An Introduction to Chemical Science | Page 9

case, and that they do not touch one another, the same thing being
true of molecules. Atoms composing sugar must be crowded nearer
together than those of salt. These atoms are probably in constant
motion in the molecule, as the latter is in the mass. If we regard this
square as a mass of matter, the dots may represent molecules; if we call
it a molecule, the dots may be called atoms, though many molecules

have no more than two or three atoms.
The following experiments illustrate the union of atoms to form
molecules, and of elements to form compounds.
12. Union of Atoms.
Experiment 6.--Mix, on a paper, 5 g. of iron turnings, and the same
bulk of powdered sulphur, and transfer them to an ignition tube, a tube
of hard glass for withstanding high temperatures. Hold the tube in the
flame of a burner till the contents have become red-hot. After a minute
break it by holding it under a jet of water. Put the contents into an
evaporating-dish, and look for any uncombined iron or sulphur. Both
iron and sulphur are elements. Is this an example of synthesis or of
analysis? Why? Is the chemical union between masses of iron and
sulphur, or between molecules, or between atoms? Is the product a
compound, an element, or a mixture?
Experiment 7.--Try the same experiment, using copper instead of iron.
The full explanation of these experiments is given on page 13.



CHAPTER IV
.
ELEMENTS AND BINARIES.
13. About Seventy Different Elements are now recognized, half of
which have been discovered within little more than a century. These
differ from one another in (1) atomic weight, (2) physical and chemical
properties, (3) mode of occurrence, etc. Page 12 contains the most
important elements.
The symbol of an element is usually the initial letter or letters of its
Latin name, and stands for one atom of the element. C is the symbol for
carbon, and represents one atom of it. O means one atom of
oxygen.[The symbols of elements will also be used in this book to
stand for an indefinite quantity of them; e.g. O will be used for oxygen
in general as well as for one atom. The text will readily decide when
symbols have a definite meaning, and when they are used in place of
words.] Write, explain, and memorize the symbols of the elements in
heavy type.

14. The Atomic Weight of an element is the weight of its atom
compared with that of hydrogen. H is taken as the standard because it
has the least atomic weight. The atomic weight of O is 16, which
means that its atom weighs 16 times as much as the H atom. Every
symbol, then, stands for a definite weight of the element, i.e. its atomic
weight, as well as for its atom.
How much bromine by weight does Br stand for? What do these
symbols mean--As, Na, N, P? If O represents one atom, how much
does O2 or 2 O stand for? How much by weight? Most elements have
two atoms in the molecule. How many molecules in 6 H? 10 N? S8?
I20?
The symbol of a compound is formed by writing in succession the
symbols of the elements of which it is composed. How many atoms in
the following molecules, and how many of each element: C2H60?
HNO3? PbSO4? MgCl2? (Hg2(NO3)2?)
15. The Simplest Compounds are Binaries.--A binary is a substance
composed of two elements; e.g. common salt, which is a compound of
sodium and chlorine. Its symbol is NaCl, its chemical name sodium
chloride. The ending ide is applied to the last name of binaries. How
many parts by weight of Na and of Cl in NaCl? What is the molecular
weight, i.e. the weight of its molecule? Name KCl. How many atoms in
its molecule? Parts by weight of each element? Molecular weight?
Does the symbol stand for more than one molecule? How many
molecules in 4 NaCl? How many atoms of Na and of Cl? Name these:
HCl, NaBr, NaI, KBr, AgCl, AgI, HBr, HI, HF, HgO, ZnO, ZnS, MgO,
CaO. Compute the proportion by weight of each element in the last
three.
A coefficient before the symbol of a compound includes all the
elements of the symbol, and shows the number of molecules. How
many in these: 6 KBr? 3 Sn0? 12 NaCl? How many atoms of each
element in the above?
An exponent, always written below, applies only to the element after
which it is written, and shows the number of atoms. Explain these:
AuCl3, ZnCl2, Hg2Cl2.
Write symbols for four molecules of sodium bromide, one of silver
iodide (always omit coefficient one), eight of potassium bromide, ten of
hydrogen chloride; also for one molecule of each of these: