Oxidation Numbers of Elements in Periodic Table | s,p,d blocks

Oxidation number of an atom is an important section in chemistry because according to the oxidation number, properties of compounds are changed widely. Some elements in the periodic table have only one oxidation number or two or more oxidation numbers. Some elements such as Manganese have several oxidation numbers. Oxidation number of an atom in a compound can be positive or negative or may be zero.

• In sodium compounds, sodium's only oxidation number is +1 (NaCl, NaBr, Na₂CO₃).
• But some types of atoms such as chlorine shows various oxidation numbers like -1, 0, +1, +3, +5, +7 oxidation numbers in compounds.

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Content

• Important facts of oxidation numbers
  • Oxidation number of an atom can be positive or negative or may be zero
  • More than one oxidation numbers of an element
  • What is the oxidation number when an element has not combined or do not form a compound
  • Oxidation number of an atom when an element has combined with the same element
• Table: oxidation numbers of according to the atomic number, first 20 elements
• Oxidation states of s block
  • Hydrogen
  • Alkali Metals - Group 1
  • Alkali Earth Metals - Group 2
• Oxidation states of p block elements
  • Group 3
  • Group 4
  • Group 5
  • Group 6
  • Group 7, Halogen series
• Oxidation states of 3d metals

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Important facts of oxidation numbers

In this chapter, we discuss very important facts about oxidation numbers with examples with different compounds.

Oxidation number of an atom can be positive or negative or may be zero

• We know metals release electrons to form cations. Therefore metals always form positive oxidation numbers.
• But non metals such as sulfur, phosphorus, chlorine can show both positive or negative oxidation numbers.

More than one oxidation numbers of an element

Some elements can show more than one oxidation states. Specially d block elements show this behavior in chemistry.

Examples to more than one oxidation numbers of a element

• Chlorine : -1 , 0 , +1 , +3 , +5 , +7
• Manganese : 0 , +2 , +4 , +7

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Oxidation number when an element has not combined or do not form a compound

When an element has not combined, it's oxidation number is 0 because no electron transfer is occurred.

Example: Gold (Ag) is found as a pure metal in the nature because Gold is very low reactive metal. In such cases, oxidation number of gold is 0.

Oxidation number of an atom when an element has combined with the same element

When an element has combined with same kind element, it's oxidation number becomes 0.

Ex: oxidation number of Bromine in Br₂ molecule is 0.

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Table: oxidation numbers of first 20 elements in the periodic table according to the atomic number

Here we are going to summarize oxidation numbers of all elements in the periodic table into one table.

NOTE: * is for rare oxidation number

Atomic Number	Element	Oxidation numbers
1	Hydrogen	-1 , 0 , +1
2	Helium	0
3	Lithium	+1
4	Beryllium	+2
5	Boron	+3
6	Carbon	-4 , -3 , -2 , -1 , 0 , +1 , +2 , +3 , +4
7	Nitrogen	-5 , -4 , -3 , -2 , -1 , 0 , +1 , +2 , +3
8	Oxygen	-2 , -1 , 0 , +1 , +2
9	Fluorine	-1 , 0
10	Neon	0
11	Sodium	+1
12	Magnesium	+2
13	Aluminium	+3
14	Silicon	0 , +4
15	Phosphorous	0 , +1 , +3 , +5
16	Sulfur	-2 , 0 , +2 , +4 , +6
17	Chlorine	-1 , 0 , +1 , +3 , +5 , +7
18	Argon	0
19	Potassium	+1
20	Calcium	+2

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Oxidation states of s block elements

Oxidation numbers of group 1 group 2 elements are studied here.

Hydrogen

Hydrogen forms three oxidation states, -1 , 0 , +1. Oxidation number 0 occurs only in hydrogen molecule.

-1 oxidation state of hydrogen atom - Examples

When hydrogen forms compounds with metals, hydrogen's oxidation number is -1. Sodium hydride (NaH) and Calcium hydride (CaH₂) are some examples.

+1 oxidation number of hydrogen atom

When carbon form compounds with non-metal elements, oxidation number of hydrogen become +1 in most occasions.

• Methane: CH₄ - Electronegativity of carbon is higher than hydrogen. So hydrogen is oxidized and oxidation number is +1.

Alkali Metals - Group 1

Alkali metals (except hydrogen) remove one electron to form +1 ion. So only oxidation number of alkali metals is +1. So Lithium, Sodium, Potassium, Rubidium, Cesium have only single oxidation number is +1.

Alkali Earth Metals - Group 2

As alkali metals, alkali earth metals only form +2 oxidation state. Beryllium, Magnesium, Calcium, Strontium, Barium have +2 oxidation number.

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Oxidation states of p block elements

In this section, we will discuss oxidation numbers of p block elements group by group. Though several elements included in one group, those elements show different oxidation numbers.

Group 3

Group three elements are Boron, Aluminium, Galleum, Indium and Thallium.

Boron

Boron shows +3 oxidation state.

• +3: BCl₃ , BF₃

Aluminium (Aluminum)

Aluminium is a metal. So it removes it's three electrons of the last shell to show +3 oxidation number. Some examples for +3 oxidation states of aluminium are AlCl₃ and Al₂O₃.

Group 4

Carbon

Oxidation numbers of carbon changes from -4 to +4.

• -4: CH₄ - Methane
• -3: CH₃CH₃ - Ethane
• -2: CH₃Cl
• -1: ClCH₂CH₂Cl
• 0 : CH₂Cl₂
• +1: CH₃-CHO - Carbonyl carbon has the +1 oxidation state.
• +2: CHCl₃
• +3: CH₃-COOH - Carboxylic acid carbon atom has the +3 oxidation number.
• +4: CCl₄ , CO₂

Group 5

Group 5 includes Nitrogen, Phosphorous, Arsenic, Antimony and Bismuth. Oxidation number variation change when going down the group.

Nitrogen

Nitrogen has various oxidation numbers from -3 to +5.

• -3: Ammonia (NH₃), ammonium chloride (NH₄Cl)
• -1: Hydroxylamine (NH₂OH)
• 0: Nitrogen Gas (N₂)
• +1: Nitrous Oxide (N₂O)
• +2: Nitric Oxide (NO)
• +3: Dinitrogen trioxide (N₂O₃)
• +4: Nitrous dioxide (NO₂), dinitrogen trioxide (N₂O₄)
• +5: Dinitrogen pentoxide (N₂O₅)

Phosphorous

Phosphorous also have various oxidation numbers from -3 to +5.

• -3: Na₃P, Ca₃P₂, PH₃
• -0: P₈ , P₄
• -+1: H₃PO₂
• +3: P₂O₃ , H₃PO₃
• +5: H₃PO₄ , Na₃PO₄ , P₂O₅

Group 6

We are going to discuss about oxidation numbers of oxygen and sulfur under group 6 elements.

Oxygen

Oxygen has the second highest electronegative value in periodic table. So in most occasions, oxidation number of oxygen is negative. Oxygen only forms positive oxidation numbers when it combine with fluorine.

• +2: In F₂
• 0: Oxygen molecule (O₂)
• -1: Example for -1 oxidation state is hydrogen peroxide (H₂O₂).
• -2: Most common oxidation number of oxygen is -2. Na₂O, MgO, H₂O are examples to -2 oxidation state of oxygen.

Sulfur

Sulfur is another p block element which has different oxidation numbers.

• -2: Na₂S , H₂S
• 0: S₈
• +4: SO₂ , H₂SO₃
• +6: H₂SO₄ , BaSO₄

Group 7, Halogen series

Group 7 has Fluorine, Chlorine, Bromine, Iodine and Astatine as elements. These metals show variable oxidation numbers.

Fluorine

FLuorine is the most electronegative element in the periodic table. Therefore. fluorine atom does not make positive oxidation numbers at all.

• 0: Fluorine gas (F₂)
• -1: Hydrogen fluoride (Hydrofluoric acid / HF)

Chlorine

Chlorine also have lot of oxidation numbers. (from -1 to +7)

• -1: HCl , NaCl
• 0: Cl₂ gas
• +1 : HOCl , NaClO
• +3: NaClO₂
• +5: NaClO₃
• +7: NaClO₄ , HClO₄

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Oxidation states of 3d metals

Most of the 3d metals have variable oxidation numbers. As an example, iron have two oxidation numbers, +2 and +3.

• +2: FeO, FeCl₂
• +3: Fe₂O₃

Zinc has only one oxidation number, +2.

3d metals variable oxidation states

3d Metal	Oxidation state
Sc	+3
Ti	+2 , +3 ,+4
Va	+2 , +3 , +4 , +5
Cr	+2 , +3 , +6
Mn	+2 , +4 , +6 , +7
Fe	+2 , +3
Co	2 , +3
Ni	+2
Cu	+1 , +2
Zn	+2

Oxidation number and colour of compound or solution

Let's take an example.

• FeCl2 - green
• FeCl3 - brown

Why chlorine has wide range of oxidation states than sulfur?

Sulfur and chlorine and are positioned at 6th and 7th groups of the periodic table. So there are six and seven electrons at their final energy levels respectively. These electrons can be combined with oxygen to form different compounds. Some examples are discussed below.

Sulfur gives its all last six electrons to make sulfuric acid molecule (+6 oxidation state). Chlorine can give seven electrons to make chloric acid to show +7 oxidation number.

Chlorine can take one electron to form chloride anion.(-1 oxidation state). Sulfur can take two electrons to form sulfide anion. (-2 oxidation state).

Common oxidation states of sulfur

Common Oxidation State	Example Compound
-2	H2S
0	Elemental sulfur (S8)
+4	SO2
+6	SO3 , H2SO4

Common oxidation states of chlorine

Common Oxidation State	Example Compound
-1	HCl
0	Cl2
+1	HOCl
+3	HClO2
+5	HClO3
+7	HClO4

Oxidation number of chlorine in Ca(OCl)₂

Charge of hypochlorite ion OCl^- is -1. So we can calculate oxidation number of chlorine.

-2 + x = -1

x = +1

oxidation number of chlorine in Ca(OCl)₂ is +1.

Do not confuse with oxidation number and charge of ion

Oxidation number and charge of ion is not equal at every time.

• Na⁺ ion's oxidation number (+1) and charge of ion(+1) are same.
• Cl^- ion oxidation number and charge of ion are same.
• But in KMnO₄, oxidation number of manganese atom is +7. But manganese did not form a +7 ion in KMnO₄.
• Chromium does not form +6 ion. But in K₂CrO₄, oxidation number of chromium is +6.

in silicon sulfide, which element would have the negative oxidation number?

Silicon sulfide (SiS₂) contain silicon and sulfur atoms. Because you want to find the negative oxidation number, find which element's electronegativity is greater?

Elecronegativity of sulfur is 2.5 and silicon's 1.8 . Therefore, sulfur is more electronegative than silicon. Thus, sulfur should have the negative oxidation number (-2).

What are the elements which have highest oxidation numbers in periodic table?

As a metal, manganese have +7 oxidation number. Some example compounds for +7 are Potassium permanganate ( KMnO₄ ), Mn₂O₇

As a non-metal, chlorine forms +7 oxidation number. HClO₄, Cl₂O₇ are examples for +7 oxidation number of chlorine.

Which element has the zero or negative oxidation number always?

Fluorine has the zero or negative oxidation number always because fluorine is the most electronegative element in the periodic table. So it always gains an electron when fluorine combines with different element to form -1 oxidation number. Fluorine form zero oxidation number in F₂ molecule.

In NaF and HF, oxidation number of fluorine is -1.

Give an example reaction to chlorine become positive oxidation number after a reaction?

Chlorine gas (oxidation number 0) reacts with cold dilute NaOH or hot concentrated NaOH to give +1 and +5 oxidation states of chlorine atom respectively.

• +1: NaClO
• +5: NaClO₃

s block elements do they have variable oxidation States

Only hydrogen shows variable oxidation numbers. All other elements show single oxidation numbers. Group 1 elements show +1 oxidation state and group 2 elements show +2 oxidation state.

In p block elements, what is the maximum oxidation number?

Group VII elements form highest oxidation numbers. From group VII, chlorine form +7 oxidation number.

Which elements have the +7 oxidation number and give examples?

Both manganese and chlorine have +7 oxidation number. In potassium permanganate ( KMnO₄) and HMnO₄, manganese oxidation number is +7. Chlorine forms chloric acid (HClO₄) to show +7 oxidation number.

Oxidation state and acidity

When oxidation number increases, acidity strength also increases. Consider acidity of different oxides of nitrogen.

N₂O and NO are neutral acidic gases and other oxides of nitrogen ( N₂O₃, NO₂, N₂O₄ and N₂O₅ ) are acidic. N₂O₅ is a strong acidic compound.

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