Atomic Size - Our Ladys High School

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Transcript Atomic Size - Our Ladys High School

The Trends in Elements in 1-20
Atomic Size
Ionisation Energies
Element
Atomic Number
Covalent
Atomic Radius
Size
Hydrogen
1
37
Helium
2
--
Lithium
3
134
Beryllium
4
129
Boron
5
90
Carbon
6
77
Nitrogen
7
75
Oxygen
8
73
Fluorine
9
71
Atomic Number versus Atomic Size
250
A
t
o
m
i
c
S
i
z
e
K
200
Na
150
Li
100
Cl
F
50
0
0
5
10
15
Atomic Number
20
25
The graph shows that
1. As we go across a period the atomic size decreases
2. As we go down a group the atomic size increases
We can explain the pattern or trend shown in this graph
by considering 2 areas
1. Number of Electron Shells (energy levels)
(The electron arrangement)
Always consider this first
2. The nuclear charge of the atom
This is given by the atomic number
Nuclear
Charge
Electron
Arrangement
Atomic Size
(Covalent radius)
Hydrogen
1+
1
37
Helium
2+
2
--
Lithium
3+
2,1
134
Beryllium
4+
2,2
129
Boron
5+
2,3
90
Carbon
6+
2,4
77
Nitrogen
7+
2,5
75
Oxygen
8+
2,6
73
Fluorine
9+
2,7
71
10 +
2,8
--
Neon
Explanation of properties
An increase in nuclear charge pulls the electrons closer
to the nucleus giving a smaller atomic size
Beryllium
4+
2,2
129
Boron
5+
2,3
90
An increase in the number of electron shells means an increase
in the atomic size
Hydrogen
1+
1
Lithium
3+
2,1
37
134
Group 1 elements
Lithium
3+
2 ,1
134
Sodium
11 +
2 ,8,1
154
Potassium
19 +
2 ,8,8,1
196
As we go down the group the number of electron shells
increases. The outer electrons are further from the nucleus,
This gives a larger atom
The extra shell of electrons also shields the outer electrons
from the attraction of the positive nucleus
This gives a larger atomic size
If you are asked to compare the size of ions remember
that the electron arrangement of an ion is not the same as the atom
Why is the ionic radius of P3- so much greater than that of Si4+
P
15+
2)8)5
but
P3- 15+ 2)8)8
The negative ion has 3 more electrons, giving a full shell
Si
14+ 2)8)4
but
Si4+ 14+
2)8
The positive ion has 4 less electrons, one less shell
The difference in size is caused by the extra shell of the negative ion
Ionisation Energies
E (g)
E+ (g)
+
e-
Ionisation energies can be explained by considering the
same points
1. Number of Electron Shells (energy levels)
(List the electron arrangement ; This will
change for the ions)
2. The nuclear charge of the atom
(This does not change when it becomes an ion)
Element
Nuclear
Charge
Electron
Arrangement
First
I.E.(kJ mol-1)
Lithium
3+
2,1
526
Beryllium
4+
2,2
905
Boron
5+
2,3
807
Carbon
6+
2,4
1090
Nitrogen
7+
2,5
1410
The trend is :
As we go across a period the first ionisation energy increases
Explanation of properties
An increase in nuclear charge pulls the electrons closer
to the nucleus giving a smaller atomic size
Lithium
3+
2,1
526 kJ
Beryllium
4+
2,2
905 kJ
The outer electrons are therefore more stongly held.
This means more energy is needed to remove the outer electron
Explanation of properties
An increase in number of shells means the outer electrons are
further from the nucleus and shielded by the extra shell
Lithium
3+
2,1
526 kJ
Sodium
11 +
2 , 8, 1
502 kJ
The outer electrons are therefore less strongly held.
This means less energy is needed to remove the outer electron
This means a smaller ionisation energy
Second Ionisation Energies
M+ (g)
M2+ (g)
+
e-
The difference in values between first and second ionisation
energies can be explained by considering the same points
1. Number of Electron Shells (energy levels)
2. The nuclear charge of the atom
Remember ions have different electron arrangements from their atoms
Explanation of Second IE
An decrease in number of shells means the outer electrons are
closer to the nucleus and no longer shielded by the extra shell
Sodium
Na
11 +
2 , 8, 1
Sodium
Na+
11 +
2,8
502 kJ
4506 kJ
The outer electrons are therefore more strongly held.
This means more energy is needed to remove the outer electron
This means a larger ionisation energy
The large increase on going from second to third ionisation
energy is when the last electron in the shell has been removed
Magnesium
Mg
12 +
2 , 8, 2
744 kJ
Magnesium
Mg+
12 +
2 , 8, 1
1460 kJ
Magnesium
Mg2+
12 +
2,8
7750 kJ
The outer electrons are therefore more strongly held.
This means more energy is needed to remove the outer electron
This means a larger ionisation energy