Mr. Chapman
Chemistry 20
Grade 9: Counting Atoms in a Compound

Rule #1: An element can have either one
or two letters for its symbol. The first letter
is always capitalized, and the second one
is never capitalized.
Example: Potassium, Oxygen, Fluorine have
the respective symbols K, O & F.
Example: Sodium, Iron and Neon have the
respective symbols Na, Fe & Ne.
Grade 9: Counting Atoms in a Compound

Rule #2: When looking at a chemical
formula, you will sometimes see small
numbers written beside some elements.
These numbers are called subscripts, and
they represent how many of that atom is
present in the chemical formula.
Example: The formula for water is H2O. This
means that there are 2 hydrogen atoms
and 1 oxygen atom in EVERY
MOLECULE OF WATER.
Grade 9: Counting Atoms in a Compound

Rule #3: If some chemical formulas, there
are brackets. There is always a subscript
outside of the bracket. To count the number
of atoms in these formulas, multiply the
number outside the brackets by everything
inside the brackets.

The formulas in brackets are called
polyatomic ions. You can find them on the
List of Common Ions, which you now need
as badly as the Periodic Table.

Example: Mg(NO3)2
Grade 10: Ionic vs. Molecular Compounds

Ionic compounds are compounds that
form between a metal and a non-metal.

Molecular compounds form between two
non-metals.

Ionic compounds form by transferring
electrons to form ions.

Molecular compounds form by sharing
electrons in order for each atom to have a
full valence shell.
Grade 10: Ionic Compounds

Recall from a few classes ago that all
atoms want to be like the nearest noble
gas – these are the group 18 elements.

Atoms will transfer electrons to each other
so that each of them can have an electron
configuration like their nearest noble gas.

Copy the white board example of sodium
and chlorine, and then hear their love story.
Grade 10: Ionic Compounds

When ions come together to form compounds,
their goal is ALWAYS TO HAVE A TOTAL
CHARGE OF ZERO AFTER COMBINING.
When a metal and a non-metal form a
compound, it is called an ionic compound.

For example, magnesium forms the Mg2+ ion
and has an overall charge of +2. Chloride ions,
Cl-, have a charge of -1. If two chloride ions
combine with one magnesium ion, their
combined charge will be 0.
Sodium and Chlorine
A touching love story of atoms & ions.
Sodium and Chlorine: Salt
Storytime: A Touching Love Story of Atoms and Ions
A chlorine (Cl) atom is walking down the street thinking as usual about being
noble like the nearest noble gas ________. “I wish I could just gain one
more electron to become like _______” the sad chlorine atom sighed.
Meanwhile, a sodium (Na) atom is walking the other way down the street
thinking as usual about being like the nearest noble gas _________. “If
only I could get rid of one of my electrons to become like _________” the
unhappy sodium atom said. Now they were both hanging their heads
because of their sadness so they accidentally bumped into each other!
“Hey,” said the sodium atom to the chlorine atom, “would you like to have
one of my electrons?” The sodium atom was used to getting rejected by
most elements so he wasn’t very hopeful.
“Why YES!” exclaimed the chlorine atom. So the sodium atom gave the chlorine
atom one electron. Being a very honest element, sodium said, “Now that I
have a charge of ____ and you have a charge of ____, I am suddenly very
attracted to you.” “Me too, after all, opposites attract!” replied chlorine. So
chlorine bonded with sodium to become a molecule of sodium chloride
(NaCl). Their family and friends were delighted that they had finally reacted
to become an ionic compound. They were proud of their new nickname,
too, as some people now called them Salt.
Grade 10: Molecular Compounds

Non-metals will share electrons with each
other in order to fill their outer valence
shells.

Bonds formed between two non-metals
through the sharing of electrons are called
covalent bonds, or molecular bonds.

Copy the whiteboard example showing the
formation of methane, CH4.