Lecture 1: Group 15 Elements (PDF)

Lecture 1 What are Group 15 Elements? Group 15 elements are also called the Nitrogen family. It includes nitrogen, phosphorus, arsenic, antimony and bismuth elements. The p-block elements are also known as the Representative Elements which are placed on the right side of the main periodic table. S...

Lecture 1 What are Group 15 Elements? Group 15 elements are also called the Nitrogen family. It includes nitrogen, phosphorus, arsenic, antimony and bismuth elements. The p-block elements are also known as the Representative Elements which are placed on the right side of the main periodic table. So, in Group 15 elements as you would move down a group, starting with the lightest element and finishing with the heavy ones; you’d notice a general flow in properties as you move down the order. For eg, Nitrogen is a gas and non-metal but as you move down the group, we encounter metalloids and then at the bottom, metal i.e. Bismuth. These trends in the periodic table help  us better understand the behavior of atoms and also helps us predict new elements. Property Nitrogen Phosphorus Arsenic Antimony Bismuth Atomic symbol N P As Sb Bi Atomic number 7 15 33 51 83 Atomic mass (amu) 14.01 30.97 74.92 121.76 209.98 Valence electron [He]2s2 2p3 [Ne]3s2 3p3 [Ar]3d10 4s24p3 [Kr]4d10 5s25p3 [Xe]4f14 5d106s26p3 configuration Melting point – 210 44.15 817 631 271 Boiling point (°C) -196 281 603(sublimes) 1587 1564 Density (g/cm3) at 1.15(g/L) 1.8 5.7 6.68 9.79 25°C Atomic radius (pm) 56 98 114 133 143 First Ionization 1402 1012 947 834 703 energy (kJ/mol) Common Oxidation -3 to +5 +5, +3, -3 +5, +3 +5, +3 +3 state(s) Ionic radius (pm) 146(-3) 212(-3) 58(+3) 76(+3) 103(+3) Electronegativity 3.0 2.2 2.2 2.1 1.9 1. Electronic Configuration  The valence shell electronic configuration plays a major role in how an element behaves. The valence electron shell configuration of group 15 elements is ns2np3.  All the group 15 elements have the same arrangement and this is why they’re similar.  The s-orbital in this group is completely filled and the p-orbitals are half filled and this makes their configuration extra stable. 2. Atomic and Ionic Radii  If you see the electronic configuration of elements in the table above, you will notice that with every step you move downwards, new orbitals are added to the atom.  This addition of new orbitals increases both the Atomic and the Ionic radii of group 15 elements.  However, we see that from Arsenic to Bismuth only a small increase in ionic radius is observed.  This is due to the presence of completely filled d and/or f orbitals in heavier members. 3. Ionization Enthalpy  Ionization Energy is the amount of energy required to remove an electron from the outermost orbit of the atom.  This is basically a measure of how hard the nucleus is holding on to the electron.  The closer the electron is to the nucleus the stronger its hold and thus the energy required is more.  As we move down the group, the radius of the atom increases, and therefore the Ionization energy decreases due to the weaker hold of the nucleus. 4. Electronegativity  The electronegativity value decreases down the group with increasing atomic size.  This again is due to the increasing distance between the nucleus and the valence shell as we move down the group. 5. Physical Properties  All the elements of the group exist in a polyatomic state.  First, Nitrogen is gas, but as you move down, there is a significant increase in the metallic character of the elements.  Nitrogen and Phosphorus are non-metals, Arsenic and Antimony are metalloids and Bismuth is a metal.   These changes can be attributed to the decrease in Ionization enthalpy and increase in atomic size. Boiling points also, in general, show an increasing trend as you move down.( What happens to the boiling point as you move down the group? The atoms increase in size, as they gain extra electron shells, and the intermolecular forces become stronger. More energy is required to break these forces, thus there are higher melting and boiling points as you go down the group). 6. Chemical Properties  The valence shells of the p-Block elements have a configuration of ns2 np3.  So, the elements here can either lose 5 electrons or gain 3.  The common oxidation states of these elements are -3, +3 and +5.  With a decrease in the Ionization enthalpy and electronegativity due to the increasing atomic radius, the tendency to gain three electrons to create a -3 oxidation state decreases down the group.  In fact, Bismuth hardly forms any compounds with a -3, oxidation state.  As we go down, the stability of the +5 state decreases and that of +3 increases due to the inert pair effect.  Q1/ Why is Group 15 called P block?  It is a p-block element since it takes the physical and chemical properties after that of other p-block elements of the eighteenth group. P-block elements are generally non-metals, while the remaining are metalloids and metals. Two of the most dissimilar nonmetallic elements are in the same group: reactive phosphorus and unreactive nitrogen. Of the other members of the group, arsenic is really a semimetal, and the two lower members of the group, antimony and bismuth, exhibit weakly metallic behavior. Nitrogen and phosphorus are both nonconductors of electricity, and both form acidic oxides, so they are unambiguously classified as nonmetals. Although they are vertical neighbors in the periodic table, the redox behavior of nitrogen and phosphorus could not be more different. Whereas the higher oxidation states of nitrogen are strongly oxidizing in acidic solution, those of phosphorus are quite stable. In fact, the highest oxidation state of phosphorus is the most thermodynamically stable and the lowest oxidation state, the least stable—the converse of nitrogen chemistry.( While both nitrogen and phosphorus can exist in a +3 and +5 oxidation state, there are factors that make the +5-oxidation state more stable for phosphorus than it is for nitrogen. Nitrogen has a strong N=N triple bond, which asks for a lot of energy to form compounds, making it less stable in its +5 oxidation state. In contrast, phosphorus can use its d orbitals to form more bonds and thus stabilize its +5 oxidation state. A classic example is that nitrogen forms only NF3, whereas phosphorus can form both PF3 and PF5. The ability to use d orbitals to form more bonds provides extra stability to the +5 oxidation state in phosphorus). Frost diagram comparing the stability of the oxidation states of phosphorus and nitrogen in acidic solution The Thermodynamic Stability of Dinitrogen: 1-If we look at the bond energies, we can see why different species are preferred for the two elements. Dinitrogen, N2, is the stable form for the element, and it is a common product from nitrogen-containing compounds in chemical reactions. This is, in large part, due to the very high strength of the nitrogen- nitrogen triple bond compared to the single (or double) bonds. 2-Thus, elemental phosphorus contains groups of singly bonded phosphorus atoms. In fact, the strong phosphorus-oxygen single bond becomes a dominant feature of phosphorus chemistry. For example, as we will see below, whereas the element nitrogen is very stable to oxidation, elemental phosphorus reacts vigorously with oxygen to give oxides. 3-The triple nitrogen-nitrogen bond energy is greater even than that for the triple carbon-carbon bond (Table 15.4). Conversely, the single bond between two nitrogen atoms is much weaker than the carbon-carbon single bond. It is this large difference between N‚N and N¬N bond strengths (742 kJ/mol– 1) that contributes to the preference in nitrogen chemistry for the formation of the dinitrogen molecule in a reaction rather than chains of nitrogen- nitrogen single bonds, as occurs in carbon chemistry. Furthermore, the fact that dinitrogen is a gas means that an entropy factor also favors the formation of the dinitrogen molecule in chemical reactions. 4-We can see the difference in behavior between nitrogen and carbon by comparing the combustion of hydrazine, N2H4, to that of ethene, C2H4. The nitrogen compound burns to produce dinitrogen, whereas the carbon compound gives carbon dioxide: The Bonding Limitations of Nitrogen: 1-Nitrogen forms only a trifluoride, NF3, whereas phosphorus forms two common fluorides, the pentafluoride, PF5, and the trifluoride, PF3. It is argued that the nitrogen atom is simply too small to accommodate more than the three fluorine atoms around it, while the (larger) lower members of the group can manage five (or even six) nearest neighbors Why there arise negative oxidation state for N? Because of the difference in the EN between H=2.1 & N=3.0. Example :-NH3 (N -III), N2H4 (-II), NH2OH(-I) ,N2 (0), N2O (+I), NO (+II), HNO2(+III), NO2(+IV) & HNO3(+V). Nitrogen can fill the outer shell to be 8es by: 1. Gains 3es forming N3- (Nitrides of alkaloids elements) 2. Forming single covalent. Bonds (e.g.NH3) or multiple (e.g. N≡N). 3. Forming covalent bonds with loosing e.g. [NH4] + 4. forming covalent bonds with gaining e.g. NH2- amide. There will be stable nitrogen compounds, the outer shell of nitrogen is incomplete (e.g.NO or NO2) each N contains one unpaired e. They have paramagnetic properties. Nitrogen forms multiple bonds differing from the other gr. elements, so it likes C & O. The bond (N-N) is weaker than that in (C-C) because of the repulsion of the non bonding electrons on the Nitrogen atoms.