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- Concepts:
- Ionization, ionization energy, shell, energy level, valence shell,
valence electron
- Skills:
- Account for the existence of energy levels using evidence from
successive ionization energies
- Use successive ionization energies for an element to determine the
group it belongs to and given other relevant data, identify the element
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- We do have confirmation for the idea of energy states in other atoms as
well, namely from ionization energy data.
- Ionization is a special case of excitation.
- It involves the complete removal of the electron from the valence shell
of a ground state atom (or molecule or ion) in the gaseous state.
- Ionization of hydrogen can be represented thus
- In hydrogen, the electron removed comes from the first shell and is
removed to such a distance that the nucleus has no influence over it.
- In other words, it involves electronic transition from n = 1 to the
level of zero potential, i.e. n = ¥.
- Similarly in the case of a couple of other elements,
- Na(g) ¾¾¾® Na+(g) +
1e-(g) n = 3 ® n = ¥
- K(g)
¾¾¾® K+(g) +
1e-(g)
n = 4 ® n = ¥
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- The process requires energy either electrical or light.
- Energy associated with this process is termed enthalpy of ionization (DHqIE) or simply ionization energy.
- X(g) ¾¾® X+(g) +
1e-(g) * DHqIE
= positive
- XY(g) ¾¾® XY+(g) +
1e-(g)
- X+(g) ¾¾® X2+(g) +
1e-(g)
- When successive electrons are removed, the stages and process of
formation of each successive ion is distinguished by calling them 1st,
2nd, 3rd ionization.
- For example
- Na(g) ¾¾® Na+(g) +
1e-(g) IE = 1st ionization
energy
- Na+ (g) ¾¾® Na2+(g) +
1e-(g) IE = 2nd ionization
energy
- Na2+ (g)
¾¾® Na3+(g) +
1e-(g) IE = 3rd ionization
energy etc.
- * Electrons removed during ionization of a gas-phase atom are also in
the gas phase. This ionization is an actual process, not a hypothetical
half-reaction, so the electron is assigned a definite state, namely “g”.
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- When the term ionization (energy) is used in isolation, 1st
ionization (energy) must be understood.
- Reverse of Ionization:
- X+(g)
+ 1e-(g) ® X(g) n = ¥ ®
n = 1, DHq = - DHqIE
- where, IE = ionization energy
- Electronic transition from ¥ ® 1 involves evolution of energy in the form of light energy
(as with every other relaxation process).
- Ionization energy therefore is the energy required to remove an electron
from a gaseous atom to produce a gaseous ion.
- Defined another way, it is the energy required to convert a mole of
gaseous atoms into a mole of positively charged gaseous ions.
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- When an electron is removed from an inner shell, there is a big increase
in the ionization energy, almost 10 fold increase, indicating the
removal of an electron that is closer to the nucleus.
- For sodium, the first jump is when the second electron is removed.
Magnesium the third, Si the fifth, which, relates to the number of
valence electrons in each atom and therefore the group the element
belongs to.
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- When successive ionization energies for any element is graphed, grouping
of electrons is observed, a grouping that is consistent from element to
element.
- Look at the successive ionization energies of sodium (see next couple of
slides).
- There is a big jump from the 1st to the 2nd
(almost a 10-fold increase).
- The change then is gradual until the 9th and then there is
almost a 5 fold increase from the 9th to the 10th.
- There isn’t that big a change between the 10th and the 11th.
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- Each group of 8 electrons (2 in the first) is an indication of an
individual shell, symbol n.
- Some also use the term orbit (in keeping with Bohr’s solar system model
of the atom) to refer to a shell.
- Successive ionization energy data thus confirms the existence shells.
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- A plot of the successive ionization energy of sodium shows a grouping of
1, 8, 2 (from the outermost to the innermost shell).
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- Similarly with potassium a grouping of 1, 8, 8, 2 is observed.
- A configuration of 2, 8, 8, 1.
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- Here the first big jump is from 4th to 5th
indicating that there are 4 electrons in the outermost shell.
- The electronic configuration therefore is 2, 8, 4.
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- 1. 90/3. The first three ionization energies for two elements, X and Y,
are
- Ionization Energies (kJ mol-1)
- First Second Third
- X 520 7,300 11,800
- Y 1,086 2,350 4,620
- Which pair of elements represents X and Y?
- A. 3Li and 6C
B. 4Be and 8O C. 2He and 4Be D. 8O and 16S
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- 1. Consider the following successive ionization energies (kJ/mol) of
element X.
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- 2. The following table shows different ionization energies for some
elements.
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- b) State what group each element belongs to. Explain your choice of
group assignment for each element using only the data given above. [4]
- c) Why doesn’t W have a 5th ionization energy associated with it? [1]
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- 4. N01/5. (b) (i) Describe how the first four ionisation energies of
aluminium vary. (You may wish to sketch a graph to illustrate your
answer.) [2]
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- 5. a) (i) Describe how the first 15 ionization energies of phosphorus
vary. (You may wish to sketch a graph to illustrate your answer.) [3]
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- (ii) Explain how the graph provides evidence for the arrangement of
electrons in different energy levels.
- b) State what you understand by ionization energy of phosphorus. Give
an equation to illustrate your answer. [3]
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Notes
