Elements possess multiple ionisation energies due to the presence of several electrons, each requiring varying amounts of energy to be removed.

Ionisation energy is defined as the energy necessary to detach an electron from an atom or ion. Each element is characterized by a unique electron configuration, where electrons inhabit distinct energy levels or shells. The first ionisation energy refers to the energy needed to remove the first electron, which is typically the outermost. This electron is generally the easiest to remove because it is located farthest from the nucleus, thereby experiencing the least amount of nuclear attraction.

Once the first electron is removed, the atom transforms into a positive ion. This alteration results in the remaining electrons being held more tightly by the nucleus due to the increased positive charge. Consequently, the second ionisation energy, which is the energy required to remove the next electron, is always greater than the first. This pattern continues for additional electrons, with each successive ionisation energy being higher than the previous one.

The rise in ionisation energy also reflects the underlying electron configuration of the atom. For instance, electrons residing in the same shell exhibit similar ionisation energies because they are approximately equidistant from the nucleus. However, when an electron is extracted from a shell that is closer to the nucleus, there is a marked increase in ionisation energy due to the stronger nuclear attraction experienced by that electron.

Moreover, ionisation energies offer valuable insights into an element’s position in the periodic table. Elements within the same group (vertical column) share similar electron configurations and, as a result, display analogous trends in ionisation energies. In contrast, across a period (horizontal row), ionisation energies typically increase due to the rising nuclear charge, which occurs without a corresponding increase in the shielding effect.

In conclusion, elements exhibit multiple ionisation energies because they contain multiple electrons, each situated in different energy levels and subjected to varying degrees of nuclear attraction. These ionisation energies are instrumental in elucidating the element’s electron configuration and its location in the periodic table.