PERIODIC LAW
A number of the elements have now been studied somewhat closely. The
first three of these, oxygen, hydrogen, and nitrogen, while having some
physical properties in common with each other, have almost no point of
similarity as regards their chemical conduct. On the other hand, oxygen
and sulphur, while quite different physically, have much in common in
their chemical properties.
About eighty elements are now known. If all of these should have
properties as diverse as do oxygen, hydrogen, and nitrogen, the study of
chemistry would plainly be a very difficult and complicated one. If,
however, the elements can be classified in groups, the members of which
have very similar properties, the study will be very much simplified.
~Earlier classification of the elements.~ Even at an early period efforts
were made to discover some natural principle in accordance with which
the elements could be classified. Two of these classifications may be
mentioned here.
1. _Classification into metals and non-metals._ The classification into
metals and non-metals most naturally suggested itself. This grouping was
based largely on physical properties, the metals being heavy, lustrous,
malleable, ductile, and good conductors of heat and electricity.
Elements possessing these properties are usually base-forming in
character, and the ability to form bases came to be regarded as a
characteristic property of the metals. The non-metals possessed
physical properties which were the reverse of those of the metals, and
were acid-forming in character.
Not much was gained by this classification, and it was very imperfect.
Some metals, such as potassium, are very light; some non-metals, such as
iodine, have a high luster; some elements can form either an acid or a
base.
2. _Classification into triad families._ In 1825 Döbereiner observed
that an interesting relation exists between the atomic weights of
chemically similar elements. To illustrate, lithium, sodium, and
potassium resemble each other very closely, and the atomic weight of
sodium is almost exactly an arithmetical mean between those of the other
two: (7.03 + 39.15)/2 = 23.09. In many chemical and physical properties
sodium is midway between the other two.
A number of triad families were found, but among eighty elements, whose
atomic weights range all the way from 1 to 240, such agreements might be
mere chance. Moreover many elements did not appear to belong to such
families.
~Periodic division.~ In 1869 the Russian chemist Mendeléeff devised an
arrangement of the elements based on their atomic weights, which has
proved to be of great service in the comparative study of the elements.
A few months later the German, Lothar Meyer, independently suggested the
same ideas. This arrangement brought to light a great generalization,
now known as the _periodic law_. An exact statement of the law will be
given after the method of arranging the elements has been described.
[Illustration: DMITRI IVANOVITCH MENDELÉEFF (Russian) (1834-1907)
Author of the periodic law; made many investigations on the physical
constants of elements and compounds; wrote an important book entitled
"Principles of Chemistry"; university professor and government
official]
~Arrangement of the periodic table.~ The arrangement suggested by
Mendeléeff, modified somewhat by more recent investigations, is as
follows: Beginning with lithium, which has an atomic weight of 7, the
elements are arranged in a horizontal row in the order of their atomic
weights, thus:
~Li (7.03), Be (9.1), B (11), C (12), N (14.04), O (16), F (19).~
These seven elements all differ markedly from each other. The eighth
element, sodium, is very similar to lithium. It is placed just under
lithium, and a new row follows:
~Na(23.05), Mg (24.36), Al (27.1), Si (28.4), P (31), S (32.06),
Cl(35.45).~
When the fifteenth element, potassium, is reached, it is placed under
sodium, to which it is very similar, and serves to begin a third row:
~K (39.15), Ca (40.1), Sc (44.1,) Ti (48.1), V (51.2), Cr (52.1), Mn(55).~
Not only is there a strong similarity between lithium, sodium, and
potassium, which have been placed in a vertical row because of this
resemblance, but the elements in the other vertical rows exhibit much of
the same kind of similarity among themselves, and evidently form little
natural groups.
The three elements following manganese, namely, iron, nickel, and
cobalt, have atomic weights near together, and are very similar
chemically. They do not strongly resemble any of the elements so far
considered, and are accordingly placed in a group by themselves,
following manganese. A new row is begun with copper, which somewhat
resembles the elements of the first vertical column. Following the fifth
and seventh rows are groups of three closely related elements, so that
the completed arrangement has the appearance represented in the table on
page 168.
THE PERIODIC ARRANGEMENT OF THE ELEMENTS
--------+-----------+-----------+-----------+-----------+-----------+
Periods | GROUP | GROUP | GROUP | GROUP | GROUP |
| 0 | I | II | III | IV |
|A B|A B|A B|A B|A B|
--------+-----------+-----------+-----------+-----------+-----------+
1 |H==1.008 | | | | |
2 |He=4 |Li=7.03 |Be=9.1 |B=11 |C=12 |
--------+-----------+-----------+-----------+-----------+-----------+
3 | Ne=20|Na=23.05 | Mg=24.36| AL=27.1| Si=28.4|
--------+-----------+-----------+-----------+-----------+-----------+
4 |A=39.9 |K=39.15 |Ca=40.1 |Sc=44.1 |Ti=48.1 |
| | | | | |
| | | | | |
--------+-----------+-----------+-----------+-----------+-----------+
5 | | Cu=63.6| Zn=65.4| Ga=70| Ge=72.5|
--------+-----------+-----------+-----------+-----------+-----------+
6 |Kr=81.8 |Rb=85.5 |Sr=87.6 |Y=89 |Zr=90.6 |
| | | | | |
| | | | | |
--------+-----------+-----------+-----------+-----------+-----------+
7 | | Ag=107.93| Cd=112.4| In=115| Sn=119|
--------+-----------+-----------+-----------+-----------+-----------+
8 |X=128 |Cs=132.9 |Ba=137.4 |La=138.9 |Ce=Yb* |
| | | | |140.25-173 |
| | | | | |
--------+-----------+-----------+-----------+-----------+-----------+
9 | Au=197.2| Hg=200| Tl=204.1| Pb=206.9| Bi=208.5|
--------+-----------+-----------+-----------+-----------+-----------+
10 | | |Ra=225 | |Th=232.5 |
--------+-----------+-----------+-----------+-----------+-----------+
| | R_{2}O | RO |R_{2}O_{3} | RO_{2} |
| | RH | RH_{2} | RH_{3} | RH_{4} |
--------+-----------+-----------+-----------+-----------+-----------+
==================part 2==============
--------+-----------+-----------+-----------+-----------+
Periods | GROUP | GROUP | GROUP | GROUP |
| V | VI | VII | VIII |
|A B|A B|A B| |
--------+-----------+-----------+-----------+-----------+
1 | | | | |
2 |N=14.04 |O=16 |F=19 | |
--------+-----------+-----------+-----------+-----------+
3 | P=31| S=32.06| Cl=35.45| |
--------+-----------+-----------+-----------+-----------+
4 |V=51.2 |Cr=52.1 |Mn=55 |Fe=55.9 |
| | | |Ni=58.7 |
| | | |Co=59 |
--------+-----------+-----------+-----------+-----------+
5 | As=75| Se=79.2| Br=79.96| |
--------+-----------+-----------+-----------+-----------+
6 |Cb=94 |Mo=96 | |Ru=101.7 |
| | | |Rh=103 |
| | | |Pd=106.5 |
--------+-----------+-----------+-----------+-----------+
7 | Sb=120.2| Te=127.6| I=126.97| |
--------+-----------+-----------+-----------+-----------+
8 |Ta=183 |W=184 | |Os=191 |
| | | |Ir=193 |
| | | |Pt=194.8 |
--------+-----------+-----------+-----------+-----------+
9 | | | | |
--------+-----------+-----------+-----------+-----------+
10 | U=238.5 | | | |
--------+-----------+-----------+-----------+-----------+
| R_{2}O_{5}| RO_{3} | R_{2}O_{7}| RO_{4} |
| RH_{3} | RH_{2} | RH | |
--------+-----------+-----------+-----------+-----------+
[* This includes a number of elements whose atomic weights lie
between 140 and 173, but which have not been accurately studied, and
so their proper arrangement is uncertain.]
~Place of the atmospheric elements.~ When argon was discovered it was seen
at once that there was no place in the table for an element of atomic
weight approximately 40. When the other inactive elements were found,
however, it became apparent that they form a group just preceding Group
1. They are accordingly arranged in this way in Group 0 (see table on
opposite page). A study of this table brings to light certain very
striking facts.
~Properties of elements vary with atomic weights.~ There is evidently a
close relation between the properties of an element and its atomic
weight. Lithium, at the beginning of the first group, is a very strong
base-forming element, with pronounced metallic properties. Beryllium,
following lithium, is less strongly base-forming, while boron has some
base-forming and some acid-forming properties. In carbon all
base-forming properties have disappeared, and the acid-forming
properties are more marked than in boron. These become still more
emphasized as we pass through nitrogen and oxygen, until on reaching
fluorine we have one of the strongest acid-forming elements. The
properties of these seven elements therefore vary regularly with their
atomic weights, or, in mathematical language, are regular functions of
them.
~Periodic law.~ The properties of the first seven elements vary
_continuously_--that is steadily--away from base-forming and toward
acid-forming properties. If lithium had the smallest atomic weight of
any of the elements, and fluorine the greatest, so that in passing from
one to the other we had included all the elements, we could say that the
properties of elements are continuous functions of their atomic weights.
But fluorine is an element of small atomic weight, and the one following
it, sodium, breaks the regular order, for in it reappear all the
characteristic properties of lithium. Magnesium, following sodium, bears
much the same relation to beryllium that sodium does to lithium, and
the properties of the elements in the second row vary much as they do in
the first row until potassium is reached, when another repetition
begins. The properties of the elements do not vary continuously,
therefore, with atomic weights, but at regular intervals there is a
repetition, or _period_. This generalization is known as the _periodic
law_, and may be stated thus: _The properties of elements are periodic
functions of their atomic weights._
~The two families in a group.~ While all the elements in a given vertical
column bear a general resemblance to each other, it has been noticed
that those belonging to periods having even numbers are very strikingly
similar to each other. They are placed at the left side of the group
columns. In like manner, the elements belonging to the odd periods are
very similar and are arranged at the right side of the group columns.
Thus calcium, strontium, and barium are very much alike; so, too, are
magnesium, zinc, and cadmium. The resemblance between calcium and
magnesium, or strontium and zinc, is much less marked. This method of
arrangement therefore divides each group into two families, each
containing four or five members, between which there is a great
similarity.
~Family resemblances.~ Let us now inquire more closely in what respects
the elements of a family resemble each other.
1. _Valence._ In general the valence of the elements in a family is the
same, and the formulas of their compounds are therefore similar. If we
know that the formula of sodium chloride is NaCl, it is pretty certain
that the formula of potassium chloride will be KCl--not KCl_{2} or
KCl_{3}. The general formulas R_{2}O, RO, etc., placed below the
columns show the formulas of the oxides of the elements in the column
provided they form oxides. In like manner the formulas RH, RH_{2}, etc.,
show the composition of the compounds formed with hydrogen or chlorine.
2. _Chemical properties._ The chemical properties of the members of a
family are quite similar. If one member is a metal, the others usually
are; if one is a non-metal, so, too, are the others. The families in the
first two columns consist of metals, while the elements found in the
last two columns form acids. There is in addition a certain regularity
in properties of the elements in each family. If the element at the head
of the family is a strong acid-forming element, this property is likely
to diminish gradually, as we pass to the members of the family with
higher atomic weights. Thus phosphorus is strongly acid-forming, arsenic
less so, antimony still less so, while bismuth has almost no
acid-forming properties. We shall meet with many illustrations of this
fact.
3. _Physical properties._ In the same way, the physical properties of
the members of a family are in general somewhat similar, and show a
regular gradation as we pass from element to element in the family. Thus
the densities of the members of the magnesium family are
Mg = 1.75, Zn = 7.00, Cd = 8.67, Hg = 13.6.
Their melting points are
Mg = 750°, Zn = 420°, Cd = 320°, Hg = -39.5°.
~Value of the periodic law.~ The periodic law has proved of much value in
the development of the science of chemistry.
1. _It simplifies study._ It is at once evident that such regularities
very much simplify the study of chemistry. A thorough study of one
element of a family makes the study of the other members a much easier
task, since so many of the properties and chemical reactions of the
elements are similar. Thus, having studied the element sulphur in some
detail, it is not necessary to study selenium and tellurium so closely,
for most of their properties can be predicted from the relation which
they sustain to sulphur.
2. _It predicts new elements._ When the periodic law was first
formulated there were a number of vacant places in the table which
evidently belonged to elements at that time unknown. From their position
in the table, Mendeléeff predicted with great precision the properties
of the elements which he felt sure would one day be discovered to fill
these places. Three of them, scandium, germanium, and gallium, were
found within fifteen years, and their properties agreed in a remarkable
way with the predictions of Mendeléeff. There are still some vacant
places in the table, especially among the heavier elements.
3. _It corrects errors._ The physical constants of many of the elements
did not at first agree with those demanded by the periodic law, and a
further study of many such cases showed that errors had been made. The
law has therefore done much service in indicating probable error.
~Imperfections of the law.~ There still remain a good many features which
must be regarded as imperfections in the law. Most conspicuous is the
fact that the element hydrogen has no place in the table. In some of the
groups elements appear in one of the families, while all of their
properties show that they belong in the other. Thus sodium belongs with
lithium and not with copper; fluorine belongs with chlorine and not with
manganese. There are two instances where the elements must be
transposed in order to make them fit into their proper group. According
to their atomic weights, tellurium should follow iodine, and argon
should follow potassium. Their properties show in each case that this
order must be reversed. The table separates some elements altogether
which, in many respects have closely agreeing properties. Iron,
chromium, and manganese are all in different groups, although they are
similar in many respects.
The system is therefore to be regarded as but a partial and imperfect
expression of some very important and fundamental relation between the
substances which we know as elements, the exact nature of this relation
being as yet not completely clear to us.
EXERCISES
1. Suppose that an element were discovered that filled the blank in
Group O, Period 5; what properties would it probably have?
2. Suppose that an element were discovered that filled the blank in
Group VI, Period 9, family B; what properties would it have?
3. Sulphur and oxygen both belong in Group VI, although in different
families; in what respects are the two similar?