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1.6 Molecular Compounds
Lewis Symbols
A Lewis symbol is a diagram composed of a chemical symbol and dots, which
represent the electrons in the outer orbit of an atom or ion. To draw the Lewis symbol,
determine how many electrons are in the outer orbit. Place electrons one at a time at
the top, right, bottom, and left side of the symbol. Then, add more electrons by pairing
them, one at a time, in the same order you placed the first four electrons.
Example: Nitrogen atom
The Lewis symbols of elements in the same group will have the same number of dots
as the group number, with the only exception being helium.
Ions can also be represented by Lewis symbols. The Lewis symbol for an ion is placed
in square brackets. The charge of the ion is placed outside the brackets.
Example: Chloride ion
Covalent Bonding and Molecular Compounds
We have seen how a metal and a non-metal form an ionic compound by transferring
electrons to form a cation and an anion. When two non-metal atoms, such as two
oxygen atoms, come together this cannot happen because one oxygen atom would
need to gain two electrons from the second oxygen. This would complete the octet for
the first oxygen atom, but would leave the second oxygen atom with only 4 electrons!
Instead of transferring electrons, two non-metals will share electrons in order to
complete their octets. The bond that forms between the atoms from shared electrons is
known as a covalent bond. Atoms that are joined together by covalent bonds are
known as molecular compounds. Until now we have treated hydrogen as a metal, but
hydrogen also acts like a non-metal.
Molecules can be represented by Lewis structures. A Lewis structure shows the
electrons being shared between the atoms as a dash, and the remaining electrons as
dots. The pairs of remaining electrons around the atom are known as lone pairs.
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Drawing Lewis structures
1. Draw the Lewis symbol for each atom in the reaction. If there are more than two
atoms put the atom with the most unpaired electrons in the middle and put the
other atoms around it.
2. Arrange the shared pairs according to the octet rule.
3. Replace the shared pairs of electrons with lines to represent the bonds.
4. Rewrite the chemical equation using Lewis symbols and Lewis structures.
[Work through examples for F2, O2, N2, H2O, and CO2, if time permits. Be certain
to point out the single, double and triple bonds]
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Naming Molecular Compounds
We have already looked at the names and formulas of ionic compounds. In molecular
compounds a different system is used. If a binary compound is formed from two
nonmetals, it is classified as a molecular compound. Even though there are fewer
nonmetals than metals, there is a wide variety of compounds formed from the
combination of two nonmetals, because two nonmetals may combine to form more than
one compound. For example, N2O, NO, and NO2 are three of the several binary
compounds that can be formed from nitrogen and oxygen. Each compound has different
properties, and so different uses.
In naming compounds formed from two nonmetals, a Greek prefix is attached to the
name of each element in the binary compound indicating the number of atoms of that
element in the molecule. If there is only one of the first type of atom, we leave out the
prefix “mono.” As in ionic compounds we change the name of the second element to
have the “-ide” ending.
Suppose you are asked to write the name for the chemical compound represented by
the formula N2O. Looking at the first element, you can see that the subscript after the
nitrogen is two, so the prefix for nitrogen is “di.” Looking at the second element, you can
see that there is only one oxygen atom, so the prefix for oxygen will be “mono.”
Therefore, the formula’s name is dinitrogen monoxide.
Prefixes Used When Naming Binary Covalent Compounds
Subscript in chemical formula
1
2
3
4
5
6
7
8
9
10
Examples:
NO2
H2O
CF4
nitrogen dioxide
dihydrogen oxide
carbon tetrafluoride
Prefix in chemical name
mono
di
tri
tetra
penta
hexa
hepta
octa
nona
deca
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Worksheet 1.6: Covalent Bonds and Molecular Compounds
1. Write the chemical formula for each of the following molecules:
(a) nitrogen
(b) carbon dioxide
(c) carbon monoxide
(d) nitrogen dioxide
(e) nitrogen monoxide
(f) dinitrogen oxide
(g) dinitrogen tetroxide
(h) sulfur dioxide
(i) diiodine pentoxide
(j) silicon tetrafluoride
(k) boron trifluoride
(l) phosphorus triiodide
(m) diphosphorus pentoxide
(n) sulfur tetrafluoride
(o) phosphorus pentachloride
(p) disulfur dichloride
(q) carbon tetrachloride
(r) sulfur trioxide
(s) sulfur hexafluoride
(t) chlorine dioxide
(u) dinitrogen pentoxide
(v) phosphorus trichloride
(w) silicon tetrachloride
(x) carbon disulphide
(y) phosphorus pentabromide
(z) carbon tetrafluoride
2. Name the compound indicated by each of the following formulas:
(a) SF6(g)
(b) N2O3(g)
(c) NO2(g)
(d) PCl3(l)
(e) PCl5(s)
(f) IF7(g)
(g) BF3(g)
(h) P2S5(s)
(i) P2O5(s)
3. Lewis structures of the following molecular compounds
(a) carbon tetrachloride
(b) chlorine dioxide
(c) phophorus trifluoride
(d) carbon disuphide