Oxidation Numbers (11.1.1)

Question 1

• Oxidation numbers provide a means of assigning electrons to individual atoms within a molecule or ion.

Answer 1

• Oxidation numbers provide a means of assigning electrons to individual atoms within a molecule or ion.

Question 2

• Oxidation numbers are assigned based on empirical rules and observation.

Answer 2

• Oxidation numbers are assigned based on empirical rules and observation.

Question 3

1. 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.
2. All alkali metals have an oxidation number of 1+.
3. All alkaline earth metals have an oxidation
   number of 2+.
4. Hydrogen has an oxidation number of 1+ with two
   exceptions: in molecular hydrogen (H2) where H
   is zero and in metal hydrides where H is 1–.
5. Oxygen has an oxidation state of 2–.
   (exceptions: molecular oxygen (O2) where O is
   zero; peroxides where O is 1–; superoxides
   where O is ½–; and when combined with F,
   where O could have a positive oxidation state).
6. In general, more electronegative elements
   achieve their preferred negative oxidation state
   (noble gas configuration).
   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electrons are withdrawn from the carbon atom.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.

Answer 3

1. 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.
2. All alkali metals have an oxidation number of 1+.
3. All alkaline earth metals have an oxidation
   number of 2+.
4. Hydrogen has an oxidation number of 1+ with two
   exceptions: in molecular hydrogen (H2) where H
   is zero and in metal hydrides where H is 1–.
5. Oxygen has an oxidation state of 2–.
   (exceptions: molecular oxygen (O2) where O is
   zero; peroxides where O is 1–; superoxides
   where O is ½–; and when combined with F,
   where O could have a positive oxidation state).
6. In general, more electronegative elements
   achieve their preferred negative oxidation state
   (noble gas configuration).
   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.

Question 4

Rules for assigning oxidation states:

Answer 4

Remember the rules for assigning oxidation numbers:
Oxidation numbers must add up to the overall charge of the molecule.
Alkaline metals always have an oxidation state of 1+.
Hydrogen will normally be assigned an oxidation number of 1+.
Alkaline earth metals are normally assigned an oxidation number of 2+.
Oxygen will normally have an oxidation state of 2−. Oxygen can have an oxidation state greater than 2− in cases where there is an elemental molecule (e.g., O2, when the oxidation state is 0.) or in rare situations when the oxygen atom is connected to a more electronegative element (e.g., peroxides).
When you have elements that don’t “fit into the scheme” for the rules for determining oxidation states, you generally allow the more electronegative element to receive as many electrons as possible to achieve a noble gas configuration.
Electronegativity is an important atomic property that is very much a part of our method for assigning oxidation state values. Many of the rules are based on the fact that certain atoms have a particular electronegativity.

Question 5

Which statement about oxidation states (or numbers) is not correct?

Answer 5

Because the electronegativity difference can be small, electronegativity is not an important consideration when determining oxidation states. (C)

Question 6

Which statement about oxidation numbers (or states) is not true?

Answer 6

The oxidation number tells you the exact number of electrons that each atom has or is missing. (B)

Question 7

Which of the following rules for determining oxidation states (or numbers) is not correct?

Answer 7

Alkaline earth metals normally have an oxidation state of +1. (B)

Question 8

What are the oxidation numbers for magnesium and oxygen (in that order) in the MgO molecule?

Answer 8

+2, −2 (B)

Question 9

What is an oxidation state?

Answer 9

It is a charge assigned to an atom. It tells you how many electrons the atom controls in a compound. (B)

This is a concise definition of an oxidation state. It is basically a mathematical tool for describing electron-atom affinity in molecules.

Question 10

Which of the following shows (in the correct order) the oxidation numbers for phosphorous (P) and oxygen (O) in the polyatomic ion PO43−?

Answer 10

+5, -2 (C)

Question 11

Which statement best summarizes the scenarios for when a molecule can have an oxidation state of 0?

Answer 11

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. (D)

This statement covers all possible situations (or scenarios) for when a molecule can have an oxidation state of 0.

Question 12

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?

Answer 12

There are four (very) polar C–H bonds in the molecule. They are polar towards the carbon atom because carbon is more electronegative. (B)

You may assume this by looking at the oxidation numbers. But remember the big clue in the question: “The electronegativity difference between carbon and hydrogen is very slight.” This strongly suggests a slightly polar bonding between the carbon and hydrogen atoms.

Question 13

Which statement best describes the primary limitation of oxidation state (or oxidation number)?

Answer 13

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. (A)

Question 14

Look at the oxidation numbers assigned for the ammonium ion, NH4+.
NH +
4
−3 + 4 (+1) = +1
ammonium ion
Which of the following statements is not correct?

Answer 14

An electron deficient atom such as nitrogen will welcome the four hydrogen atoms because of the electronegativity difference. (B)

The nitrogen atom will welcome the four hydrogen atoms (partially) because of their electronegativity difference. Nitrogen is more electronegative than hydrogen. But the nitrogen in this molecule has an oxidation state of −3, which means it is electron rich (not electron deficient).