[عاطف خليفة]

* (7) Oxidation numbers:
- Valency
The number of single (unpaired)electrons in the valency shell of the atom.

Ex:
Nitrogen (N7) trivalent
- Unpaired electrons are counted in Valency because they only electrons of bonds.

* Oxidation number

“ The number that refers to the electric charges the atom would have in compound”.

- Rules for assigning oxidation numbers
* 1- In ionic compounds

· In cations: It is equal to its Valency preceded by (+ sign)
· In Anion: It is equal to its Valency preceded by - sign.
· (+ ve) sign refers to loosing electrons .
(- ve) sign refers to gaining electrons .
Ex

K1+ , Mg2+ , Cl1- , O2-
* 2- In Covalent Compounds

a) Diatomic molecules of similar atoms in electronegativity the oxidation number of each atom = zero (why?).
Diatomic molecules of different atoms in electronegativity the shared electrons are assigned to the more negative atom

N.B.

1- Oxidation number of hydrogen + 1 (except metal hydrides)

Ex:
NaH - KH. it equals -l

2- Oxidation number of oxygen = -2
(except few compounds such as H2O2 and Na2O2 = -1
,KO2 it equals -1/2 , OF2 = +2


* Basic rules

1) In neutral molecule algebraic sum of oxidation numbers of all atoms = 0

2) The oxidation number of an element in elementary form = 0

3) The oxidation number of polyatomic ion = The charge on the ion.

Oxidation number is preferred to Valency as it helps to explain the change in the electron structure of the reacted atoms and helps to tell the type of chemical change during chemical reactions

N.B.
Oxidation loosing electrons(increase(+ve)charges).
Reduction Gaining electrons(increase(-ve)charges).

Ex.1
- Explain the type of change(oxidation and reduction).

fecl2-----------fecl3
(1) Iron

Fe Cl2 = 0 reactants.

= Fe + 2C1 = 0 as Cl = -1

= Fe + [2 (-l)] = 0



FeCl3= 0
Fe + (3x-1) = 0





So the iron has been oxidized

Ex.2

Find the type of change that occurred to Cr and Fe in :-
K2Cr2O7 + 6FeCl2 + 14HCl → 2KCl+2CrCl3 + 6FeCl3 +7H2O
1-Chromium

2-Iron

* Gradation of oxidation numbers

1) Groups IA, IIA, IIIA
Their electron configuration nS1 , nS2 , np1
They lose the valence electrons to form positive ions
Their oxidation numbers are + 1, +2, +3
Ex.: Na1+Cl - Mg2+ Cl2 - A13+Cl3

2) Groups IVA, VA, VIA, VII A
( gain electrons forming -ve ions.)

1- The highest oxidation number equals group number

2- The lowest oxidation number = group number -8

3) Group zero: Their oxidation number = Zero




* The discovery of freon

Ammonia, sulphur dioxide and propane were used as cooling agents.

But :- Ammonia is poisonous
Sulphur dioxide is poisonous and corrosive.
propane is vigorouslly inflammable.

- Thomas Medgely

discover the freon andobserved that(in studying periodic table)
Non-metals forms gaseous compounds at normal temp. and their ability to ignite decrease on moving to the right in the periodic table
In addition halogen compounds [ CCl4 ] were used to put out fire.
Medgely synthesized freons

- Freons
Compounds containing carbon - fluorine and chlorine such as CF4, CCl2 F2.
Freons are used in fridges and air conditioners.

N.B
Freons Cause the decay of the ozone layer which protects the earth from harmful rays (ultraviolet)
- so scientist look for other agents