Trends and Exceptions in Group 15 Oxidation States
In chemistry, groups of elements refer to different columns of elements in the periodic table. There are in total 18 groups that are numbered in the periodic table from left to right. The f-block column of the periodic table is still not numbered as a group.
Out of the 18 numbered groups, we are going to talk about group 15. Group 15 consists of the p-block elements (the block in the periodic table is an arrangement or set of elements on the basis of their valence electrons). The p-block is situated on the right-hand side of the table. The group 15 elements include nitrogen, phosphorus, arsenic, antimony, and bismuth.
The members of group 15 show similar patterns in their electronic configuration, especially in their outermost shell. All these elements have 2 electrons in their subshell, and all of their outermost shells consist of 5 electrons.
The two most important elements of this group are Nitrogen and Phosphorus. Nitrogen, which occurs in a free state as a gas that is diatomic, constitutes more than 70% of the volume of air. Phosphorus is an element of life- its forms are present in our RNA and DNA. It is also present in bone marrows.
The Trends in Chemical Reactivity of the Group 15 Elements
Here we will discuss the trends in chemical reactivity of the group 15 elements, i.e. nitrogen, phosphorus, arsenic, antimony, and bismuth.
Elements of a group in the periodic table have similar patterns in their configuration of electrons. This similarity results in the formation of different trends in the chemical reactivity of the elements. Similarly, the elements of group 15 have a similar electron configuration- all of them have 2 electrons in their subshells and 5 electrons in their outer shells. This leads to the patterning of trends in their reactivity. Like as we go down the group (from Nitrogen to Bismuth), the metallic character of the elements increases and the ionization enthalpy (the amount of energy required to lose an electron) character of the elements decreases.
The covalent character (sharing of electrons) of the atoms decreases as we move down the group.
Group 15 elements are used in forming hydrides (compounds of hydrogen with any other element). Formation of NH3 (Ammonia), BiH3 (Bismuthine), PH3(Phosphine), AsH3 (Arsine), and SbH3 (Stibine) occurs.
Group 15 elements also form Halides (compounds formed with halogen atoms and any element). The halide is usually trihalides and pentahalides, e.g. Phosphorus trichloride (PCl3) and Phosphorous pentachloride (PCl5).
Group 15 elements form oxides like Nitric Oxide, Bismuth Oxychloride, etc. The oxides are formed by the oxidation process (loss of electrons during a reaction) under different oxidation states of p block elements. H2
Since the atomic size increases as we move down, the melting point also gradually increases. Nitrogen has the least melting point in the group, and as we move to Arsenic and Phosphorus, the melting point starts to increase. There is a decrease in the trend from antimony onwards. This occurs because of the loose structure of atoms in the bonding.
The boiling point of the elements regularly increases as we move down, and so does the density of the elements.
Oxidation States of Group 15 Members
The number of electrons in the outermost shell of the group 15 members is 5. In order to make it an octet configuration, it requires 3 more electrons. Therefore, it needs to gain 3 more electrons or share 3 electrons with the help of the covalent bonds. Therefore, the more common oxidation for these elements is the -3 oxidation which means adding 3 more electrons. The tendency to produce the -3 oxidation decreases as we move down the group. This happens because of the increase in metallic character and the atomic size of the elements. -3 oxidation state is used by reacting the group 15 members with hydrogen to produce ammonia, phosphane, arsane, stibane, and bismuthine. As we go down the group, these hydrides become more toxic and less stable.
The +3 oxidation and +5 oxidation states occur by sharing electrons. In +3 oxidation, this sharing can occur through covalent bonds, in the case of- nitrogen, arsenic, and phosphorus. E.g. Nitrogen trichloride, phosphorus trichloride, arsenic trichloride, or ionic bonds, in the case of antimony and bismuth. E.g. Antimony trifluoride and Bismuth trifluoride. As we move down the group, the covalency character of the elements decreases.
The +5 oxidation state for nitrogen forms the N2O5. The true +5 oxidation occurs in phosphorus, arsenic, and antimony. Phosphorus even produces oxyhalides.
The Other Oxidation States for Group 15 Elements
Nitrogen can form various oxides under oxidation states +2, +4, and very unstable +6.
Antimony can produce a compound under the oxidation state of +2.
Phosphorus in phosphoric acid has the +1 oxidation state, and in hypo phosphoric acid, it has an oxidation state of +4.
General properties of Group 15
Following are the compounds included in the nitrogen family :
Nitrogen(N)
Phosphorus(P)
Arsenic(As)
Antimony(Sb)
Bismuth(Bi)
The electron configuration of all the elements in group 15 is ns2np3in their outer shell, where n is the principal quantum number.
Periodic Trends
Following are the trends followed by all the Group 15 elements :
Electronegativity: Electronegativity of an atom is the ability of that atom to attract electrons. It decreases down the group.
Ionization Energy: The ionization energy of an atom is the amount of energy it requires to remove an electron from an isolated atom/molecule. It decreases down the group.
Atomic Radii: It increases in size down the group.
Electron Affinity: The ability of an atom to accept an electron is known as electron affinity. It decreases down the group.
Melting Point: It is the amount of energy required to break bonds in order to change a substance to a liquid phase from a solid phase. It increases down the group.
Boiling Point: It is the amount of energy required to break bonds in order to change a substance to a phase from the liquid phase. It increases down the group.
Metallic Character: It refers to the level of reactivity of the metal. It increases down the group.
FAQs on Oxidation State of Group 15 Elements: Complete Guide
1. What are the common oxidation states shown by Group 15 elements?
Group 15 elements, having five valence electrons, generally show a range of oxidation states. The most common ones are -3, +3, and +5. However, the stability of these states varies as you move down the group. For instance, nitrogen can exhibit almost all oxidation states from -3 to +5.
2. Why are Group 15 elements also called pnictogens?
The name 'pnictogen' comes from the Greek word 'pnigein', which means 'to choke' or 'to suffocate'. This name was given to Group 15 because its first element, nitrogen gas, is known to cause suffocation. The elements of this group—Nitrogen, Phosphorus, Arsenic, Antimony, and Bismuth—are therefore collectively known as pnictogens.
3. What types of oxides do Group 15 elements typically form?
The elements of Group 15 form two main types of oxides: trioxides (with the general formula E₂O₃) and pentoxides (with the formula E₂O₅). The acidic character of these oxides decreases down the group. For example, oxides of nitrogen and phosphorus are acidic, arsenic and antimony oxides are amphoteric, and bismuth oxide is basic.
4. Why does the stability of the +3 oxidation state increase as we move down Group 15?
This trend is explained by the 'inert pair effect'. As we descend the group, the two electrons in the s-orbital of the valence shell become reluctant to participate in chemical bonding. This is due to the poor shielding of the nuclear charge by inner d- and f-orbitals. Consequently, only the three p-electrons are easily available for bonding, making the +3 oxidation state more stable for heavier elements like Bismuth (Bi).
5. Why can't nitrogen form pentahalides like NCl₅, even though it can show a +5 oxidation state?
Nitrogen is unable to form pentahalides because it lacks vacant d-orbitals in its valence shell (the second energy level). To form five bonds, an atom needs to expand its octet, which requires available d-orbitals for bonding. Since nitrogen does not have these, it cannot form compounds like NCl₅. Other elements in the group, like phosphorus, can form PCl₅ as they have accessible d-orbitals.
6. How does the inert pair effect influence the oxidation states of heavier Group 15 elements?
The inert pair effect is the tendency of the two s-electrons in the outermost shell to remain unshared in compounds. This effect becomes much more significant for heavier elements. In Group 15, this means that losing all five valence electrons to show a +5 state becomes very difficult for Antimony (Sb) and Bismuth (Bi). They prefer to lose only their three p-electrons, which is why the +3 oxidation state is the most stable and common for them.
7. What is meant by disproportionation in the context of oxidation states?
Disproportionation is a specific type of redox reaction where an element in an intermediate oxidation state is simultaneously oxidised and reduced to form two different products. In Group 15, certain oxidation states of nitrogen and phosphorus are known to undergo this. For example, phosphorous acid (H₃PO₃), where phosphorus is in the +3 state, can disproportionate into phosphine (PH₃, -3 state) and phosphoric acid (H₃PO₄, +5 state).
8. Which element in Group 15 is the most metallic, and why?
Bismuth (Bi) is the most metallic element in Group 15. Metallic character increases as we move down a group in the periodic table. This occurs because the atomic size increases and the ionisation enthalpy decreases, making it easier for the atom to lose its outermost electrons—a key characteristic of metals.
