Compounds
A molecule is a
group of atoms in
which the atoms
are bond together
by 1 or more
pairs of electrons.
Chapter 3.1
If H20 is water what is H204?
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Drinking……
Water is a compound with elements chemically
bonded in a formula, H2O
H is for hydrogen;
O is for oxygen;
2 means there are two ions of hydrogen
there is only one ion of oxygen
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Note: do not write a one for the oxygen in the formula
MgO
1
C6H12O6
6
2 SO2
4
Mg(OH)2
2
3Sn(Cr2O7)2
42
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Remember that atoms “WANT” to
have full outer shells

When two atoms get close together, their
unpaired valence electrons interact
and there are two ways they can get that:
1.
2.
They can steal (lose/gain) electrons
They can share electrons with other atoms
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Made up of positive and negative ions.
 Metal + Non-Metal
 Electrons are given and taken
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So each atom in the compound attempts to have a ‘full’ outer
shell of valence electrons.
Unpaired electrons are involved in the bonding and the lone pair
electrons are not.
 Sodium
(Na) & Chlorine (Cl)
 metals lose electrons and non-metals gain electrons
 Ionic Compounds form when electrons are transferred from
positive (+) ions to negative (-) ions.
Magnesium Sulfide - MgS
+2
12 p
12 n
2-
16 p
16 n
The number of bonds an atom wants to form is determined by the number
of e- in the outer shell that an element wants to donate or borrow.
- In other words its combining capacity (ion charge)
Example:
Ca = +2
therefore donates 2 electrons
Cl = -1
therefore borrows 1 electron
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lithium and oxygen form an ionic bond in the compound
Li2O
+
+
lithium
oxygen
Electrons are transferred from
the positive ions to negative
ions
Lithium oxide, Li2O
2-
+
Ionic Lattice

We think of (and write) ionic compounds in lowest terms,
but in reality they are found as a network – called an
“ionic lattice”
Covalent
Compounds
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made up of negative ions
Non-metal + Non-metal
Share electrons
Negative charge
Negative charge
Fluorine Gas – F2
9p
10 n
9p
10 n
9p
10 n
POLYATOMIC ION
A group of atoms that tend to
stay together and act as one
charged ion
Some Compounds with
Polyatomic Ions
More Compounds with Polyatomic Ions
Table 5.7