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Objectives: Objectives:
Review: Naming binary covalent compounds; Electronegativity; Lewis diagrams - rules
Non-polar and polar covalent compounds; Contrast covalent compounds and Ionic compounds;
Polyatomic Ions; Cations and Anions; Lewis diagrams for Covalent Ions;
Review:
1.
Rules for naming binary covalent compounds
a.
The first word is the name of the first element in the compound with a prefix indicating the number of atoms
of this element in the compound. The second word is the root of the name of the second element with the
ending -ide and a prefix indicating the number of atoms of this element in the compound,
b.
The prefixes are 1 - mono; 2 - di; 3 - tri; 4 - tetra; 5 - penta; 6 - hexa; 7 - hepta; 8 - octa; 9 - nona; 10 - deca.
2.
Electronegativity is a measure of attraction of valence electrons (electrons likely to be involved in bonding of atoms)
by the nucleus.
3.
Electronegativity increases as we move along a row to the right of the periodic table and it also increases as we
move up along a column.
4.
Procedure for drawing a Lewis diagram
a.
Draw the least electronegative element at the centre and all other elements symmetrically around; hydrogen
(or halogens) is never at the centre.
b.
Compute:
i.
n = # x 8 + ## x 2, where # is the number of non hydrogen atoms; ## is the number of hydrogen
atoms;
ii.
v = total number of valence electrons
iii.
s = n - v is the number of shared electrons
iv.
b = s/2 is the number of bonds
v.
lp = (n - 2 x s)/2 is the number of lone pairs
c.
Place the bonds and lone pairs so that the octet rule is not violated and all bonds and lone pairs are drawn
in the diagram.
Polar and Non-polar covalent compounds
Definition: A nonpolar bond is a bond in which shared electrons are symmetrically placed between bonded atoms. Truly
2
nonpolar molecules are diatomic covalent molecules, e.g., O , etc. In other words, we may say that in nonpolar bonds electrons are
equally shared.
Definition: A polar bond is a bond in which shared electrons are not symmetrically placed between bonded atoms because
they are attracted to one atom more than to the other atom. For example, the bond between hydrogen and fluorine is a polar bond
in which the fluorine atom attracts the electrons more than the hydrogen atom does and therefore, the two shared electrons are
located closer to the fluorine atom in HF molecules. An atom with higher electronegativity becomes a negative pole and the atom with
lower electronegativity becomes a positive pole. In extreme cases, a polar bond may resemble a bond between a positive and a
negative ion in an ionic compound more than it resembles a nonpolar bond. The distinction between an ionic bond and a covalent
bond is not absolute. We may think of it as a continuum
ionic bond
very polar covalent bond

slightly polar covalent bond
nonpolar covalent bond
It is important to note that covalent compounds consist of molecules (as particles) while ionic compounds consist of two different
2
particles: positive and negative ions. For example, carbon dioxide consists of CO particles. On the other hand, magnesium chloride
consists of two different particles: Mg and Cl . There are twice as many Cl particles than there are Mg particles in this compound.
2+
-
-
2+
2
2
Having said this, in computation of molar mass and moles we will refer to molar mass of MgCl ; we will say that 1 mol of MgCl has
2
2
6.02 x 10 MgCl molecules; we may say that 1 mol of MgCl corresponds to 2 mol of Cl. Although we know it is not really true, we
23
2
think of MgCl as a unit called molecule for the purpose of calculations and writing of simple conversion factors.
Properties of ionic and covalent compounds.
As we have discussed before, the interaction between atoms is electrical. Nonpolar molecules dont interact electrically with each other
because they are truly neutral. Consequently, molecules move freely and independently of each other. They dont attach to other
molecules. This behaviour is very characteristic of gases. Molecules are generally far from each other and only occasionally collide
with each other much like billiard balls.
Molecules with polar bonds interact electrically with each other because one side of the molecule is positive and the other side is
negative. The force of interaction between polar molecules may be weak and then the substance is usually a gas. When the force of
interaction between molecules is strong then the substance is usually a liquid (for example water, etc.). The average distance between
the molecules in liquids is 10 times smaller than the average distance between molecules in gases. The molecules move freely but
are forced to keep distances between them small.
An ionic compound consists of two ions that interact strongly. Most ionic compounds are solids. The ions occupy edges of geometrical
shapes, e.g., a cube in case of ordinary salt NaCl. Ions are only free to vibrate around fixed positions. The average distances between
ions and the average distances between molecules in liquids are similar.
When ionic compounds are mixed with polar compounds, they may dissolve. For example, when we mix an ordinary salt (ionic
2
2
compound NaCl) with water (polar molecules H O) the forces between poles of H O molecule and Na or Cl ions must be stronger
+
- than the attractive force between the two ions because the salt crystals disappear. We will discuss this issue a little later.
Note that all negative ions are called anions and all positive ions are called cations.
Polyatomic Ions
So far we have discussed binary compounds. In binary ionic compounds, each ion had only one atom. We will expand our discussion
to compounds that involve polyatomic ions, i.e ions that have covalent bonds between two or more atoms. You have to learn their
names and formulas.
4 You need to remember a positive ion called ammonium ion (NH ) and two negative ions: hydroxide ion (OH) and
+
-
cyanide ion (CN) . The ammonium ion and hydroxide ion are the two most common ions found in nature.
- You also need to remember a host of negative ions called oxyanions. To make remembering easier we organized them in a
table below according to their charge:
Charge of 1
Charge of 2
Charge of 3
-
-
-
Formula
Name
Formula
Name
Formula
Name
3
3
4
(NO )
nitrate ion
(CO )
carbonate ion
(PO )
phosphate ion
-
2-
3-
2
4
3
(NO )
nitrite ion
(SO )
sulfate ion
(PO )
phosphite ion
-
2-
3-
3
3
(ClO )
chlorate ion
(SO )
sulfite ion
-
2-
2
(ClO )
chlorite ion
-
3
(BrO )
bromate ion
-
2
(BrO )
bromite ion
-
3
(IO )
iodate ion
-
2
(IO )
iodite ion
-
Patterns:

-ate ions have one more oxygen atom than -ite ions polyatomic ions formed with halogens/nitrogen gain one electron; these ions have two or three oxygen atoms. polyatomic ions formed with sulfur/carbon gain two electrons; the carbonate ion has three oxygen atoms and sulfur-based
ions have three or four oxygen atoms polyatomic ions formed with phosphorus gain three electrons; the ions have three or four oxygen atoms
Lewis Diagrams of Polyatomic Ions
A procedure for constructing Lewis diagrams is the same as the procedure for constructing Lewis diagrams for neutral molecules with
one exception. We have to change the computation of the total number of valence electrons v. Otherwise, we compute n, s (the
number of shared electrons), the number of bonds and the number of lone pairs as before.
A negative ion is negative because it gained one or two or three valence electrons. Consequently, the total number of valence
electrons is the sum of the valence electrons of all the atoms in a neutral compound plus the electrons gained. For example, (OH) has
-
two atoms. A neutral oxygen atom has 6 valence electrons and a neutral hydrogen atom has 1 valence electron. Thus the total number
of valence electrons is 7 in OH. A negative ion (OH) has gained one electron and consequently, the value of v for (OH) is 7 + 1 = 8.
-
-
Example 1. Draw a Lewis diagram for (OH)
-
n = 8 + 2 = 10
v = 6 + 1 +1 = 8 (oxygen has 6 valence electrons; hydrogen has 1 valence electron; this polyatomic ion gained 1 valence electron as
indicated by its charge 1-)
s = n - v = 2
bonds = s/2 = 1
lp = (10 - 2 x 2)/2 = 3
Custom is to put square brackets around the diagram and indicate the charge of the ion.
4
Example 2. Draw a Lewis diagram for (PO )
3-
n = 5 x 8 = 40 v = 5 + 4 x 6 +3 = 32 (phosphorus has 5 valence electrons; each of the four oxygen atoms has 6 valence electrons and this polyatomic
ion gained 3 electrons as indicated by its cha