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Chemical Compounds
When atoms approach other in a chemical reaction, the electrons of the atoms interact to form
chemical bonds. Compounds are substances composed of more than one element, chemically
combined.
E. g. HCl, H2O, NH3
In general, compounds can be classified into two types: Molecular and ionic.
Molecular Compounds
A molecular compound is made up of discrete units called molecules, which typically consist of
two or more of nonmetal atoms held together by covalent bonds. A covalent bond, the most
common kind of chemical bond, results when two atoms share electrons.
Even some elements exit as molecules rather than as atoms.
Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine, iodine, sulfur and phosphorus all exist
as molecules whose atoms held together by covalent bond. Therefore, we have to write them as
H2, N2, O2, F2, Cl2, Br2, I2, S8 and P4 when using any of these elements in a chemical equation.
Chemical Formulas
A compound is represented by giving its chemical formula, a notation that uses atomic symbols
with numerical subscripts to convey the relative proportion of atoms of different elements in the
substance.
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Some Common Types of Formulas
There are different ways to represent a molecule: it will have a name, it has a chemical formula
which displays the number and types of atoms that make up the molecule, and it has a structural
formula which shows how the various atoms are attached to one another.
i) An empirical formula is the simplest formula for a compound; it shows the types of atoms
present and the ratio of atoms in a compound. Compounds with different molecular formulas
can have the same empirical formulas and such substances will have the same percentage
composition. E g. formaldehyde (CH2O), Acetic acid (C2H4O2), and glucose (C6H12O6) all have
the empirical formula. Generally, empirical formula does not tell us much about a molecule.
ii) A molecular formula is based on an actual molecule of a compound. It gives the exact
number of different atoms of an element in a molecule. In some cases, the empirical formula and
the molecular formula are identical
E.g. formaldehyde CH2O. In other cases, the molecular formula is a multiple of the empirical
formula unit E.g. C6H12O6 = (CH2O)6
Empirical and molecular formulas tell us the combining ratio of the atoms in the compound, but
show nothing about how the atoms are attached to each other.
There are other types of formula that will show the connectivity of atoms in a molecule.
iii) A structural formula shows the order in which atoms are bonded together in a molecule and
by what types of bonds. The covalent bonds in the structure formula are represented by lines
(). Each line represents one bond.
iv) A condensed structural formula: a less cumbersome way of showing how the atoms are
connected
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To show three-dimensional structures of molecules is by structural models
ball-and-stick
type
space-filling
type
1) ball-and stick-model, the centers of the bonded atoms are represented by small balls, and the
bonds between atoms by sticks. Such model help us to visualize distances between the centers of
atoms (bond length) and the geometrical shapes of molecules.
2) A space-filling model shows that the atoms in a molecule occupy space and that they are in
actual contact with one another. This model is the most accurate representation of the size and
shape of a molecule because it constructed to scale.
Ionic Compounds
~ A ionic compound is made up of positive and negative ions (cation and anion) joined together
by ionic bonds.
A ionic bond result not from sharing electrons but from a transfer of one or more electrons from
one atom to another then joint together by electrostatic forces of attraction. For example,
the arrangement of Na+ions and Cl-ions in a crystal of sodium chloride. Each Na+ion is
surrounded by six neighboring Cl-ions, and each Cl-ion is surrounded by six neighboring Na+
ions. Thus, there is no discrete “molecule” of NaCl. Instead, the entire crystal is an ionic solid.
The formula unit of an ionic compound is the smallest electrically neutral collection of ions. The
ratio of atoms (ions) in the formula unit is the same as in the chemical formula
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Organic and Inorganic Compounds
Chemical compounds also can be classified as organic or inorganic. Organic compounds are
those formed by carbon and hydrogen (hydrocarbon) or carbon and hydrogen together with
oxygen, nitrogen, and a few other elements. Inorganic compounds are compounds composed of
elements other than carbon. Except a few simple compounds of carbon, including carbon
monoxide, carbon dioxide, carbonates and cyanides are generally considered to be inorganic.
Nomenclature: Naming Simple Compounds
In order to communicate clearly, a systematic way of naming chemical compounds –
nomenclature- has been developed and continually been revised by International Union of Pure
and Applied Chemistry (IUPAC). The common or trivial names of some compounds such water
(H2O), ammonia (NH3) etc. are still commonly used among scientists, you should known both
their common and systematic names.
Here we are only concerned about inorganic nomenclature. Organic compounds have
their own set of nomenclature rules, you will learn them in organic chemistry section.
I. Binary compounds: are those composed of only two elements.
1) Binary compounds of metals and nonmetals.
If one element is metal and the other a nonmetal; the binary compound is made up of ions. The
simplest are monatomic ions. A monatomic ion is an ion formed from a single atom. Common
monatomic ions of are listed in Table 4.1
Table 4.1 Common Monatomic Ions of Man-Group Elements
IA
IIA
IIIA
IVA
VA
VIA
IIA
+
Period 1
H,
H(hydride)
Period 2
Li+
Be2+
B
C
N3O2F+
2+
3+
2Period 3
Na
Mg
Al
Si
P
S
Cl+
2+
3+
2Period 4
K
Ca
Ga
Ge
As
Se
Br+
2+
3+
2+
4+
2Period 5
Rb
Sr
In
Sn , Sn
Sb
Te
I+
2+
+
3+
2+
2+
3+
Period 6
Cs
Ba
Tl , Tl
Pb , Pb
Bi
* Elements shown in shade do not normally form compounds having monatomic ions.
When use old group number- Roman numeral plus letter A or B, the charge on a cation for maingroup element, in most cases, equals the group number ( the Roman numeral). For example,
aluminum, which is in group IIIA, has an ion with charge 3+. Some exceptions are noted for
elements of high atomic number such as lead, in group IVA, whose common ion is Pb 2+. The
charge on an anion euals the group number minus 8. Oxygen, in group VIA, has an anion charge
of 2- (6-8= -2).