11.1.1 Oxidation Numbers Flashcards
Oxidation Numbers
- Oxidation numbers provide a means of assigning electrons to individual atoms within a molecule or ion.
- Oxidation numbers are assigned based on empirical rules and observation.
note
- Oxidation numbers must add up to the overall charge.
- a. Neutral elements are assigned an oxidation number of zero.
- b. Oxidation numbers of elements in polyatomic ions must sum to the total charge of the polyatomic ion.
- c. Oxidation numbers of elements in neutral molecules must sum to zero.
- All alkali metals have an oxidation number of 1+.
- All alkaline earth metals have an oxidation number of 2+.
- Hydrogen has an oxidation number of 1+ with two
exceptions: in molecular hydrogen (H 2 ) where H is zero and in metal hydrides where H is 1–.
- Hydrogen has an oxidation number of 1+ with two
- Oxygen has an oxidation state of 2–. (exceptions: molecular oxygen (O 2 ) where O is zero; peroxides where O is 1–; superoxides where O is 1⁄2–
- Rule numbers 1b and 5 are used to assign oxidation states to the nitrate ion.
- Beginning with oxygen, rule number five states that oxygen has an oxidation state of 2–. Since there are three oxygen atoms in nitrate, and each has a 2– oxidation state, the total is 6–.
- Rule 1b states that elements in an ion must have oxidation states that sum to the total charge of the ion. This means that nitrogen has an oxidation state of 5+, since the total charge of the ion is 1–.
- An atom can have different oxidation states, depending on the atoms to which it is bonded.
- In the three molecules to the left, carbon has three different oxidation states: 4+, 0, and 4–. These different states reflect the degree to which electrons are withdrawn from the carbon atom. How electron deficient or rich an atom within a molecule is provides insight into how the molecule might behave in chemical reactions.
Which statement about oxidation states (or numbers) is not correct?
Because the electronegativity difference can be small, electronegativity is not an important consideration when determining oxidation states.
Which of the following rules for determining oxidation states (or numbers) is not correct?
Alkaline earth metals normally have an oxidation state of +1.
Which of the following shows (in the correct order) the oxidation numbers for phosphorous (P) and oxygen (O) in the polyatomic ion PO43−?
+5, −2
Look at the oxidation numbers assigned for the ammonium ion, NH4+.
NH4+ −3 + 4 (+1) = +1 ammonium ion
Which of the following statements is not correct?
An electron deficient atom such as nitrogen will welcome the four hydrogen atoms because of the electronegativity difference.
Which statement best describes the primary limitation of oxidation state (or oxidation number)?
In molecules, electrons are not just in atomic orbitals. They are also in molecular orbitals. Therefore, oxidation states (or numbers) do not necessarily represent the electron reality for each atom.
Which statement best summarizes the scenarios for when a molecule can have an oxidation state of 0?
A molecule can have an oxidation state of 0 when the molecule consists of only elemental atoms or the composite oxidation state values for the individual atoms add up to 0.
What is an oxidation state?
It is a charge assigned to an atom. It tells you how many electrons the atom controls in a compound.
Which statement about oxidation numbers (or states) is not true?
The oxidation number tells you the exact number of electrons that each atom has or is missing.
Let’s look at the methane molecule, CH4.
We learned that the electronegativity difference between carbon and hydrogen is very slight. Which statement about this molecule is not correct?
There are four (very) polar C–H bonds in the molecule. They are polar towards the carbon atom because carbon is more electronegative.
What are the oxidation numbers for magnesium and oxygen (in that order) in the MgO molecule?
+2, −2
