We may remark here the gradual increase of valency as we pass from left to right in the periodic table. Lithium is a monad, with its congeners; the elements of the beryllium group are dyads; boron a triad; carbon a tetrad; phosphorus acts as pentad as well as triad ; sulphur, as a pseudo-monad, a dyad, and a tetrad; and sulphur, selenium and tellurium burn in fluorine, forming very stable hexafluorides, SF_G, SeF₆ and TeF₆; while iodine forms a monochloride and a trichloride, and also a pentafluoride.

Passing back to the boron group, if it is desired to form anhydrous chloride, it is necessary either to heat the element, or its oxide mixed with charcoal, in a current of chlorine, or, except in the case of boron, to prepare a double salt of the chloride with ammonium chloride, and to volatilise the latter after driving off the water; the aqueous chlorides are formed by dissolving the oxides or hydroxides in hydrochloric acid. Thallium forms monohalides, sparingly soluble in cold water, and thereby attaches itself to the copper group.

Almost the same remarks apply to the elements of the carbon group; solutions of the chlorides, with exception of those of carbon and silicon, are obtained from the element and hydrochloric acid or from the hydroxide, and they cannot be dried without reacting wholly or partially with water. For instance, titanium chloride, on careful addition of water, can become ClTi(OH)₃, Cl₂Ti(OH)₂, Cl₃Ti(OH), all of which are intermediate products between the tetrachloride and the tetrahydroxide 5 such compounds are termed "basic chlorides." Anhydrous stannic chloride is a fuming liquid, formed by the distillation of a mixture of the metal with mercuric chloride or by heating the metal in a stream of chlorine. Lead tetrachloride is a very unstable liquid, formed from the tetracetate, Pb(C₂H₃O₂)₄, by converting it into the double ammonium salt with a mixture of ammonium chloride and concentrated hydrochloric acid; this salt, (NH₄)₂PbCl₆, is then decomposed by concentrated sulphuric acid, when the tetrachloride separates as a heavy liquid. It at once decomposes into PbCl₂ + Cl₂ on warming; hence PbO₂, when warmed with hydrochloric acid, undergoes the change: PbO₂ + ₄HC1. Aq = PbCl₂ + Aq + Cl₂.

Tin and lead resemble elements of the zinc group in forming dichlorides. On dissolving tin in hydrochloric acid the dichloride is formed; and a solution of the tetrachloride, when exposed to the action of nascent hydrogen, yields the lower chloride. This action may be thus formulated :-Sn|-Cl₄.Aq + ₂H = Sn^II|=Cl₂.Aq+ ₂H|-Cl.Aq. Stannous chloride is a white, soluble salt, crystallising with water of crystallisation. Lead dichloride, on the other hand, is sparingly soluble in cold water ; it is formed when a soluble lead salt, such as the nitrate, is mixed with the solution of a chloride: Pb(NO₃)₂.Aq + 2NaCI. Aq = PbCl₂ + 2NaCl.Aq. The bromide and the iodide are also sparingly soluble, and are similarly produced.

With arsenic and the remaining members of that group we may notice the same characters : the anhydrous chlorides produced by the action of chlorine on the element, or, when it is not available, on a mixture of the oxide with carbon at a red heat; the aqueous solution produced by dissolving the oxide or hydroxide in hydrochloric acid. Basic • or oxy-chlorides are also known, e.g. ClAsO, CISbO, and ClBiO, from the trichlorides; and OPCl₃, and OSbCl₃, from the pentachlorides, on reacting with a small amount of water.