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Tuesday, Mar 08, 2016

 

 

 

 

 

/files/user/923/file/chbondionic%20(1).pdf    ionic learning guide

/files/user/923/file/chbondcovalent%20(1).pdf   covalent learning guide

/files/user/923/file/chbondpracticepg%20(1).pdf   practice learning guide

 

 

 

 

MORE NOTEs

Writing Chemical Formulas

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What number is never used as a subscript in a chemical formula?

 

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Pharmacist

Chemical formulas represent compounds.

Oxidation numbers are used to determine the ratio in which elements combine to form compounds.

Oxidation numbers tell the number of electrons an atom gained or lost when forming the compound.

The plus or minus indicates if electrons were lost or gained.

Since electrons have a negative charge, and atom with a negative oxidation number will gain electrons. That means an atom with a positive oxidation number will lose electrons.
 

 

Nonmetals and polyatomic ions almost always gain electrons - have negative oxidation numbers.

Metals almost always lose electrons - have positive oxidation numbers.
 

The number indicates how many electrons.

Example:

Aluminum has an oxidation number of +3. It will lose 3 electrons when forming compounds.

Oxygen has an oxidation number of -2. It will gain 2 electrons when forming compounds.
 

 

Common Oxidation Numbers

Element

Oxidation #

Element

Oxidation #

Element

Oxidation #

Aluminum
Al

+3

Bromine
Br

-1

Calcium
Ca

+2

Carbon
C

-4 or +4

Chlorine
Cl

-1

Chromium
Cr

+3

Copper
Cu

+2 or +1

Fluorine
F

-1

Gold
Au

+3

Helium
He

0

Hydrogen
H

+1

Iodine
I

-1

Iron
Fe

+3 or +2

Lead
Pb

+2

Lithium
Li

+1

Magnesium
Mg

+2

Mercury
Hg

+2

Neon
Ne

0

Nickel
Ni

+2

Nitrogen
N

-3

Oxygen
O

-2

Phosphorus
P

-3

Potassium
K

+1

Silicon
Si

+4

Silver
Ag

+1

Sodium
Na

+1

Sulfur
S

-2

Tin
Sn

+4 or +2

Uranium
U

+6

Zinc
Zn

+2
 

 

Notice that some of the elements on the table have more than one possible oxidation number. You will soon learn how to know which one to use. However, the first listed on the table is the most common.
 

link to a local webpage with useful information

 

Understanding Chemical Formulas

  • Chemical formulas are composed of a positive half and a negative half.

Water is a compound you know to have a formula of H2O.

  • The element with the positive oxidation number is always written first.

H

  • The element with the negative oxidation number is always written second.

HO

  • The total of the oxidation numbers in a compound must equal zero.

Hydrogen's oxidation number is +1 and oxygen's is -2.
With one H and one O, the total is not 0, it is -1.

  • Subscripts, small numbers to the lower right of the chemical symbol, represent the number of atoms of that element present in the compound.

The subscript of 1 is never written in a chemical formula.
It is understood since the chemical symbol is there.
Add subscripts after a chemical symbol, when needed, to make the oxidation numbers total zero.

H2O

  • Multiply subscript by oxidation number for the total oxidation number of each element in a formula.

For hydrogen: (oxidation number +1) (subscript 2) = +2 total
For oxygen: (oxidation number -2) (subscript 1) = -2 total

  • The formula H2O is the correct formula.  
 

 
 

 

The easiest way to think of writing chemical formulas is to
use the oxidation number (without the + or -) of one
element as the subscript of the other element.

 

 

Using oxidation numbers to write chemical formulas

Remember

-   subscripts of 1 are never written in a formula!
 

 

 
 

 

steps

Identify the oxidation number for the element making up each half (positive & negative) of the compound.

Use the oxidation number (without the plus or minus) for each half as the subscript for the other half.

Do not write a subscript of 1.
Reduce the subscripts, if needed.
 

 

After doing this, be sure the subscripts will not reduce.
If both subscripts are divisible by the same number,
they must be reduced to have the formula in its proper form.

Example: Ca2O2 must be reduced to CaO
 

 

Practice Writing Chemical Formulas:

Write the formulas for the compounds formed when these elements combine. Do not look at the answers before you have written all the formulas. If one of your formulas differs from the answer, try to find out why. If you have questions, ask your facilitator.

  1. hydrogen and oxygen
  2. sodium and chlorine
  3. chlorine and magnesium
  4. potassium and sulfur
  5. aluminum and oxygen
  6. gold and fluorine
  7. iodine and hydrogen
  8. phosphorus and silver
  9. phosphorus and aluminum
  10. sulfur and silver
  11. oxygen and uranium
  12. tin (IV) and oxygen
  13. iron (III) and oxygen
  14. copper (I) and chlorine
  1. copper (II) and chlorine

Text Box: HomeText Box: Home
Roman numerals indicate the oxidation number of the positive element in problems 12 - 15.
 

 

 

 
 

 



Resources
chbondcovalent 1 ( 33.9 KB )
chbondionic 1 ( 34.3 KB )
chbondpracticepg 1 ( 37.8 KB )

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