| element | ionisation enthalpy/ kJ mol–1 |
| Na | 494 |
| F | 1680 |
| Cu | 745 |
The ionisation energies given show that there is considerable variation in the magnitude of this quantity.
How can this be explained in terms of our model for electron arrangement in the atom?
Ionisation enthalpies are lower for atoms where the outer electron is in a higher energy orbital.
How can this be explained in terms of our model for electron arrangement in the atom?
Ionisation enthalpies are lower for atoms where the outer electron is in a higher energy orbital.
Higher energy orbitals extend farther from the nucleus.
(i) the nuclear charge
adding protons lowers orbital energy
due to attractive forces between protons and outer electrons
(ii) the presence of other electrons on the atom
adding core electrons increases orbital energy
due to increased shielding of the outer electrons from the nucleus
addition of electrons in the same shell increases orbital energy
due to repulsive forces between electrons in the same region in space
For elements in the same period of the periodic table, ionisation energy increases across the row.
number of protons (nuclear charge) increases going across the row
number of core (shielding) electron shells stays the same, but there are more electrons in the outer shell going across the row
the effect of an increase in nuclear charge is greater than the repulsive forces due to additional electrons so that orbital (electron) energy decreases.
For elements in the same group of the periodic table, ionisation energy decreases going down the group.
number of protons (nuclear charge) increases
number of core (shielding) electron shells increases
number of outer electrons is the same
the repulsive forces due to the additional electron shells outweigh the attractive forces due to an increase in nuclear charge so that orbital (electron) energy increases.