Chemical bonding is the joining of atoms to form new
substances. The properties of the
new substance is different from those of the elements that make it up.
The force of attraction that holds the atoms together is the chemical
bond.
Valence electrons are the electrons located in the
outermost shell of the atom. These
are the only electrons that are involved in chemical bonding.
Most atoms form bonds by either gaining, losing or
sharing electrons until they have 8 valence electrons.
The outer shell is considered full if it has 8 electrons, except Hydrogen
which is full if it has 2 valence electrons.
There is a special group of elements called the Nobel
Gases. These elements are also
called the inert gases. This is
because they do not readily react with anything because their outer shell are
already full. Inert means, ‘will
not react’. These elements are,
Helium, Neon, Argon, Krypton, Xenon, and Radon.
IONIC
BONDS
The ionic bond is the force of attraction between
oppositely charged ions. In ionic
bonding, electrons are transferred between atoms. The atom that loses electrons becomes a positive ion.
The atom that gains electrons becomes a negative ion.
When writing an ion, you write the symbol, the charge, and the ion
number. For example,
Mg+2
The ions that make up an ionic compound are bonded
together in a regular repeating 3 dimensional pattern called a crystal lattice.
Ionic bonding usually occurs between atoms of metals and atoms of
nonmetals. Energy is needed to remove electrons from metal atoms tp form
positive ions. Energy is released
when most nonmetal atoms gain electrons to form negative ions.
COVALENT BONDS
The covalent bond is the force of attraction between the positive
nuclei of the atoms and the negative electrons shared by the atoms.
In covalent bonding electrons are shared between atoms.
Covalent bonding usually occurs between atoms of nonmetals.
Covalently bonded atoms form a particle called a
molecule. A molecule is the
smallest unit of a compound that has all the properties of the compound.
Electron-dot diagrams are a simple way to represent
the valence electrons in an atom. (see
page 337)
Diatomic elements are the only elements found in
nature as diatomic molecules. They
consist of two atoms of the same element covalently bonded together.
They are the simplest molecules. For
example, Oxygen, Nitrogen, Fluorine, Chlorine, and Bromine, and Iodine.
METALLIC BONDS
The metallic bond is the force of attraction between a
positive metal ion and the electrons in the metal. In metallic bonding, the outermost energy levels of the metal
atoms overlap. This allows the
valence electrons to move throughout the metal.
Many properties of metals such as, conductivity of heat and electricity,
ductility, and malleability, result from the freely moving electrons in the
metal.
SYMBOLS &
FORMULAS
A chemical formula is a group of symbols and numbers that show the
make-up of a compound. The numbers are written as subscripts, and show how many
atoms there are of the element directly in front of it.
The number 1 is never written as a subscript.
Just having the symbol there means 1 atom.
WRITING FORMULAS
USING VALENCE NUMBERS
1. Write
the symbols
2. Write in the valence number with
the charge. (see above, ionic
bonds)
3. Drop the charges.
4. Drop the number 1.
5. Using the cross-over rule, cross
over the remaining numbers to the bottom of the
opposite
element, writing them as subscripts. If
the numbers are the same they
cancel each
other out and you only write the symbols.
NAMING COMPOUNDS
Compounds composed of a metal and a nonmetal are
written with the metal first. The
nonmetal is written second with the ending changed to ‘ide’.
Compounds composed of 2 nonmetals are written left to right as they
appear on the periodic table. Prefixes
are used to show the number of atoms, especially for the second element.
Common prefixes are, mono = 1, di = 2, tri = 3, tetra = 4 penta = 5, and
hexa = 6. Compounds composed
of 3 elements with the last element being Oxygen may have endings of either ‘ite’,
or ‘ate’. Some compounds made
of metal elements from groups 3-12 may use roman numerals to show the positive
charge of the metal.
MODELS OF IONIC
COMPOUNDS
1. Write the formula.
2. Make a data table
3. Draw the atom with the most
electrons in the outer shell first
*There are
exceptions to this rule.
4. Draw the remaining atom(s)
directly opposite the single electron of the first atom.
5. Circle the electron that’s
going to leave.
6. Draw an arrow to where its going
to go.
7. Draw in the transferred electron
differently.
8. Write the ions.
MODELS OF COVALENT COMPOUNDS
1. Write
the formula.
2. Make a data table
3. Draw the atom with the most
electrons in the outer shell first
There are
exceptions to this rule.
4. Draw the outer shell of the
remaining atom directly through the single electron of
the first atom,
draw in the shared electron differently, complete that atom.
5. Repeat this procedure for any
remaining atoms.
6. Whenever there is more than 1
carbon in the formula, connect the carbons in a
straight line,
sharing electrons with each other first. This
should leave you with
exactly enough
single electrons to complete the model.
**Remember, in both models it always works out that all outer shells will be
complete.
CALCULATING FORMULA & MOLECULAR MASS
1. Write the formula.
2. Look up the atomic weight for
each element in the compound.
3. Multiply the atomic weight by
the number of atoms of that element.
4. Do this for elements in the
compound.
5. Add all your answers together to
get the final answer.
EXAMPLE:
Na(OH)2
Na = 23 x 1 = 23
O = 16 x 2 = 32
H =
1 x 2 = 2
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