we have a series of elements exhibiting a gradation of properties in the order of the atomic weights. The table on the next page shows this for some properties.

A complete list of the properties of the members of the nitrogen family will not agree so well as in the case of the halogens. When, however, we can compare corresponding compounds having the same number of atoms to the molecule, a fair degree of regularity exists.

376. Exercises.

1. What means are there of kindling fires without the use of phosphorus?

2. How much phosphorus is there in 440 grams (about one pound) of bone-ash, if 70% of the ash is Cas(PO4)2?

3. Express the hydrolysis of antimony and bismuth chlorides by equilibrium equations. Would hydrolysis be favored by the presence of much or little acid? Much or little water? How get the basic chlorides into solution?

4. Write the formulas of the following substances: normal barium phosphate, primary ammonium phosphate, potassium hypophosphite, strontium metaphosphate, and silver pyrophosphate.

5. Why is it undesirable that arsenic compounds should be used to color wall papers? Explain.

6. Show by an equation the reduction of arsenic trioxide by nascent hydrogen to arsine. The combustion of arsine.

7. When a bath of Wood's metal is being melted the unmelted metal floats upon the liquid portion. Compare the specific gravity of the solid with that of the liquid. Will the liquid expand or contract on solidifying?

ELEMENT.

Nitrogen. Phosphorus. Arsenic. Antimony. Bismuth.

PROPERTIES

OF TRIOXIDES.

N2O3 is

an-P2O3 is anhy-As2O3

i s Sb2O3

is Bi203 has

hydride of dride of phos- both weak- like As2O3. only basic nitrous acid. phorous acid. ly acid and

Weaker than weakly
nitrous acid.

properties.

basic.

unites PH3 unites with ASH3 does Sb Hз does BiHз is not
HCl, acids with dif- not unite not unite known to
ficulty.
with acids. with acids. exist.

HYDROGEN COMPOUNDS.

NH3
with
HI, etc., to
form salts.

CHAPTER XXVII.

THE PERIODIC SYSTEM.

377. Natural Families.- During the first half of the nineteenth century various attempts were made to classify the elements. Chemists recognized the fact that there were "natural families of elements, and that the members of the same family (called homologous elements), while bearing a general resemblance to one another, yet showed a regular gradation of properties in the order of the atomic weights.

We have already described two of these families, viz., the halogen family (cf. § 335) and the nitrogen family (cf. § 375).

Other natural families exist. Thus, sulphur and oxygen, with the rarer elements selenium (Se) and tellurium (Te) form a group of homologous elements. The atomic weights are,

O, 16; S, 32; Se, 79; Te, 127.

Again, the elements lithium, sodium, and potassium, with the rare elements rubidium and casium, form the well-known 66 alkali" group of metals. The atomic weights of the three more common metals of the family are,

Li, 7; Na, 23; K, 39.

Here, as in the case of the halogens, we find a continuous gradation of properties in the order of the atomic weights.

THE PERIODIC ARRANGEMENT.

347

Lithium hydroxide, LiOH, is a weaker base than sodium hydroxide, NaOH; while potassium hydroxide, KOH, is the strongest base of the three.

378. The Periodic Arrangement.

Although the connection between the properties and the atomic weights of elements in the same group had been recognized for years, it was left to the Russian chemist Mendelejeff and the German chemist Lothar Meyer to discover, in 1869 to 1871, a new and peculiar relation between the properties of all elements and their atomic weights. This relation is the basis of the Periodic System.

Let us write the symbols of the first sixteen elements in the order of the atomic weights. Omitting hydrogen, which for the present stands almost unrelated, we have,

A study of the elements from lithium to fluorine, inclusive, shows that there is a regular gradation of properties. The strongly metallic properties of lithium are weaker in glucinum, and yet weaker in boron. The hydroxide of boron is, in fact, called boric acid. In carbon we have an element with faintly electro-negative, i. e., non-metallic, properties; the elements nitrogen and oxygen are still more electro-negative; until in fluorine we have a typical

non-metal, and probably the most electro-negative substance known.

The increase of atomic weight from 7 to 19 has thus continuously diminished the electro-positive, or metallic, character possessed by lithium, and has increased the electro-negative character typified by fluorine.

But after fluorine the gradation of properties does not continue; for the element sodium, the next greater in atomic weight, is, like lithium, one of the most typical metals. There is, in fact, a sudden reversion to type"; for sodium belongs in the same natural family with lithium.

Let us, then, proceed in the order of atomic weight, writing the second set of seven elements under the first set, as in the table in $379. Sodium will be under lithium, magnesium under glucinum, etc. We find the same gradation of properties with increase of atomic weight from sodium to chlorine as we found from lithium to fluorine.

Magnesium, like sodium, is a metal; but magnesium hydroxide is a weaker base than sodium hydroxide. Aluminum, the next in the order of atomic weight, is also metallic; but its hydroxide is either a base or an acid, according to circumstances. In silicon, the next element, metallic properties are wanting; silicon forms silicic acid. Next come phosphorus and sulphur, undoubted non-metals; and then chlorine, the first homologue of fluorine.

The element following its atomic weight is 39.

chlorine is potassium; Potassium is a typical