Man's physical universe

xanabras

578 MAN IS MASTERING HIS MATERIAL WORLD

that of spider-web filaments; other metals crumble readily. What

causes these differences in the properties of the elements? According

Fig. 277. The argon atom

has eight electrons in the

outer layer. It does not

to the present theory of atomic structure,

the differences in the properties of the elements

are explained in terms of the differences

in the number of electrons in the

outermost levels of the atoms.

When there are eight electrons in the outermost

level, the atoms show no attraction for

each other and refuse to combine with other

atoms of the same kind or with atoms of

different elements. The elements helium,

neon, argon, krypton, xenon, and radon are

all gases ; even the very heavy element radon

is a gas; and these gases consist of molecules

react with other elements.

which have only one atom to the molecule,

In contrast with the rare gases, carbon

and silicon form by far the largest number

of the known compounds of the elements.

These atoms are assumed to have four

electrons in the outer level, and because of

this fact they are the most gregarious of

all the elements. Four carbon atoms may

cooperate, each sharing one of its outside

electrons with one carbon atom, producing an

arrangement somewhat similar to the one

shown in Fig. 279. By cooperating, each

atom is given a stable configuration. In the

diamond, which is pure carbon, we see how

Fig. 279. The carbon

atoms in the diamond are

arranged very compactly.

Fig. 278. Carbon

atoms share pairs of electrons.

well the carbon atoms have joined each

other to form a very hard,

material.

compact, inert

Silicon has an outside structure similar

to that of carbon and likewise forms hard

crystals. Silicon will also combine with

carbon to form the beautiful, very hard

crystals of carborundum.

Silver has only one electron in the outermost

level. Seven other silver atoms would

have to share their electrons to enable one

silver atom to fill its level with eight electrons,

but in the meantime each of these

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