Formation of Compounds
Chemical reactions between atoms result in the
formation atoms that achieve a noble gas electron
configuration. Knowing this allows us to predict
the formulas of many compounds.
There are two ways that atoms can achieve a
noble gas electron configuration:
1. Losing or gaining electrons - Results in
ionic compounds where the ions are held together
by ionic bonds.
2. Sharing of electrons - Results in covalent
compounds where covalent bonds hold the atoms
together.
In forming ions elements lose or gain electrons so
that they achieve the same number of electrons as
a noble gas.
Consider sodium, Na. Sodium has 11 protons and
11 electrons. The closest noble gas is Neon, Ne,
which has 10 electrons. Sodium can achieve a
noble gas electron configuration by losing 1
electron.
Na -------> Na+ + e+11 -11
+11 -10 -1
0
+1
Rule- Elements in Groups 1A, 2A, and 3A lose
electrons to become positive ions. The number of
electrons lost corresponds to the group number.
The charge on the ion also corresponds to the
group number.
Example - Cs
Cs+
54 eSr
Sr2+
36 eAl
Al3+
10 eRule - Nonmetals in Groups 5A, 6A, and 7A gain
respectively 1, 2, and 3 e- to achieve a noble gas
electron configuration. The atoms achieve a
negative charge equal to the number of electrons
they have gained.
Example - Br
Br36 eS
S218 eP
P318 e-
Forming Ionic Compounds
When metals react with non-metals they form
ionic compounds called salts.
Salts are formed because the metals lose
electrons and the nonmetals gain electrons. Thus
each element becomes an ion. The solid salt is
held together by the electrical attraction of the
positive and negative charged ions. In forming
salts the number of electrons lost must equal the
number of electrons gained. Knowing this allows
us to predict the formulas of the salt.
Consider the reaction of potassium with chlorine.
Potassium loses 1 e- and becomes a K+ ion.
Chlorine gains 1 electron and becomes a Cl- ion.
Thus there is a balance between the number of
electrons lost and gained. The resultant
compound is made up of K+ and Cl- ions. The
formula of the salt is KCl.
Consider potassium reacting with sulfur. K loses
1 e- becoming K+ and S gains 2e- becoming S2-. In
order balance electrons the salt must contain 2 K+
ions for every S2- ion. Thus the formula is K2S.
Predict formulas for salts formed by reacting
A. Ca with Br
B. Al with O
C. Na with N
Drawing Lewis Structures for atoms
Write the symbol for the element. Using the
periodic table determine the number of valence
(outer level) for the element. Draw dots around
the symbol to represent the electrons. 1 dot
equals 1 electron.
Example - Sodium, Na
Na
Nitrogen, N
N
Bromine, Br (Do not show electrons
for the transition electrons. Show only electron in
Groups 1A through 8A.)
Br
Neon, Ne
Ne
Covalent Compounds
Nonmetals (including Hydrogen) can react with
each other by sharing electrons to form covalent
compounds. By sharing electrons each atom can
achieve a noble gas configuration. Shared
electrons are counted as belonging to both
atoms. This sharing of electrons that holds the
atoms together is called a covalent bond. The
electrons shared are the valence or outer level
electrons.
Example -
Water, H2O
Carbon tetrachloride, CCl4
Methane, CH4
Rules of Drawing Lewis Structures of covalent
compounds
1. Decide what atoms are bonded together. If
atoms are bonded to a central atom, it will be
listed once.
2. Count the total number of valence (outer level )
electrons in all of the atoms and take into
consideration the charge if the substance is an
ion. (Ex- SO32- has 6 + 3x6 +2 = 26 electrons)
3. Place a pair of electrons between the central
atom and each of the other atoms.
4. Distribute the remaining electrons so that each
atom has a filled outer level.
5. When there are not enough electrons, make
double and triple bonds between atoms.
Applying the rules - Draw Lewis structures for the
following substances.
1. H2S
2. HBr
3. PH3
4. O2
5. CO2
6. ClO3-
7. ethane, C2H6