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"To the dull mind all nature is leaden. To the illumined mind the whole world burns and sparkles with light." -- Ralph Waldo Emerson

CHEMICAL BONDS

Why Atoms Combine

Concept Pages:

Physical Science "Key Concepts" Home Page
 

 

Section Review Questions
Chapter Review Questions

 
Compounds
• Are what most matter we are familiar with is.

• Are made from more than one type of atom, i.e., from more than one element.

• A particular compound will always be found to have exactly the same ratio of the elements that it is made of.

• Have properties unlike their constituent elements.

 
Formulas
Use the chemical symbols for the elements in a way to describe the ratios of elements that make up a particular compound.
• Called The Chemical Formula

e.g.:
Na = Sodium
Cl = Chlorine

• NaCl = Sodium chloride, or Table salt
Fe = Iron
O = Oxygen

• Fe2O3 = Iron (III) oxide, or Rust
(the (III) will be discussed later)
Chemically Stable Atoms
• Have outer electron energy levels that are full.
i.e. The maximum number of electrons that can be in that shell are in that shell.

e.g. 2 in the first, 8 in the second, etc.
The Noble gases - Group 18 or VIIIA
• Have full outer shells and are called "inert",

That is,

They don't react readily to form compounds.
All other elements have incomplete outer shells:
- 1 to 7 electrons -

• And lose, gain, or share electrons to make their outer shells "full" and become chemically stable.
Chemical Bonds
A Chemical Bond • Is the way elements combine to form compounds.
 
A Bond • Is the force of attraction between elements:

The force that causes them to "stick together".
• It is the result of the process in which they lose, gain, or share electrons to make their outer shells "full".

 

Kinds of Chemical Bonds

Ions and Ionic Bonds
Ions
• Are atoms or molecules that have an electromagnetic charge.
• They have this charge because they have gained or lost electrons.
Ionic Bonds
• Are bonds between ions that are the result of their having opposite electromagnetic charges.
• Which causes them to be attracted the same way opposite magnetic poles are.

e.g.:

Na + Cl   ® [ Na ]+    +  [ Cl ]-  ® NaCl

Mg + Cl   ® [ Mg ]2+  + 2   [ Cl ]-   ®  MgCl2

Mg + O    ® [ Mg ]2+  +   [ O ]2-   ®  MgO
 

Covalent Bonds
• Are bonds in which the atoms or molecules share electrons:

e.g.:
Cl + Cl   ® Cl Cl   ® Cl2
A Molecule
• Is generally a group of atoms combined by covalent bonds.
Polar Molecules
• Are molecules that have different charges on opposite sides:
H+ + Cl-  ®   H+ Cl-   ®  +H Cl-

And surprisingly, water:
2H+  +   O-    ®   +H O-
                           H+

Formulas and Names of Compounds

Oxidation Numbers
• A positive or negative number assigned to an element to show its combining ability in a compound.
 

e.g.:

 Na loses an electron and has a 1+ oxidation number.

so:
All of group 1(IA) has a 1+ oxidation number.

and so:
• Group 2 (IIA) has 2+
• Groups 3 (IIIB) - 12 (IIB) have oxidation numbers of 1+, 2+, and/or 3+
• Many have more than one.

And so:
• Group 13 (IIIA) has a 3+

Can you finish the rest?

• Group 14 (IVA)?
• Group 15 (VA)?
• Group 16 (VIA)?
• Group 17 (VIIA)?
• Group 18 (VIIIA)?

Binary Compounds
• Composed of 2 elements:

like:
HCl, CO2, and H2O
Formulas:
Write, in order:

        1) Element with positive oxidation number:
         

          [Al]3+

        2) Element with negative oxidation number:

          [S]2-

        3) Add subscripts to balance:

          [Al]3+  +    [S]2-    ®    Al2S3
           (2)              (3)
 

It helps to remember the "least common multiple" rule from math:

In the previous example, it's "6"

Naming Binary Compounds
For the second element, add the suffix -ide to the root of the element name:

e.g.:
chlorine becomes chloride

nitrogen becomes nitride
 

• For those containing elements with more than one oxidation number:

A Roman Numeral,

(I, II, or III)

is used to designate the oxidation number,
 

(1+, 2+, or 3+)

Some Elements with Variable Oxidation Numbers

Copper(I)

Cu+

Copper(II)

Cu2+

Iron(II)

Fe2+

Iron(III)

Fe3+

Chromium(II)

Cr2+

Chromium(III)

Cr3+

Lead(II)

Pb2+

Lead(IV)

Pb4+

 
Compounds with Polyatomic Ions

 
• Have covalently bonded molecules with an electromagnetic charge,

• That often stick together as if they were an atom.

 
•To write formulas, follow rules for binary compounds, but add the following step:

3a) Write parentheses around the polyatomic ion when you need to indicate more than one in the formula:
H+     +     SO42   ®   H2SO4

Cu(II)    +     NO3    ®  Cu(NO3)2

Some Polyatomic Ions

Charge

Name

Formula

1+

Ammonium

NH4+

1-

Acetate

C2H3O2-

 1-

Chlorate

ClO3-

 1-

Hydroxide

OH-

 1-

Nitrate

NO3-

2-

Carbonate

CO32-

 2-

Sulfate

SO42-

3-

Phosphate

PO43-

   
Hydrates
• Compounds that have water chemically attached to their ions:

e.g.:
CoCl2    •     6H2O
is cobalt chloride hexahydrate

•     "hex" means "6"

Updated 10/14/07