CHEMISTRY TEXTBOOK

Problem 7.1 : Elements of group 16

generally show lower values of first

ionisation enthalpy compared to the

elements of corresponding period of group

15. Why?
    Solution : Group 15 elements have extra
    stable, half filled p-orbitals with electronic
    configuration (ns^2 np^3 ). Therefore more
    amount of energy is required to remove an
    electron compared to that of the partially
    filled orbitals (ns^2 np^4 ) of group 16 elements
    of the corresponding period.

Table 7.3 : Atomic and physical properties of group 17 elements

Element numberAtomic

Atomic

mass

g/mol

Atomic

radius

(pm)

Ionic

radius

E(pm)

Ionization

enthalpy

(∆iH 1 )

kJ/mol

Electro

negativity

Electron

gain

enthalpy

kJ/mol

Density

g/cm^3

M.P.

(K)

B.P.

(K)

F 9 19.00 64 133 1680 4.0 -333 1.5 54.4 84.9

Cl 17 35.45 99 184 1256 3.2 -349 1.66 172.0 239.0

Br 35 79.90 114 196 1142 3.0 -325 3.19 265.8 332.5

I 53 126.90 133 220 1008 2.7 -296 4.94 386.6 458.2

At 85 210 - - - 2.2 - - - -

Table 7.4 : Atomic and physical properties of group 18 elements.

Element numberAtomic

Atomic

mass

g/mol

Atomic

radius

(pm)

Ionization

enthalpy

(∆iH 1 )

kJ/mol

Electron

gain

enthalpy

kJ/mol

Density

g/cm^3

M.P.

(K)

B.P.

(K)

Atmospheric

content

(% by volume)

He 2 4.00 (^120237248) 1.8 × 10 -4 - 4.2 5.24 × 10 -4
Ne 10 20.18 160 2080 116 9.0 × 10 -4 24.6 27.1 1.82 × 10 -3
Ar 18 39.95 190 1520 96 1.8 × 10 -3 83.8 87.2 0.934
Kr 36 83.80 200 1351 96 3.7 × 10 -3 115.8 119.7 1.14 × 10 -4
Xe 54 131.30 220 1170 77 5.9 × 10 -3 161.3 165.0 8.7 × 10 -6
Rn 86 222.00 - 1037 68 9.7 × 10 -3 202 211 -
Try this...

• Observe Table no 7.3 and
  explain the trend in following
  atomic properties of group 17 elements.
  i. Atomic size, ii. Ionisation enthalpy,
  iii. electronegativity, iv. electron gain
  enthalpy


• Oxygen has less negative electron gain
  enthalpy than sulfur. Why?

iii. Electronegativity : In a group (16, 17 and
18) the electronegativity decreases down the
group.

• 
  

  Oxygen has the highest electronegativity
  next to fluorine amongst all the elements.

  
  


• 
  

  Halogens have very high electronegativity.
  Fluorine is the most electronegative element
  in the periodic table.

  
  

iv. Electron gain enthalpy : In the groups 16
and 17 electron gain enthalpy becomes less
negative down the group.

However in group 16, oxygen has less
negative electron gain enthalpy than sulfur
due to its small atomic size.

• 
  

  In group 17, fluorine has less negative
  electron gain enthalpy than that of chlorine.
  This is due to small size of fluorine atom.

  
  


• 
  

  Group 18 elements (noble gases) have
  no tendency to accept electrons because of
  their stable electronic configuration (ns^2 np^6 )
  and thus have large positive electron gain
  enthalpy.