second ionization energy of nitrogen is less than second ionization energy of oxygen why?
Answers
Answer:
Well, I would use NIST to check these. I looked up the first two ionization energies (by typing the atomic symbol) and got...
IE
1
(
N
)
=
14.534 eV
IE
1
(
O
)
=
13.618 eV
IE
2
(
N
)
=
29.601 eV
IE
2
(
O
)
=
35.121 eV
or...
IE
1
(
N
)
≈
1402 kJ/mol
IE
1
(
O
)
≈
1314 kJ/mol
IE
2
(
N
)
≈
2856 kJ/mol
IE
2
(
O
)
≈
3389 kJ/mol
Here, you can see that the first ionization energy of oxygen atom is lower. Consider the electron configurations.
N
:
[
H
e
]
2
s
2
2
p
3
↑
↓
−−−−−
↑
↓
−−−−−
↑
↓
−−−−−
.
2
p
↑
↓
−−−−−
.
2
s
O
:
[
H
e
]
2
s
2
2
p
4
↑
↓
−−−−−
↑
↓
−−−−−
↑
↓
−−−−−
.
2
p
↑
↓
−−−−−
.
2
s
Since oxygen has a paired
2
p
electron, that one will repel the others, making it easier to remove than any of the others.
Easier to remove
→
lower ionization energy.
The second ionization energy of oxygen is higher than for nitrogen. In principle, they should be the same for two seemingly identical
2
p
electrons, BUT we would neglect an important factor...
It is because oxygen atom is smaller due to a higher effective nuclear charge
Z
e
f
f
=
Z
−
S
, where
S
is approximated to be the number of core electrons and
Z
is the atomic number.
Z
e
f
f
,
N
≈
7 protons
−
2 core e
−
≈
5
Z
e
f
f
,
O
≈
8 protons
−
2 core e
−
≈
6
So, it is more difficult to remove the electron from the smaller atom, whose electrons are more tightly bound to the nucleus.
Harder to remove
→
higher ionization energy.
Easier to remove → lower ionization energy. The second ionization energy of oxygen is higher than for nitrogen. It is because oxygen atom is smaller due to a higher effective nuclear charge Zeff=Z−S , where S is approximated to be the number of core electrons and Z is the atomic number