The p-Block Elements: Group 15 - Trends in Physical and Chemical properties

Modern Periodic Table

The modern periodic table as conceived by Dimitri Mandeleev arranges all the elements known to man on the basis of its atomic number, which is unique to every element. The results of such an arrangement was periodic table. The elements with similar properties were arranged into a column called a group. So 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 behaviour 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 configuration [He]2s2 2p3 [Ne]3s2 3p3 [Ar]3d10 4s24p3 [Kr]4d10 5s25p3 [Xe]4f14 5d106s26p3
Melting point

Boiling point (°C)

– 210

-196

44.15

281

817

603(sublimes)

631

1587

271

1564

Density (g/cm3) at 25°C 1.15(g/L) 1.8 5.7 6.68 9.79
Atomic radius (pm) 56 98 114 133 143
First Ionization energy (kJ/mol) 1402 1012 947 834 703
Common Oxidation state(s) -3 to +5 +5, +3, -3 +5, +3 +5, +3 +3
Ionic radius (pm) 146(-3) 212(-3) 58(+3) 76(+3) 103(+3)
Electronegativity 3.0 2.2 2.2 2.1 1.9

Some of the trends in the modern periodic table with respect to group 15 of the p-Block elements are discussed below.

Electronic Configuration
The valence shell electronic configuration plays a major role in how an element behaves. The valence shell configuration of group 15 elements is ns2np3. All the elements of group 15 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.

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 asmall increase in ionic radius is observed. This is due to the presence of completely filled d and/or f orbitals in heavier members.

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.

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.

Physical Properties

All the elements of the group exist in a polyatomic state. The first, Nitrogen is a 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. Except Nitrogen, all the other elements have allotropes.

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 decrease in the Ionization enthalpy and electronegativity, due to 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 -3 oxidation state. As we go down, the stability of +5 state decreases and that of +3 increases due to inert pair effect.

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Practise This Question

Nitrogen has the electronic configuration 1s2,2s2 2p1x 2p1y 2p1z and not 1s2,2s2 2p2x 2p1y 2p0z which is determined by