Oxidation State of Group 16
Group 16 belongs to the p-block of the periodic table as their last electron enters in the p orbital. Elements of group 16 are oxygen(O), Sulphur(S), Selenium(Se), Tellurium(Te) and Polonium(Po). These elements are termed as chalcogens/ ore-forming elements. Polonium is a radioactive element and is unstable, selenium and tellurium are metalloids, and rest all are considered as non-metals. The general electronic configuration of these elements present in the Group 16 is ns²np⁴. The valency of the electrons of the elements present in group 16 is considered to be 6 which means that 2 more electrons are required to achieve the octet state.
The Oxidation State of Sulfur
Sulfur is bigger in size with an empty d-orbital 3d which allows it to expand its valency. In addition to -2 oxidation state sulfur exhibits +2,+4, and +6 oxidation states respectively. Out of these +4 and +6 are common oxidation states.
The Oxidation State of Oxygen
The oxygen state refers to the number of electrons gained or forfeited by the element in order to achieve a noble gas configuration. An element can have only one or more than one oxidation state depending on the stability of the compound formed. Oxidation states of elements very much depend upon the electronic configuration of elements as well as their electronegativity. Electronegativity is the ability of the element to attract electrons towards itself. The smaller is the element, the greater will be the electronegativity.
Oxygen is highly electronegative and shows -2 oxidation state(gains 2 electrons) in most of the metallic oxides. Extra space is not available in oxygen for the accommodation of unpaired electrons due to its small size and absence of d-orbitals. But there are exceptions. Oxygen shows the +2 oxidation state in OF₂, +1 oxidation state in O₂F₂, and -1 oxidation state in hydrogen peroxide(H₂O₂).
The analysis of the above table is crucial to understand the oxidation states of group 16 elements.
As we move from top to bottom in a group of the periodic table the size of the atoms increases and the electronegativity decreases. Also as we move from left to right in a period of the periodic table the size of the atom decreases. The electronegativity decreases from oxygen to tellurium and thus their oxidation state increases to the same extent. This also points towards the decrease in the covalent character from top to bottom.
Oxygen (O)is the most abundant element on earth. Its percentage in the atmosphere is considered to be around 23%. Oxygen combines with most of the elements except the noble gases and metals like ɡold. It also acts as a good oxidizing agent.
It shows anomalous behavior in group 16 due to its small size and high electronegativity. Oxygen has the highest electronegativity after the element fluorine. It is considered to be paramagnetic with two unpaired electrons even though it has an even number of electrons(16). Also, it can still expand its covalence up to 4.
Oxidation States of Selenium and Tellurium
Selenium and Tellurium seem to exhibit -2,+2,+4,+6 oxidation states respectively.
The Oxidation State of Polonium
Polonium exhibits +4 and +2 oxidation states.
Polonium is a radioactive metal that has a half-life of only 13.8 days which means that it will reduce to it's half quantity in approximately 14 days. Polonium exists as a decay product of thorium and uranium minerals in nature.
As one goes down from top to bottom the inert pair effect comes in role causing the pair of electrons from the outermost s orbital to not take part in the bonding of the electrons. Thus +6 oxidation state is not stable for lower elements in group 16. Also, the stability of the -2 oxidation state decreases down the group due to a decrease in the electronegativity of the elements.
Q1. H₂O has Strong Hydrogen Bonding Whereas H₂Te/H₂S Does not Despite Belonging to the Same Group. Why?
This is attributed to the high electronegativity of oxygen which helps in the formation of H-bond with it. Hydrogen bonds are formed between highly electronegative elements like Fluorine/nitrogen/oxygen and hydrogen. The more is the electronegativity of the element the more will be the strength of the H-bond formation. Oxygen has a higher electronegativity as compared to sulfur and tellurium so sulfur does not form H-bond.
2. What is the Oxidation State of Sulfur and Oxidation State of Oxygen in the Sulfate ion?
SO₄-2 is known as the Sulphate Polyatomic Ion.
The oxidation state of sulfur is +6 and the oxidation state of all the four oxygen is -2 in SO₄-2.
One can calculate this by the following method-
The sum of all the oxidation states of atoms present in sulfate ion must be -2
Thus Let the oxidation state of sulfur be x
x + 4*(-2)= -2
The oxidation state of oxygen can also be calculated by putting the oxidation state of sulfur as +6.
Using this method one can easily find oxidation states of different elements in polyatomic ions.