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NAMIMG COMPOUNDS
In the early days of chemistry, there was no system for the naming of compounds.
Chemists used common names like bicarb of soda, quicklime, milk of magnesia, Epsom salts, spirits of salt,
and laughing gas to describe compounds.
As the number of named compounds increased it was obvious that if such common names were used,
confusion would result. In 1787, in order to solve the problem, a scientist named Lavoisier established the
principles for a systematic naming process.
There are, however, some familiar compounds that are always referred to by their common names. The
systematic name for H20, for example, is never used. What would you prefer to call it: dihydrogen
monoxide or water?
There are two types of compounds (ionic and covalent) that require naming, and different rules apply for
naming each type.
Ionic compounds
• Ions fonn when atoms gain or lose outer-shell electrons. Whereas atoms are neutral, ions are
charged.
• If an atom loses electrons, then its ion is positive (there are more protons than electrons).
• If an atom gains electrons then the charge of its ion will be negative (there are more electrons
than prcltOilS
• When the ions come together to form compounds, they combine in a ratio that gives the
compound a total charge of zero. There must be enough negative charges to balance the
positive charges and vice versa.
• Sodium ions have a+1 charge and chloride ions have a -1 charge. Add these charges
together:+1 + (-1) = 0.
Thus, one of each ion join to give a compound with a total charge of zero. The formula is
NaCI, and the name of this compound is sodium chloride.
sodium
(2,8, 1)
chlorine
(2, 8, 7)
sodium ion
(2, 8)
chloride ion
(2, 8, 8)
0
Na
Cl
An electron is
transferred from
sodium to chlorine.
These positive and
negative ions are
attracted to each
other and form a
crystal where the
ions are stacked
to maximise attraction.
Cl-
For example, Na+ is called
•
simple 11egative ion is named by taking the first part of the parent element's name and
adding thesuffix ciqe. Cl- was originally a chlorine atom but is now an ion and is given the
new name $@1M!. Likewise, Br- (originally bromine) is called bromide, 02- (originally
is called oxide and N3is called nitride.
o2- (2,8)
Na+ (2,8)
s2- (2,8,8)
Ca2+ (2,8,8)
Ns.. (2,8)
Calcium nitride
Note that the charges of the ions are not included in compound formulas, but the numbers of each ion (the
subscript numbers) are included. These subscript numbers indicate how many of each ion are in the formula.
For instance, MgClz indicates that there are one magnesium ion and two chloride ions in the formula.
Metal ions
The formation of ions was covered in THE PERIODIC
Group I (the
· metals) always
ions with a+ I charge.
• Metals in Group II (the alkaline earth metals) always form ions with a +2 charge.
• Metals in Group III always form ions with a +3 charge.
• Metals in Group IV often have a +4 charge but can also have a charge of +2. Assume +2 unless you are told
otherwise.
• Transition metals have a variety of ionic charges, but most form ions with a +2 charge.
• If a metal has more than one common ion, the charge it takes is shown with Roman numerals.
For example, copper(I) = Cu+, copper(II) = Cu2+, iron(II) =FeZ+, iron(III) = Fe3+.
• The metals in Groups V and VI also have charges that can vary.
Thesa atoms gain electrons to get a
noble gas electronic configuration.
2,8
2,8
2,8,7
2,8,6
2,8, 8
2, 8,8
e
In these bonds, no electro.tis ate
Instead, atoms
order to gain a noble gas electron configuration. These links are called
e
Figure 2.2.5 shows how pairs of electrons are shared in an ammonia (NH3) molecule.
m
methane. CH4
ammonia, NH 3
These covalent compounds show atoms
sharing electrons to gain noble gas
electronic configurations.
f!jl nS Mn"lft H i:slfmi ®U'W' even though there are no ions
present.
The
rules apply:
• The
in the chemical formula is named first, using the
• The
in the formula is named as if it were a negative ion.
•
(see the table on following page).
• If the first element exists as a single atom, no prefix is used. For example, C02 is called carbon
dioxide.
To avoid awkward pronunciations, the final 'o' or 'a' of the prefix is often dropped when the
element name begins with a vowel. For example, CO is called carbon monoxide, not carbon monooxide.
Some examples of covalent compounds and their names are:
• C02 = carbon dioxide
• CO = carbon monoxide
• N205 = dinitrogen pentoxide
• CC14 = carbon tetrachloride
• NH3 =nitrogen trihydride (commonly known as ammonia)
• CH4= carbon tetrahydride (commonly known as methane)
2
di-
3
tri-
4
tetra-
5
pent-
6
hex-
7
hept-
8
oct-
9
non-
10
dec-
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