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Chapter 5: Molecules - LCHS Physical
Science |
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Objectives: The learner will...
...explain how static electricity
relates to atomic structure
....write chemical formulas
...name chemical compounds |
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Vocabulary:
ionic bond: a bond formed by
a metal giving electron(s) to a non-metal
covalent bond: a bond formed by non-metals sharing electron(s)
formula: a symbolic way of showing a molecule/compound's
elements and ratios
molecular mass: sum of atomic masses in a single molecule
polyatomic ion: multiple atoms that bond covalently and act
as a single atom |
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ATOMIC BONDS
Typically, metals (on the left of the periodic table)
will give up electrons to non-metals (on the right of the periodic
table) which prefer to take electrons. When a metal bonds with a
non-metal by trading electrons this is called an ionic bond.
In the standard example below, sodium (Na) gives up it's outermost
electron, becomes a positively charged ion (1 more proton than electrons),
and is now "content" with a full outer shell. Chlorine
(Cl) takes this electron to complete and fill its outer shell becoming
a negative ion (1 more electron than proton now). The positive sodium
ion (Na+) is now attracted to the negative chloride ion (Cl-) because
opposite charges attract.
When non-metals combine with non-metals they form a covalent
bond. A covalent bond occurs when electrons are shared because
neither atom wants to give, only take (e.g. O2, CO2).
NAMING IONIC COMPOUNDS
Most ionic compounds (metal + non-metal) are fairly easy to name.
The metal is pronounced first followed by the non-metal modified
with an "ide" suffix. The examples below show how easy
it is.
NaCl = Sodium + Chlorine = Sodium Chloride
MgS = Magnesium + Sulfur = Magnesium Sulfide
AlN = Aluminum + Nitrogen = Aluminum Nitride
FORMULAS
The ratios that ionic compounds combine in is determined by their
charges. A Calcium atom (+2) wants to give 2 electrons
and a Fluorine (-1) atom wants to take only 1. To balance out the
charges, 2 Chlorines will each donate one and the Calcium will take
them both to make CaCl2 (Calcium Chloride). If
the charges between the metal and non-metal ions are the same (but
opposite), then only one of each is required (e.g. Al +3 and N -3
make AlN). If they are different then you can cross the charges
to get the subscripts, e.g. Al+3 and O-2
make Al2O3 (the charge becomes the subscript
of the other). If the charge is a +1 or -1 the subscript can
be 1 but is traditionally left off, e.g. H2O1
is H2O. An exception would be a +4 and
-2 combination. In this case you would factor out a common denominator
(2) before crossing.
MOLECULAR MASS & %MASS
The concept of molecular mass and % mass is a very important concept
to learn especially if you plan on taking chemistry, biochemistry,
or just following chemical recipes. You will first need the periodic
table to obtain individual atomic masses (amus), then combine
this with the atomic ratios provided by the formula. According to
the formula Al2O3, Aluminum Oxide would contain
2 aluminum (mass = 27 amu each) and 3 oxygen (mass = 16 amu each)
for a total molecular mass of (2 x 27) + (3 x 16) or 102.
The %mass of Aluminum in Aluminum Oxide (Al2O3)
is (2 x 27) / 102 or .529 or 52.9%.
POLYATOMIC IONS
Often in a reaction certain non-metals will group together and
stay with each other through thick and thin. They bond
covalently and act as a single atom and will donate (in rare
cases, take) an electron as a unified group. These collectives
are known as polyatomic ions. When making a compound simply
treat the polyatomic as a single atom and cross the charges,
etc as you would before. Aluminum Carbonate would be Al+3
+ [CO3]-2 or Al2[CO3]3
| Polyatomic Ions |
Formula |
| +1 ions |
[NH4] (ammonium) |
| -1 ions |
[OH] (hydroxide)
[NO3] (nitrate)
[ClO3] (chlorate) |
| -2 ions |
[CO3] (carbonate)
[SO4] (sulphate) |
| -3 ions |
[PO4] (phosphate) |
TRY
THIS ONLINE COMMON CHEMICAL QUIZ
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| Practice: |
Help: |
PB1
What is the name of Al2S3? |
metal=Aluminum
non-metal=Sulfur |
PB2
How many N in Mg 3 N_ ?
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use periodic
table
criss-cross |
PB3
How many Al in Al_ Cl 3 ?
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use periodic
table
criss-cross |
PB4
What is the formula of Aluminum Fluoride?
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use periodic
table
criss-cross |
PB5
What is the molecular mass of Na 3 P 1 ?
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Na3P
= 3 Na and 1 P
use periodic table |
PB6
What % (by mass) of Al is in Al 1 Cl 3 ?
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AlCl3 = 1 Al
and 3 Cl
use periodic table
Al / ( AlCl3) x 100% |
PB7
What is the ionic charge of Mg? |
use periodic table
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PB8
What is the ionic charge of [PO4]?
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polyatomic chart |
PB9
How many Li in Li_ [SO4] 1 ?
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criss-cross
treat polyatomic as
single atom/charge |
PB10
What is the name of Al2[SO4]3?
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metal=Aluminum
polyatomic chart |
answer bank:
(some are bogus)
2
AlF3
2
aluminum sulfide |
-3
aluminum sulfide
99.94307
aluminum sulfate |
1
20.24
2 |
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Scientist Spotlight:
Sir
Francis Bacon 1561-1626 He believed that understanding could
not start from first principles (which only lead further astray),
but must be obtained by performing a series of experiments and making
generalizations by induction.
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Resources (Study Links/Study Tips/Reading Lists)
TRY
THIS ONLINE COMMON CHEMICAL QUIZ
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Chapter 5: Molecules - LCHS Physical
Science |
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