the trends in elements in 1-20 atomic size ionisation energies
TRANSCRIPT
The Trends in Elements in 1-20
Atomic Size
Ionisation Energies
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
Element Atomic Number Atomic SizeCovalent Radius
0
50
100
150
200
250
0 5 10 15 20 25
Atomic Number versus Atomic Size
Li
F
Na
Cl
K
Atomic Number
Atomic Size
We can explain the pattern or trend shown in this graph
by considering 2 areas
1. Number of Electron Shells (energy levels)
2. The nuclear charge of the atom
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
(The electron arrangement)
Always consider this first
This is given by the atomic number
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
1 +
2 +
3 +
4 +
5 +
6 +
7 +
8 +
9 +
10 +
Nuclear Charge
1
2
2 , 1
2 , 2
2 , 3
2 , 4
2 , 5
2 , 6
2 , 7
2 , 8
Electron Arrangement
37
--
134
129
90
77
75
73
71
--
Atomic Size(Covalent radius)
Explanation of properties
An increase in nuclear charge pulls the electrons closer to the nucleus giving a smaller atomic size
An increase in the number of electron shells means an increasein the atomic size
Hydrogen 1 371 +
Lithium 2 , 1 1343 +
Beryllium 2 , 2 129
Boron 2 , 3 90
4 +
5 +
Group 1 elements
The extra shell of electrons also shields the outer electronsfrom the attraction of the positive nucleusThis gives a larger atomic size
Lithium 2 ,1 134
Sodium 2 ,8,1 154
Potassium 2 ,8,8,1 196
3 +
11 +
19 +
As we go down the group the number of electron shellsincreases. The outer electrons are further from the nucleus,This gives a larger atom
If you are asked to compare the size of ions rememberthat 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 P3- 15+ 2)8)8 but
The negative ion has 3 more electrons, giving a full shell
Si 14+ 2)8)4 Si4+ 14+ 2)8 but
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
Ionisation energies can be explained by considering the same points
1. Number of Electron Shells (energy levels)
2. The nuclear charge of the atom
E (g) E+ (g) + e-
(List the electron arrangement ; This willchange for the ions)
(This does not change when it becomes an ion)
Element Electron Arrangement
FirstI.E.(kJ mol-1)
NuclearCharge
Lithium 2 , 1 5263 +
Beryllium 2 , 2 9054 +
Boron 2 , 3 8075 +
Carbon 2 , 4 10906 +
Nitrogen 2 , 5 14107 +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 2 , 1 526 kJ3 +
Beryllium 2 , 2 905 kJ4 +
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 2 , 1 526 kJ3 +
Sodium 2 , 8, 1 502 kJ11 +
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
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
M+ (g) M2+ (g) + e-
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
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
Sodium 2 , 8, 1 502 kJ11 +Na
Sodium 2 , 8 4506 kJ11 +Na+
The large increase on going from second to third ionisation
energy is when the last electron in the shell has been removed
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
Magnesium 2 , 8, 2 744 kJ12 +Mg
Magnesium 2 , 8, 1 1460 kJ12 +Mg+
Magnesium 2 , 8 7750 kJ12 +Mg2+