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Writing Chemical Formulas

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Allison Wolfe

on 27 March 2014

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Transcript of Writing Chemical Formulas

Writing Chemical Formulas
Covalent Compounds
In order to write a formula for covalent compounds you need to use prefixes.
mono--1 hexa----6
di------2 hepta---7
tri------3 octa----8
tetra---4 nona---9
penta--5 deca---10
Dichlorine pentaoxide
Di= 2 so therefore there are 2 chlorine in this compound.
You read the second word the same as the first.
Penta = 5 so there are 5 oxygen atoms in this compound.
Only the prefixes go in the front of the compound names, if you are trying to make a formula you will need to use the prefixes as a sub script for the element.
Ex. Cl2 O5

Ionic Compounds
Very simple ways to write a formula for an ionic compound is called the criss-cross method.
You will need a periodic table to find the charges of the elements with the METAL LISTED FIRST.
For example: Na+ S2-

When you criss-cross the each elements charge becomes the other elements subscript.
Na2 S
If the subscript is a one you don’t have to put a 1 because it is already implied. Also you don’t have to bring down the positive/negative charge just the number.

Bonding Diagrams
How to draw bonding diagrams for covalent compounds.
To tell how many valence electrons are in an element you look at the group number.
If it is in groups 14-18 you just subtract 10 and that is how many valence electrons you have.
Ex. Oxygen is in the 16th group on the periodic table.
So oxygen has 6 valence electrons.
When ever drawing a diagram you have to use dots around the elements symbol to represent the valence electrons.
Ex. Oxygen : Where the numbers are is the order in which you put the electrons

3 6
5 8
Using the dot diagram you can combine two
elements to make them a compound.
*REMEMBER* and element needs 8 electrons to
"be happy"

Because those two electrons
do not want to be lonely they attach
to each other.

Notice how these two are alone on both
First we need to move them
so they can be happy.
one line is equal to 2 electrons
Oxygen still doesn't have 8 electrons.
Now we combine the elements
so they each have 8 electrons.
Because one electron does not want
to be alone but it has another lonely
electron on the other oxygen they can combine
By having 2 lines and four electrons on each
oxygen, it all adds up to 8 electrons so it makes them happy.
The way to draw the dot form is the same
as the form shown earlier known as the structural
form. Just instead of lines you still use dots.

How to draw bonding diagram for Ionic compounds
Ionic bonds transfer electrons so they do not have to share them like the covalent ones.
You still draw the dots around the element symbol but you just transfer the electron.
Sodium and Fluorine

Na F

Na F

Sodium only has one electron
because it is in the 1st group.
Fluorine has 7 because it is in the 17th group

It is alot easier for sodium to lose
1 electron rather than gain 7.
This space indicates that Fluorine only needs 1
electron to be happy.
At this point you circle the electron that
sodium will lose.
Then draw an arrow to where the electron
is going to fill the 8th electron space.

Types of Compounds
Ionic Compounds
When naming an ionic compound you always take the ending on the second element and change it to

KBr = Potassium Bromide
Cal2= Calcium Iodide
Na ( SO4)= Sodium Sulfate

on the last example the ending did not change because the second element was a polyatomic ion.
When an element has more than one possible charge which is usally d block elements you will use roman numerals to indicate the amount of said element is in a compound

Fe2 ( CrO4)3
when naming this you can use an algebraic equation.

2(x)+3 (-2) = 0

The 'x' represents the charge of the
d-block element

The 2 represents the number of that specific
element in the compound
The 3 represents the number of the
amount of the polyatomic ion that is in
the compound.
The -2 is the charge on the Polyatomic
2(x)+3(-2)=0 This is where you get the
2x-6=0 roman numeral.
2x=6 Iron (III) Chromate
Molecular Compounds
For Molecular (covalent) compounds you need to use the Greek Prefixes. You always take the ending on the second element and replace it with -ide.

mono--1 hexa----6 *Need to memorize di-----2 hepta---7 the prefix and
tri------3 octa----8 number.*
tetra---4 nona---9
penta--5 deca---10
N2O4 = Dinitrogen tetraoxide
CO= Carbon monoxide
SiO2= Silicon dioxide
If both element have a 1 subscript which
is also not written, only the
second element uses the
prefix -mono-
Prefixes determine the subscripts.
Trisulfur hexafluoride= S3F6
carbon pentaoxide=CO5
tetraphosphrous dioxide=P4O2
Does the compound have a metal?

when naming place the metal first
Does it have a transition metal
(D-block element)
Then use Roman Numerals to
tell the number of valence
electrons that element has.
Do not use Roman Numerals

Use prefixes to tell the number of
atoms in the compound
mono-1, di-2, tri-3, tetra-4,
penta-5, hexa-6
Iron(III) oxide
Copper(II) Chloride
Maganese (IV) oxide
Sodium Chloride
Magnesium Nitride
Sodium Fluoride
dinitrogen trioxide
nitrogen trichloride
phosphorus pentoxide
Types of Bonds
Covalent Bond-----
Two nonmetals share electrons so both
have 8 valence electrons. Exception: H
Must use prefixes in the name.
Do NOT reduce the formulas

Ionic Bond-----
Contain a metal and a nonmetal.
Metal is listed first, followed by nonmetal.
Change the name of the nonmetal to -ide.
Examples: nitride, sulfide, fluoride,

Metallic Bond-----
Formed between mobile electrons and metallic atoms
Electrons move freely around the atom

Hydrogen Bond-----
A bond between hydrogen and Fluorine, Oxygen, OR Nitrogen. ex. H2O
Polar Bond-----
Where electrons are unevenly shared between atoms
atom that is electronegative will pull the electrons closer to itself.
If the atoms are different they will have a different pull on electrons.

Non-Polar Bond-----
Electrons are shared evenly between two atoms
Atoms are the same and have the same pull on electrons
Increases across the periodic table.
Decreases down the periodic table
How to tell the difference.
Different atoms around the central atom means it is always polar.
Same atoms around the central will always be non-polar.
ex. H2 ---- H H... non-polar
ex. HF ---- H F
In the second example there is an arrow which is pointing to the atom that is more electronegative. This shows which way the shared electrons are being pulled
Binary Compound
Made of two elements it
could be metals and
non-metals or 2 non-metals.
Molecular Geometry


Diatomic molecule:
molecules made up of
2 of the same atoms.

Valence Shell Electron Pair Repulsion Theory (VSEPR):
The electron pairs (both shared and unshared) in the outermost energy level trying to get as far apart from each other as they can.
This determines the shape of the molecule.
1. Linear ( Linear Triatomic)
2 atoms: H H
3 atoms: O C O
Central Atom:
is the atom with the most open spaces to receive electrons.
2. Bent Triatomic
3 atoms: O
3. Trigional Planar
4 atoms: H C H
You do not have to
draw the triangle. It's just
to show how it makes a triangle.
4. Pyramid.

5. Tetrahedral. ( Only Carbon Family.)


6. Trigional Bipyramidal.

Intermolecular Forces
Hydrogen Bond-
A bond between hydrogen and Fluorine, Oxygen, OR Nitrogen. ex. H2O
gas at room temp.

Dipole Dipole Force-
Formed between polar covalent molecule because of electronegativity difference between non-metals.

Van Der Waals Force ( Londen Dispersion Force)-
Formed between non-polar covalent molecules and noble gases.
Electron density affects directly it's strength.

Ionic Solid-
ionic compounds between metals and non-metals because of electrostatic attraction
Electrostatic attraction affects the strength.
Strength determines boiling point.
London Dispersion Forces
Dipole - Dipole
Hydrogen Bonding
Intramolecular Force: any force that holds
and atom together.

What You will Learn Today
How to write chemical formulas.( Covalent & ionic)
Bonding Diagrams ( Covalent & Ionic)
How to name a compound.
VSEPR theory
Intermolecular forces
Types of Bonds

Letters to the Editor
Dear Editor(s),

I am having trouble with understanding where the dots on the dot diagrams are supposed to go on each element symbol. What if all I have 3 left over electrons but already connected two in the middle of the diagram?
Answer: When you have 3 left over electrons is a sign you are doing something wrong, or you just haven't finished the problem. For example : ClCN you want to space out all of your elements first.
Cl C N
Next you write out the valence electrons in Lewis dot form

Cl C N
Now you connect the elements based on lonely electrons
Cl C N
You cannot have one electron by itself and you can only have two on the sides with no lines, plus carbon in the center still does not have 8 electrons and one electron left. This is when you would have a triple bar connecting Carbon and nitrogen.
Cl C N
Dear Editor,

If I am given just two elements but no charge how am I supposed to do the criss-cross method?

A: You need to find the charge of each element. You find a charge by looking at the group number on the periodic table. When an element looses an electron it gains a positive charge, when one gains an electron it has a negative charge.
For example: Magnesium is in group 2 it would rather lose 2 electrons so it has a +2 charge.
Chlorine is in group 17 it would rather gain 1 electron so it has a -1 charge.
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