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Determining Properties of Elements Using the Periodic Chart
To keep things simpler, we are going to focus on the first twenty elements in the periodic chart.
These elements generally follow a simple octet rule when bonding to form compounds and they
contain the most important elements in biology – most notably carbon, oxygen, hydrogen, and
nitrogen – also phosphorus, potassium, magnesium, sodium, and calcium. Use the periodic chart
to complete the requested information for each element.
For each of the following elements, 1) fill in the blank lines, 2) draw a shell model diagram of
one atom following the octet rule, 3) draw one atom showing the element’s bonding pattern (how
it would appear in a structural diagram). The first one is done for you.
1) OXYGEN
proton number
_____8____
electron number in neutral atom
_____8____
neutron number in an isotope of molar mass 17 grams _____9____
O
2)
3)
HELIUM
proton number
charge of an atom with 1 electron
molar mass of an isotope with 2 neutrons
__________
__________
__________
NITROGEN
proton number
number of electrons it can share
neutron number in an isotope of molar mass 14 grams
__________
__________
__________
4)
5)
6)
7)
CHLORINE
proton number
charge of an atom that has gained 1 electron
molar mass of an isotope with 18 neutrons
__________
__________
__________
CALCIUM
proton number
number of electrons it wants to give away
neutron number in an isotope of molar mass 40 grams
__________
__________
__________
CARBON
proton number
electron number in a neutral atom
neutron number in an isotope of molar mass 13 grams
__________
__________
__________
SODIUM
proton number
number of electrons that it wants to give away
molar mass of an isotope with 10 neutrons
__________
__________
__________
8)
9)
10)
POTASSIUM
proton number
electron number in a neutral atom
neutron number in an isotope of molar mass 39 grams
__________
__________
__________
MAGNESIUM
proton number
number of electrons that it wants to give away
neutron number in an isotope of molar mass 25 grams
__________
__________
__________
NEON
proton number
number of electrons that it wants to share
molar mass of an isotope with 11 neutrons
__________
__________
__________
Writing Molecular Formulas
The molecular formula specifies the actual number of atoms of each element in a molecule.
The conventional form for writing a molecular formula is to write the symbol for each element
followed by a subscript indicating the actual number of those atoms present in a molecule. When
only one atom of an element is present, the subscript is omitted. For example, the molecular
formula for water, H2O, specifies that there are two hydrogen atoms and one oxygen atom
present in each molecule of water. Remember that the Hill system (or Hill notation) dictates that
the number of carbon atoms in a molecule is indicated first, the number of hydrogen atoms next,
and then the number of all other chemical elements subsequently, in alphabetical order. When
the formula contains no carbon, all the elements, including hydrogen, are listed alphabetically.
For ionic compounds, the positive ion is always listed first.
For each of the following compounds, draw a structural diagram that shows the molecule. Then
write the molecular formula and indicate if the bonds are covalent or ionic. The first one is done
for you.
1) a molecule with 1 carbon atom and 4 hydrogen atoms
H
H
C
H
CH4
H
2) a molecule with 2 hydrogen atoms
3) a molecule with 3 carbon atoms and six hydrogen atoms
covalent
4) a molecule with 1 hydrogen atom and 1 fluorine atom
5) a molecule with 2 sulfur atoms
6) a molecule with 1 carbon atom and 2 oxygen atoms
Balancing Equations
When a chemical reaction occurs, it can be described by a chemical equation. An equation
shows the chemicals that react (called the reactants) on the left-hand side, and the chemicals that
they produce (called the products) on the right-hand side. The chemicals can be represented by
their names or by their chemical symbols. Unlike mathematical equations, the two sides are
separated by an arrow that indicates that the reactants form the products and not the other way
round.
The law of conservation of mass says that matter cannot be created or destroyed during a simple
chemical reaction. This means that if we have specific reactants on the left hand side of an
equation, we must have products on the right hand side that account for every atom that went
into the reaction.
Take a look at this chemical word equation:
Aluminum + Oxygen
Aluminum Oxide
This is the equation for the burning of aluminum in oxygen. If we convert each of the chemical
names into the appropriate symbols, we get the following:
Al + O2
Al2O3
Note that oxygen gas is diatomic, which means that one oxygen molecule consists of two oxygen
atoms. A molecule of aluminum oxide consists of two aluminum atoms combined with three
oxygen atoms.
You can see by looking at it that there is something wrong with this equation. If you count the
number of atoms of each type on each side, you will see that there is only one aluminum atom on
the left side whereas there are two on the right. There are two oxygen atoms on the left side, as
compared to three on the right side. This clearly doesn't balance.
We can balance the equation by multiplying the different atoms and molecules on each side by
different amounts. Firstly, multiply the aluminum atoms on the left side by 2:
2 Al + O2
Al2O3
Now there is the same number of aluminum atoms on each side of the equation. We could also
multiply the number of oxygen molecules on the left side by one and a half (1.5), which would
give three oxygen atoms on the left side (1.5 x 2 = 3) to match the three oxygen atoms on the
right side:
2 Al +
1.5 O2
Al2O3
This is now balanced, but that 1.5 is a horrible thing to have in an equation. You cannot have one
and a half molecules. We can solve this problem by multiplying everything throughout by 2:
4Al + 3O2
2Al2O3
If you count the number of atoms on each side, you will find that there are four aluminum atoms
and six oxygen atoms on each side. Notice that we balanced the equation ONLY by changing the
coefficients that are in front of each of the reactants or products telling you how much Al2O3 or
how much O2 there is. You can change quantities to balance an equation but don’t ever change
the molecular formulas themselves! (In other words, never change the subscript numbers.)
Try balancing each of the following equations. This is often best learned by a sort of ‘trial and
error’ approach. Use the atomic model kits to visualize each equation as you balance.
1)
Al
+
HCl
AlCl3
+
H2
2)
KOH
+
H3PO4
K3PO4
+
H2O
3)
KNO3
+
H2CO3
K2CO3
+
HNO3
4)
Na
+
Cl
NaCl
5)
SnO2
+
H2
Sn
+
H2O
6)
SeCl6
+
O2
SeO2
+
Cl2
7)
Zn
+
HCl
ZnCl2
+
H2
8)
CH4
+
O2
CO2
+
H2O
9)
NH3
+
O2
NO
+
H2O
10)
C2H6
+
O2
H2O
+
CO2
Critical Thinking Questions
1) Explain briefly why elements like helium and neon are generally considered ‘non-reactive’.
Which would be ‘more reactive’ – a nitrogen atom or a sodium atom? Why?
2) The atomic mass should roughly equal the number of protons plus the number of neutrons.
Why is it not exact? Why are the mass numbers in the periodic chart not whole numbers?
3) Why is the molecular formula for water written H2O and not OH2 ?
4) Look at the Periodic Table and focus only on the first twenty elements. List all of the
elements from the first twenty that you think are most likely to form ionic bonds. List all of
the elements that you think are most likely to form covalent bonds. Do some elements belong
on both lists? Describe your reasoning and the criteria you used to put elements into each of
your lists.
Most Likely to Form Ionic Bonds
Most Likely to Form Covalent Bonds