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Syllabus
Syllabus
What does this mean?
What does this mean?
Ionisation Energy
Ionisation Energy
You’ll remember the definition of first Ionisation Energy from Topic 3 (!).
“First Ionisation Energy (1st IE) is the energy _________________________________________________________________ ______________________________________________________________________________________________________________”
Write an equation for the First Ionisation Energy of Chlorine
Write an equation for the Second Ionisation Energy of Chlorine
QUESTION
Why is the Second Ionisation Energy of every element always larger than 1st IE? Why is 3rd IE always larger than 2nd IE? etc
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The Convergence Limit
The Convergence Limit
You’ll also remember all about the Convergence Limit from Topic 2 (!)
Energy levels (shells) become closer together as we move away from the nucleus.
Eventually they converge forming a continuum. At this point no extra energy is needed to promote an electron. So the electron has left the atom and the atom has been ionised.
We call the principal quantum number of the convergence limit n= ∞
In the case of Hydrogen or Helium that only have electrons in n=1, the energy needed to promote an electron from n=1 to n = ∞ is the ionisation energy.
QUESTIONS
1. Write the equation that links frequency, wavelength and the speed of light.
2. Write the equation that links energy, Planck’s constant and frequency.
3. Combine these two equations in the form E =
Using the equations
The emissions spectrum of Helium has its lowest wavelength at around 390 nm. But this is only the lowest wavelength visible line. The lowest wavelength of em radiation is around 50 nm.
Energy = hc/ λ
= 6.626 x 10-34 x 2.998 x 108 / 50 x 10-9
= 3.94 x 10 -18 J
=3.94 x 10 -21 kJ
This is for only one atom, to convert to kJ/mol we need to multiply by Avogadro’s Number
Energy = 3.94 x 10 -21 kJ x 6.022 x 10 23 mol-1 = 2370 kJ/mol
QUESTION
1. Calculate the 1st Ionisation Energy of Hydrogen if the shortest wavelength of its emission spectrum is 91 nm
Successive Ionisation Energy
Successive Ionisation Energy
Each of the 3 elements shows a gradual rise in IE (as the atom/ion gets more positive) and then a sudden jump.
It is these sudden jumps that provide evidence of shells.
An inner shell has its electron closer to the nucleus, and less shielded buy inner shells.
So the attraction is greater and the Ionisation energy greater too.
Oxygen has a jump after 6 similar IEs – suggesting it has 6 electrons in its outer shell – it is in Group 6.#
Silicon jumps after 4 similar IEs– it is in Group 4 etc
Because the IEs increase so much they are often plotted on Logarithmic scales to reduce the y-axis.
QUESTION
Which groups must these elements be in?
Trends in 1st IE
Trends in 1st IE
QUESTIONS
1. What evidence is there that the first shell takes a maximum of 2 electrons and the second shell a maximum of 8 electrons?
2. What evidence is there that s orbitals contain only 2 electrons?
3. What evidence is there that electrons pair in orbitals?
4. Why is the general trend always upwards?
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