Untitled - Kelly Walsh High School

132 CHEMISTRY FOR THE UTTERLY CONFUSED
Don’t Forget!
An ionic compound is neutral (same number of positive and negative charges)
and that the formula has the lowest whole number ratio of elements.
Mg
The total number of electrons lost must equal the total number of electrons
gained. The reaction of aluminum with oxygen produces aluminum oxide. The
aluminum has three valence electrons to lose. The oxygen has six valence electrons
and needs two. The lowest common factor between 3 and 2 is 6. It requires
two aluminum atoms, losing three electrons each, to supply six electrons. It will
require three oxygen atoms, gaining two electrons each, to account for the six
electrons. The resultant compound, aluminum oxide, has the formula Al 2O 3.
All ions present have an octet of electrons.
The formation of ions requires energy. The reaction cannot occur if insufficient
energy is available. The source of the necessary energy is the lattice energy. The
lattice energy is defined as the energy required to separate the ions in 1 mol of
an ionic solid. There are ways of estimating the lattice energy, but we often find
lattice energies by applying Hess’s law. You learned how to apply Hess’s law in
Chapter 6. A Born-Haber cycle represents the application of Hess’s law to the
determination of lattice energies.
9-4 Covalent Bonds
Cl
Cl
Mg 2+
Covalent bonding is the sharing of one or more pairs of electrons by two atoms.
The covalent bonds in a molecule; a covalently bonded compound are represented
by a dash. Each dash is a shared pair of electrons. These covalent bonds
may be single bonds, one pair of shared electrons as in H-H; double bonds, two
shared pairs of electrons as in H 2C:CH 2; or triple bonds, three shared pairs of
electrons, :NæN:. It is the same driving force to form a covalent bond as an ionic
bond—completion of the atom’s octet. In the case of the covalent bond, the sharing
of electrons leads to both atom utilizing the electrons towards their octet.
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Cl
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Cl