P Block Elements

7.42 

To draw the resonating structure of NO₂ No. of valence electrons for N = 5

No. of valence electrons for O = 2 * 6 = 12

Total no. of valence electrons = 5 + 12 = 17

The N and O atoms are arranged in such a way that the less electronegative atom N is placed as the central atom.

Electron pairs are placed between bonds and distributed around the atoms to form octet as shown. Both the O atoms have their complete octet but N has only 5 electrons.

A pair of electrons is transferred from O to the bond between O and N. Both the O atoms have their complete octets and N has 7 electrons.

The resonating structures of NO₂ can be shown as:

To draw the resonating structure of N₂O₅

No. of valence electrons for N = 2 * 5 = 10

No. of valence electrons for O = 5* 6 = 30

Total no. of valence electrons = 10 + 30 = 40

Just like in NO_2, the valence electrons are distributed between bonds and then around atoms to complete their octet.

(Formal charge of each atom is enclosed in bracket)

When the valence electrons around the atoms, the formal charge of N atoms are found to be +1 while that of the center O is +2. But N is less electronegative than O. So, the electrons can be re-arranged as

The centre O now has formal charge 0 and the terminal O atoms have formal charge -1 which the N atoms have +1 which is more appropriate since O is more electronegative than O.

The resonance structures of N₂O₅ can be shown as:

7.37 

Chemical reactivity of group 15 elements towards hydrogen, oxygen, halogens, and metals are discussed below.

Reactivity towards hydrogen: Group 15 elements react with H to form hydrides of type EH3 where E=N, P, As, Sb or On moving down from NH₃ to BiH₃, the stability of the hydrides decreases. For example, the P-H bond in PH₃ is less stable than the N-H bond in NH₃. The strength of the E-H bond gets weaker as the size of the central atom increases.

Stability order of E-H bond (where E is group 15 elements) can be represented as

N-H > P-H > As-H >Sb-H > Bi-H

Reactivity towards oxygen: Group 15 elements react with O to form oxides of the type E₂O₃ and E₂O₅ where E = N, P, As, Sb or Bi. The oxide of the element with higher oxidation state is more acidic than those with lower oxidation Down the group, the acidic character of the oxides decreases.

Reactivity towards halogens: Group 15 elements form halides of the type EX₃ and EX₅ where E = N, P, As, Sb or Bi. However, N does not form pentahalides EX₅ as it lacks d-orbital. All group 15 elements form trihalides but trihalides of N are

Reactivity towards metals: Group 15 elements react with metals to form compounds of general formula M₃E₂ where E= N, P, As, Sb or The binary compounds formed by the reaction of group 15 elements with metal show oxidation state -3.

7.1

The general characteristics of Group 15 elements are:

Electronic configuration: All Group 15 elements have 5 electrons in their valence The general electronic configuration of these elements is ns² np³

Oxidation state: Group 15 elements have 5 valence electrons and they require 3 more electrons to complete their However, the gaining of 3 electrons is difficult

Atomic size: Atomic size increases as we move down the group due to increase in the number of

Ionisation enthalpy: Ionisation enthalpy decreases as we move down the group because of increase in atomic

Electronegativity: Electronegativity decreases on moving down the group due to increase in atomic