What is oxidation number?

What do those little +1, +2, -1, -2, etc. numbers mean?

Atoms are Lazy and Want to be Happy (Who doesn’t?)

• Atom’s goal is to fill its outer (valence) energy level

• ex. Cl (chlorine) has 7 electrons in its outermost energy level - it wants to gain one more to be happy (remember Octet rule?)

• Na (sodium) on the other hand, has 1 electron in its outermost energy level. It has a choice: gain 7 more electrons or lose 1.

Na will lose its lone electron and will become Na+

Cl will gain one electron (from sodium in this case) and become Cl-

NaCl, a neutral compound is formed!!!

Negative Charge

• If an atom gains an electron the atom becomes a negatively charged ion; written 1-

• Likewise, if an atom gains 2 electrons the atom becomes a doubly negative charged ion; written 2-

*This charged number is AKA oxidation number (the # of electrons an atom gains or loses when bonding)

chlorine atoms have: Positive Protons +17 Negative Electrons -17 Neutral Atom 0

If a chlorine atom gains an electron it becomes a negatively charged ion: Positive Protons +17 Negative Electrons -18 Negative Ion 1-

oxygen atoms have: Positive Protons +8 Negative Electrons -8 Neutral Atom 0

If an oxygen atom gains 2 electrons it becomes a negatively charged ion: Positive Protons +8 Negative Electrons -10 Negative Ion 2-

Positive Charge

• If an atom loses an electron the atom becomes a positively charged ion; written 1+

• Similarly, if an atom loses 2 electrons the atom becomes a doubly positive charged ion; written 2+

sodium atoms have: Positive Protons +11 Negative Electrons -11 Neutral Atom 0

If a sodium atom loses an electron it becomes a positively charged ion: Positive Protons +11 Negative Electrons -10 Positive Ion 1+

On the periodic table,All of the alkali metals have 1 valence electron and 1+ charge

On the periodic table, All of the alkaline earth metals have 2 valence electrons and 2+ charge

On the periodic table, All of group 13 have 3 valence electrons and a 3+ charge

On the periodic table, All of group 14 have 4 valence electrons and a 4+ charge

On the periodic table, All of group 15 have 5 valence electrons and a 3- charge

On the periodic table, All of group 16 have 6 valence electrons and a 2- charge

On the periodic table, All of the Halogens (Group 17) have 7 valence electrons and a 1- charge

On the periodic table, All of the noble gases have full electron shells and 0 charge

Neutral Compounds• Most compounds like NaCl are neutral

- or - they have no charge.

• Remember: Na+ + Cl- NaCl w/ no charge

• (+1) + (-1) = 0

• Sometimes more than one of the same

element can help to create a compound.

• If you know the the charges of the atoms you can determine how many atoms will be needed to create neutral compound.

• ex: if you know that Ti has a 3+ charge; when combined with Cl- in order to make a neutral compound you must have 3 Cl for every 1 Ti

• looks like this: TiCl3

(3+) + (1-) + (1-) + (1-) = 0 charge