Ionization energy of atoms is a vital concept to navigate for the cape unit 1 course.

1. CAPE UNIT 1

2. IONIZATION ENERGY
• Ionization Energies
• Ionization is the process by which an electron is removed from an atom or a molecule.The process
is endothermic, because energy is required to break the force of attraction between the electron
and the central positive nucleus.
• Ionization Energy
• The first ionization energy is the energy needed to remove 1 electron from each atom of an
element in 1 mole of gaseous atoms, to form 1 mole of gaseous ions with a +1 charge.

3. Successive Ionization Energy
•
• The second ionization energy is the energy needed to remove 1 electron from each ion of
an element in 1 mole of gaseous +1 ions to form 1 mole of gaseous ions with a +2 charge.
•
• The successive ionization energy is the energy each time you remove an electron.When
talking about successive ionization energies, we often draw a graph for a particular
element of the first ionization energy, second ionization energy, third ionization energy,
and so on.
•
• Gas state symbol (g) is used because gaseous atoms are used when ionization energies
are recorded.

6. Element Symbol Atomic number
First ionisation
energy /kJ mol–1
Sodium Na 11 496
Magnesium Mg 12 738
Aluminium Al 13 578
Silicon Si 14 789
Phosphorus P 15 1012
Sulfur S 16 1000
Chlorine Cl 17 1251
Argon Ar 18 1521

8. First Ionization Energy of PeriodThree
Elements
• The graph shows how the first ionization energy varies across period 3.
• First ionization energy generally increases across period 3.
• However, the trend needs a more detailed consideration than the trend in group 2.
• This is because first ionization energies:
 decrease from magnesium to aluminum then increase again, and
 decrease from phosphorus to sulfur then increase again.
•

9. • General increase across the period
• Going across period 3: there are more protons in each nucleus so the nuclear charge increases …
• therefore, the force of attraction between the nucleus and outer electrons is increased, and …
• there is a negligible increase in shielding because each successive electron enters the same shell
…
• so, more energy is needed to remove an outer electron.
• Magnesium to aluminum
• Look at their electronic configurations:
• Magnesium: 1s2 2s2 2p6 3s2
• Aluminum: 1s2 2s2 2p6 3s2 3p1
• The outer electron in magnesium is in an s sub-shell. However, the outer electron in aluminium
is in a p sub-shell, so it is higher in energy than the outer electron in magnesium.This means
that less energy is needed to remove it.

10. • Phosphorus to sulfur
• Look at their electronic configurations:
•
• Phosphorus: 1s2 2s2 2p6 3s2 3p3
• Sulfur: 1s2 2s2 2p6 3s2 3p4
• It’s not immediately obvious what’s going on until you look at the arrangements of the electrons. The 3p
electrons in phosphorus are all unpaired. In sulfur, however, two of the 3p electrons are paired. There is
some repulsion between paired electrons in the same sub-shell, so the force of their attraction to the
nucleus is reduced. This means that less energy is needed to remove one of these paired electrons than is
needed to remove an unpaired electron from phosphorus.

12. Factors affecting Ionization Energy
• Factors affecting Ionization Energy
• 1. Size of the positive nuclear charge
• As the nuclear charge increases, its attraction for the outermost electron increases and more
energy is required to remove an electron.
• This means that the ionization energy increases.
• 2. Size of atom (distance of outermost electron from the nucleus)
• As atomic size increases, the attraction of the positive nucleus for the negative electron
decreases and less energy is required to remove an electron.
• This means that the ionization energy decreases.

13. Factors affecting Ionization Energy
• 3. Screening (shielding) effect of inner shell electrons
• The outermost electron is screened (shielded) from the attraction of the nucleus by the repelling
effect of the inner electrons.
• As shielding increases, the attraction of the positive nucleus for the negative electron decreases
and less energy is required to remove an electron.
• This means that the ionization energy decreases.

14. The size of the ionization energy is determined by the strength of the attraction between the outer shell electrons and the central
nucleus. The stronger the attraction, the harder it is to remove the electron and the higher the ionization energy.
The following factors determine the strength of the attraction between the nucleus and outer electrons:
● Atomic Radius– the higher the atomic radius, the lower the ionization energy. This is because a higher atomic radius means that the
outer electrons are further from the nucleus, and hence the attractive pull from the nucleus is lower.
● Nuclear Charge– the higher the nuclear charge, the higher the ionization energy. This is because the greater the positive charge of
the nucleus, the stronger the attraction for the outer electrons.
● Number of Inner Shells– the more inner shells present, the lower the ionization energy. By inner shells, we mean the shells
between the outer shell and the nucleus. For example, if the outer electron is in shell 4, then there are 3 inner shells to inner shells
repel the outer electrons, lessening the pull towards the nucleus. This effect is called shielding.
For example, Zn has a higher atomic radius and more inner shells than Na, so has a lower ionization energy. This is despite the fact
that Zn has more protons in its nucleus than Na, so there is a greater nuclear charge. The first two factors override the nuclear
charge.

16. Successive Ionization Energy
• Successive Ionization Energies
• Trends in Successive Ionization Energies
• Successive ionization energies means that more and more electrons are removed, each from an ion
that is becoming increasingly positive. We can look at successive ionization energies to understand
shell structure:
 Successive ionization energies increase between shells. Each time a new shell is broken into, there
is a sudden rise in ionization energy. For example, for sodium, which has one outer electron, there is a
big jump in ionization from the first to second. This is because the second electron is being removed
from a shell that is much closer to the nucleus, meaning there is stronger attraction from the nucleus.

17. • Successive ionization energies increase within each shell. Each
time an electron is removed, even if you haven’t broken into a
new inner shell, the ionization energy increases because you are
removing an electron from an increasingly positive ion. For
example, for magnesium, which has two outer electrons, the
second ionization energy is greater than the first, because it is
harder to remove an electron from an Mg2+ ion than an Mg+
atom, because there is a stronger attraction back.

19. Evidence for Shell Structure
• Every time there is a sudden jump in ionization energy, we can tell that
another shell has been entered. So for the diagram below, we can see that
there is 1 electron in the outer shell, and then 8 electrons in the next inner
shell, and so on.
• You can also find out which group of the periodic table the element belongs
to by counting how many electrons are removed before the first sudden rise
in ionization energy. For this diagram, we can tell it is a Group 1 element,
as it has 1 outer electron.

21. Accordingtothisgraphforsodium,thereis1electronremovedbeforethefirstsuddenrise,then8beforethenextriseandfinally1
electronsotheelectronicstructureforsodiumis: