.00 (Neoytterbium)
Lutecium Lu 174 Yttrium Y 89.0
Magnesium Mg 24.32 Zinc Zn 65.37
Manganese Mn 54.93 Zirconium Zr 90.6
The elements are usually divided into two classes, the metallic and the
non-metallic elements; the following are classed as non-metals, and the
remainder as metals:--
Hydrogen Oxygen Boron Neon
Chlorine Sulphur Carbon Krypton
Bromine Selenium Silicon Xenon
Iodine Tellurium Phosphorus Helium
Fluorine Nitrogen Argon
Of these hydrogen, chlorine, fluorine, oxygen, nitrogen, argon, neon,
krypton, xenon and helium are gases, bromine is a liquid, and the
remainder are solids. All the metals are solids at ordinary temperatures
with the exception of mercury, which is liquid. The metals are mostly
bodies of high specific gravity; they exhibit, when polished, a peculiar
brilliancy or metallic lustre, and they are good conductors of heat and
electricity; the non-metals, on the other hand, are mostly bodies of low
specific gravity, and bad conductors of heat and electricity, and do not
exhibit metallic lustre. The non-metallic elements are also sometimes
termed metalloids, but this appellation, which signifies metal-like
substances (Gr. [Greek: eidos], like), strictly belongs to certain
elements which do not possess the properties of the true metals,
although they more closely resemble them than the non-metals in many
respects; thus, selenium and tellurium, which are closely allied to
sulphur in their chemical properties, although bad conductors of heat
and electricity, exhibit metallic lustre and have relatively high
specific gravities. But when the properties of the elements are
carefully contrasted together it is found that no strict line of
demarcation can be drawn dividing them into two classes; and if they are
arranged in a series, those which are most closely allied in properties
being placed next to each other, it is observed that there is a more or
less regular alteration in properties from term to term in the series.
When binary compounds, or compounds of two elements, are decomposed by
an electric current, the two elements make their appearance at opposite
poles. Those elements which are disengaged at the negative pole are
termed electro-positive, or positive, or basylous elements, whilst those
disengaged at the positive pole are termed electro-negative, or
negative, or chlorous elements. But the difference between these two
classes of elements is one of degree only, and they gradually merge into
each other; moreover the electric relations of elements are not
absolute, but vary according to the state of combination in which they
exist, so that it is just as impossible to divide the elements into two
classes according to this property as it is to separate them into two
distinct classes of metals and non-metals. The following, however, are
negative towards the remaining elements which are more or less
positive:--Fluorine, chlorine, bromine, iodine, oxygen, sulphur,
selenium, tellurium.
The metals may be arranged in a series according to their power of
displacing one another in salt solutions, thus Cs, Rb, K, Na, Mg, Al,
Mn, Zn, Cd, Tl, Fe, Co, Ni, Sn, Pb, (H), Sb, Bi, As, Cu, Hg, Ag, Pd, Pt,
Au.
Elements which readily enter into reaction with each other, and which
develop a large amount of heat on combination, are said to have a
powerful affinity for each other. The tendency of positive elements to
unite with positive elements, or of negative elements to unite with
negative elements, is much less than that of positive elements to unite
with negative elements, and the greater the difference in properties
between two elements the more powerful is their affinity for each other.
Thus, the affinity of hydrogen and oxygen for each other is extremely
powerful, much heat being developed by the combination of these two
elements; when binary compounds of oxygen are decomposed by the electric
current, the oxygen invariably appears at the positive pole, being
negative to all other elements, but the hydrogen of hydrogen compounds
is always disengaged at the negative pole. Hydrogen and oxygen are,
therefore, of very opposite natures, and this is well illustrated by the
circumstance that oxygen combines, with very few exceptions, with all
the remaining elements, whilst compounds of only a limited number with
hydrogen have been obtained.
_Compounds._--A chemical compound contains two or more elements;
consequently it should be possible to analyse it, i.e. separate it into
its components, or to synthesize it, i.e. build it up from its
components. In general, a compound has properties markedly different
from those of the elements of which it is composed.
_Laws of Chemical Combination._--A _molecule_ may be defined as the
smallest part of a substance which can exist alone; an _atom_ as the
smallest part of a substance which can exist in combination. The
molecule of every compound must obviously contain at least two atoms,
and generally the molecules of the elements are also polyatomic, the
elements with monatomic molecules (at moderate temperatures) being
mercury and the gases of the argon group. The laws of chemical
combination are as follows:--
1. _Law of Definite Proportions._--The same compound always contains the
same elements combined together in the same mass proportion. Silver
chloride, for example, in whatever manner it may be prepared, invariably
consists of chlorine and silver in the proportions by weight of 35.45
parts of the former and 107.93 of the latter.
2. _Law of Multiple Proportions._--When the same two elements combine
together to form more than one compound, the different masses of one of
the elements which unite with a constant mass of the other, bear a
simple ratio to one another. Thus, 1 part by weight of hydrogen unites
with 8 parts by weight of oxygen, forming water, and with 16 or 8 x 2
parts of oxygen, forming hydrogen peroxide. Again, in nitrous oxide we
have a compound of 8 parts by weight of oxygen and 14 of nitrogen; in
nitric oxide a compound of 16 or 8 x 2 parts of oxygen and 14 of
nitrogen; in nitrous anhydride a compound of 24 or 8 x 3 parts of oxygen
and 14 of nitrogen; in nitric peroxide a compound of 32 or 8 x 4 parts
of oxygen and 14 of nitrogen; and lastly, in nitric anhydride a compound
of 40 or 8 x 5 parts of oxygen and 14 of nitrogen.
3. _Law of Reciprocal Proportions._--The masses of different elements
which combine separately with one and the same mass of another element,
are either the same as, or simple multiples of, the masses of these
different elements which combine with each other. For instance, 35.45
parts of chlorine and 79.96 parts of bromine combine with 107.93 parts
of silver; and when chlorine and bromine unite it is in the proportion
of 35.45 parts of the former to 79.96 parts of the latter. Iodine unites
with silver in the proportion of 126.97 parts to 107.93 parts of the
latter, but it combines with chlorine in two proportions, viz. in the
proportion of 126.97 parts either to 35.45 or to three times 35.45 parts
of chlorine.
There is a fourth law of chemical combination which only applies to
gases. This law states that:--gases combine with one another in simple
proportions by volume, and the volume of the product (if gaseous) has a
simple ratio to the volumes of the original mixtures; in other words,
the densities of gases are simply related to their combining weights.
_Nomenclature._--If a compound contains two atoms it is termed a binary
compound, if three a ternary, if four a quaternary, and so on. Its
systematic name is formed by replacing the last syllable of the
electro-negative element by _ide_ and prefixing the name of the other
element. For example, compounds of oxygen are _oxides_, of chlorine,
_chlorides_, and so on. If more than one compound be formed from the
same two elements, the difference is shown by prefixing such words as
mono-, di-, tri-, sesqui-, per-, sub-, &c., to the last part of the
name, or the suffixes -_ous_ and -_ic_ may be appended to the name of
the first element. For example take the oxides of nitrogen, N2O, NO,
N2O3, NO2, N2O5; these are known respectively as nitrous oxide, nitric
oxide, nitrogen trioxide, nitrogen peroxide and nitrogen pentoxide. The
affixes -_ous_ and _sub_- refer to the compounds containing more of the
positive element, -_ic_ and _per_- to those containing less.
An _acid_ (q.v.) is a compound of hydrogen, which element can be
replaced by metals, the hydrogen being liberated, giving substances
named _salts_. An _alkali_ or _base_ is a substance which neutralizes an
acid with the production of salts but with no evolution of hydrogen. A
base may be regarded as water in which part of the hydrogen is replaced
by a metal, or by a radical which behaves as a metal. (The term
_radical_ is given to a group of atoms which persist in chemical
changes, behaving as if the group were an element; the commonest is the
ammonium group, NH4, which forms salts similar to the salts of sodium
and potassium.) If the acid contains no oxygen it is a _hydracid_, and
its systematic name is formed from the prefix _hydro_- and the name of
the other element or radical, the last syllable of which has been
replaced by the termination -_ic_. For example, the acid formed by
hydrogen and chlorine is termed hydrochloric acid (and sometimes
hydrogen chloride). If an acid contains oxygen it is termed an
_oxyacid_. The nomenclature of acids follows the same general lines as
that for binary compounds. If one acid be known its name is formed by
the termination -_ic_, e.g. carbonic acid; if two, the one containing
the less amount of oxygen takes the termination _-ous_ and the other the
termination -_ic_, e.g. nitrous acid, HNO2, nitric acid, HNO3. If more
than two be known, the one inferior in oxygen content has the prefix
_hypo_- and the termination -_ous_, and the one superior in oxygen
content has the prefix _per_- and the termination -_ic_. This is
illustrated in the four oxyacids of chlorine, HClO, HClO2, HClO3, HClO4,
which have the names hypochlorous, chlorous, chloric and perchloric
acids. An acid is said to be monobasic, dibasic, tribasic, &c.,
according to the number of replaceable hydrogen atoms; thus HNO3 is
monobasic, sulphuric acid H2SO4 dibasic, phosphoric acid H3PO4 tribasic.
An acid terminating in -_ous_ forms a salt ending in -_ite_, and an
oxyacid ending in -_ic_ forms a salt ending in -_ate_. Thus the chlorine
oxyacids enumerated above form salts named respectively hypochlorites,
chlorites, chlorates and perchlorates. Salts formed from hydracids
terminate in -_ide_, following the rule for binary compounds. An _acid_
salt is one in which the whole amount of hydrogen has not been replaced
by metal; a _normal_ salt is one in which all the hydrogen has been
replaced; and a _basic_ salt is one in which part of the acid of the
normal salt has been replaced by oxygen.
_Chemical Formulae._--Opposite the name of each element in the second
column of the above table, the symbol is given which is always employed
to represent it. This symbol, however, not only represents the
particular element, but a certain definite quantity of it. Thus, the
letter H always stands for 1 atom or 1 part by weight of hydrogen, the
letter N for 1 atom or 14 parts of nitrogen, and the symbol Cl for 1
atom or 35.5 parts of chlorine.[8] Compounds are in like manner
represented by writing the symbols of their constituent elements side by
side, and if more than one atom of each element be present, the number
is indicated by a numeral placed on the right of the symbol of the
element either below or above the line. Thus, hydrochloric acid is
represented by the formula HCl, that is to say, it is a compound of an
atom of hydrogen with an atom of chlorine, or of 1 part by weight of
hydrogen with 35.5 parts by weight of chlorine; again, sulphuric acid is
represented by the formula H2SO4, which is a statement that it consists
of 2 atoms of hydrogen, 1 of sulphur, and 4 of oxygen, and consequently
of certain relative weights of these elements. A figure placed on the
right of a symbol only affects the symbol to which it is attached, but
when figures are placed in front of several symbols all are affected by
it, thus 2H2SO4 means H2SO4 taken twice.
The distribution of weight in chemical change is readily expressed in
the form of equations by the aid of these symbols; the equation
2HCl + Zn = ZnCl2 + H2,
for example, is to be read as meaning that from 73 parts of hydrochloric
acid and 65 parts of zinc, 136 parts of zinc chloride and 2 parts of
hydrogen are produced. The + sign is invariably employed in this way
either to express combination or action upon, the meaning usually
attached to the use of the sign = being that from such and such bodies
such and such other bodies are formed.
Usually, when the symbols of the elements are written or printed with a
figure to the right, it is understood that this indicates a molecule of
the element, the symbol alone representing an atom. Thus, the symbols H2
and P4 indicate that the molecules of hydrogen and phosphorus
respectively contain 2 and 4 atoms. Since, according to the molecular
theory, in all cases of chemical change the action is between molecules,
such symbols as these ought always to be employed. Thus, the formation
of hydrochloric acid from hydrogen and chlorine is correctly represented
by the equation
H2 + Cl2 = 2HCl;
that is to say, a molecule of hydrogen and a molecule of chlorine give
rise to two molecules of hydrochloric acid; whilst the following
equation merely represents the relative weights of the elements which
enter into reaction, and is not a complete expression of what is
supposed to take place:--
H + Cl = HCl.
In all cases it is usual to represent substances by formulae which to
the best of our knowledge express their molecular composition in the
state of gas, and not merely the relative number of atoms which they
contain; thus, acetic acid consists of carbon, hydrogen and oxygen in
the proportion of one atom of carbon, two of hydrogen, and one of
oxygen, but its molecular weight corresponds to the formula C2H4O2,
which therefore is always employed to represent acetic acid. When
chemical change is expressed with the aid of molecular formulae not only
is the distribution of weight represented, but by the mere inspection of
the symbols it is possible to deduce from the law of gaseous combination
mentioned above, the relative volumes which the agents and resultants
occupy in the state of gas if measured at the same temperature and under
the same pressure. Thus, the equation