Oxidation States and Trends in Physical and Chemical Properties — Definition

Definition

The Group 15 elements, often called the pnictogens, are a fascinating family in the periodic table, including Nitrogen (N), Phosphorus (P), Arsenic (As), Antimony (Sb), and Bismuth (Bi). Understanding their 'oxidation states' and how their 'physical and chemical properties' change as you move down the group is crucial for NEET. Let's break it down simply.

First, what are 'oxidation states'? An oxidation state (or oxidation number) is a number assigned to an element in a compound that represents the number of electrons an atom has gained, lost, or shared when forming chemical bonds with other atoms.

It's essentially a way to keep track of electron distribution in compounds. For Group 15 elements, their general outer electronic configuration is $ns^2 np^3$. This means they have 5 valence electrons.

To achieve a stable octet, they can either gain 3 electrons (forming a -3 oxidation state), or lose electrons. Losing all 5 valence electrons would lead to a +5 oxidation state, and losing only the three p-electrons would lead to a +3 oxidation state.

Nitrogen, being the first member, is unique due to its small size, high electronegativity, and absence of d-orbitals, allowing it to exhibit a wide range of oxidation states from -3 to +5, including positive fractional states in some compounds like azides.

However, it typically doesn't show a +5 oxidation state in simple ionic compounds because it cannot expand its octet.

Now, let's talk about 'trends in physical properties'. As we move down Group 15 from Nitrogen to Bismuth, several properties change predictably:

1
Atomic and Ionic Radii — These generally increase down the group because new electron shells are added with each successive element, increasing the distance of the valence electrons from the nucleus.
2
Ionization Enthalpy — This is the energy required to remove an electron. It generally decreases down the group because the outermost electrons are further from the nucleus and experience greater shielding, making them easier to remove.
3
Electronegativity — This is the ability of an atom to attract shared electrons in a bond. It generally decreases down the group as atomic size increases and the nucleus's pull on valence electrons weakens.
4
Metallic Character — This increases down the group. Nitrogen and Phosphorus are non-metals, Arsenic and Antimony are metalloids (showing properties of both metals and non-metals), and Bismuth is a metal. This change is due to the decreasing ionization enthalpy and electronegativity, making it easier for heavier elements to lose electrons.
5
Melting and Boiling Points — These generally increase from N to As, then decrease for Sb and Bi. This trend is complex and influenced by factors like atomic size, crystal structure, and intermolecular forces.

Finally, 'trends in chemical properties' refer to how these elements react with other substances:

1
Reactivity towards Hydrogen — All Group 15 elements form hydrides of the type $EH_3$ (e.g., $NH_3, PH_3$). The stability of these hydrides decreases down the group ($NH_3 > PH_3 > AsH_3 > SbH_3 > BiH_3$) due to decreasing bond dissociation enthalpy (E-H bond becomes weaker as E-atom size increases). Their reducing character, however, increases down the group.
2
Reactivity towards Oxygen — They form oxides of the type $E_2O_3$ and $E_2O_5$. The acidic character of the oxides decreases down the group ($N_2O_3$ and $P_2O_3$ are acidic, $As_2O_3$ and $Sb_2O_3$ are amphoteric, and $Bi_2O_3$ is basic). The stability of the +5 oxidation state decreases, while the stability of the +3 oxidation state increases down the group (inert pair effect).
3
Reactivity towards Halogens — They form halides of the type $EX_3$ and $EX_5$. The stability of $EX_5$ decreases down the group, with $BiF_5$ being the only known pentahalide for Bismuth, and $BiCl_5$ being unstable. Nitrogen does not form pentahalides due to the absence of d-orbitals.
4
Reactivity towards Metals — These elements react with metals to form binary compounds in which they exhibit the -3 oxidation state (e.g., $Ca_3N_2, Ca_3P_2$).

By understanding these fundamental concepts and trends, you'll be well-equipped to tackle NEET questions on Group 15 elements.