12. 1. Ionization energy
If an electron obtains enough energy it jumps beyond the highest energy level and becomes a free electron: the atom itself becomes a positive ion. This electron transition is represented as follows: n = 1 to n = (. Notation n = ( refers to the “world” outside the atom – beyond the control of the nucleus; any electron outside an atom has no potential energy (energy = 0). With hydrogen the ionization energy can be calculated from the frequency at the point in the emission spectrum where the lines in the Lyman series converge. (see extension in topic 2 notes)
(http://www.physchem.co.za/Atomic/Hydrogen%20Spectrum.htm: shows energy levels)
There are different types of ionization energies:
first ionization energy (IE1):
2. the minimum energy needed to remove one (first/highest energy level)/most loosely held electron electron from 1 mole of gaseous (or free/by itself) atoms to make a gaseous cations;
X (g) (( X+ (g) + e-
3. the more strongly the electron is attracted to the nucleus, the greater the amount of energy needed 4. ionization energy is usually measured in kilojoules per mole Kj mol-1 of atoms; 5. ionization energies are positive values (endothermic process) as energy is needed to remove an electron.
successive ionization energy:
• energy needed to remove second, third,... electrons from 1 mole of gaseous ions, e.g.
second ionization energy (IE2) X+ (g) (( X2+ (g) + e-
third ionization energy (IE3): X2+ (g) (( X3+ (g) + e-
7. the lower the energy level an electron is on, the closer it is to the nucleus, the more strongly it is attracted, the more energy is needed;
8. in the same atom, successive ionization energies increase as a result of a combination of the following factors:
9. the electron-electron repulsion decreases (this repulsion puts electrons onto higher energy levels) and this increases the attraction from the nucleus, but also because
10. the first electron is removed from an atom whilst the second and successive electrons are removed from ions with an increasing positive charge !!!!
11. nucleus has greater effect on smaller number of electrons and they are held more tightly
These ionization energies provide evidence on how an atom is structured and on the number of electrons on the energy levels.
12. 2 Successive ionization energies: evidence for the main energy levels and the number of electrons that can occupy an energy level.
All the successive ionization energies for the same element can be shown on a graph of successive ionization energies against electrons removed.
In the space below sketch the graph for sodium.
Notice that the scale for ionization energies is a logarithm scale so that the large differences between the ionization energies can be shown onto the same graph.
| | | |Observations: |Explanation: | |1 electron has a much smaller ionization energy than the rest which |This electron must be on a much higher energy level than the rest; most of the| |means it needs a lot less energy for the electrons to escape to n = (. |time it is the furthest away from the nucleus and held less strongly. | |8 electrons require a lot more energy than the 2 electrons at the |there are 8 electrons in the same lower energy level closer to the nucleus. | |higher energy levels | |...