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A formula is a single symbol or a group of symbols that represents a substance.
The symbols in a formula identify the elements present in the substance.
Thus NaCl is the formula for sodium chloride (common table
salt) and so identifies the elements sodium and chlorine as the
constituents of salt.
Subscripts are used in formulas to indicate the
relative numbers of atoms of each type in the compond, but only if more
than one atom of a given element is there.
The formula for water,
H20, tells us that each molecule contains two atoms of
hyrdogen and one atom of oxygen, or that there are two atoms of Hydrogen, and one atom of Oxygen.
In other words that there are two atoms of
hydrogen per one atom of oxygen (2 atoms H / 1 atom O)
The formula of sodium
chloride, NaCl, indicates the presence of equal numbers of atoms of the
elements sodium and chlorine (1 atom Na / 1 atom Cl). Note that in the
formulas for water and sodium chloride the subscript 1 is omitted, which is
the usual practice.
The formula for aluminum sulfate,
Al2(SO4)3, specifies two atoms of Aluminum, Al, for each three sulfate,
SO4, groups. Each sulfate group contains one atom of sulfur
and four atoms of oxygen. Hence, the formula shows a total of two atoms
of aluminum, three atoms of sulfur, and
twelve atoms of oxygen.
A molecular formula gives the actual numbers of atoms of ecah element
in a molecule. An Empirical Formula
(sometimes called a simplest formula) gives the simplest
whole-number ratio of the atoms that it can find. Such as water where
the formula shows that there are 2 atoms of hydrogen and one atom of
oxygen. Since the simplest ratio of hydrogen atoms to oxygen atoms in
water is 2 to 1, the empirical formula is also H2O. The
molecular formula of benzene, C6H6, identifies the
benzene is one carbon atom per one hydrogen atom. Thus the empirical
formula of benzene is CH.
Many compounds do not contain discrete molecules but instead are composed
of particles called ions.
Ions are atoms or groups of atoms that are electrically charged.
They usually arise in the combination of a metal and a nonmetal.
Thus the composition of compounds such as NaCl,
Al2(SO4)3, KOH, and CuSO4
cannot by identified in terms of the nubmer of atoms of each type in a
molecule, but only in terms of a formula unit, as such compounds do not
contain physically distinct and electrically neutral molecules. The
formula units of such compounds are empirircal formulas because they give
only the simplest whole-number ratios of elements in the compound. The
formula for copper sulfate, CuSO4, indicates that there is
exactly one atom of copper for each atom of sulfur and each four atoms of
oxygen present in a sample of the compound, but is an empirical formula
since copper sulfate does not contain distict mulecules composed of a
copper atom, s sulfur atom, and four oxygen atoms. The total numbers of
atoms of each element in any sample of copper sulfate will be
proportional to these numbers.
The mulecular formula of the most common form of sulfur is S8,
showing that each molecule of this element consists of eight atoms. The
molecular formulas for elemental hyrogen and oxygen are H2 and
O2, while the formula for neon is Ne. Note that H2
and 2H do not mean the same thing. H2 represents a
molecule of hydrogen consisting of two atoms of the element, chemically
combined. The expression 2H, on the other hand, is that the two hydrogen
atoms are not in combination as a unit but are separate particles.