Ionisation Energy and the Periodic Table
Ionisation energy is the energy that is needed to remove one electron from an atom. The ionisation energy of one atom is different from another atoms.
From our previous topic on electronic configuration, we focused our attention on the electron conﬁguration of neutral atoms. In a neutral atom, the number of protons is the same as the number of electrons.
But what happens if an atom gains or loses electrons? Does it mean that the atom will still be part of the same element?
A change in the number of electrons of an atom will not change the identity of the atom . However, the charge of the atom will change. If electrons are added, then the atom will become more negative. If electrons are taken away, then the atom will become more positive. The atom that is formed in either of these cases is called an ion. Simply put, an ion is a charged atom.
An ion is a charged atom. A positively(+) charged ion is called a cation e.g. Na+, and a negatively(-) charged ion is called an anion e.g. F −. The charge on an ion depends on the number of electrons that have been lost or gained.
Let’s look at the following examples. Notice the number of valence electrons in the neutral atom, the number of electrons that are lost or gained, and the ﬁnal charge of the ion that is formed.
The second ionisation energy is the energy that is needed to remove a second electron from an atom, and so on. As an energy level becomes more full, it becomes more and more diﬃcult to remove an electron and the ionisation energy increases.
From the Periodic Table of the Elements, a group is a vertical column of the elements, and a period is a horizontal row.
In the periodic table, ionisation energy increases across a period, but decreases as you move down a group.
The lower the ionisation energy, the more reactive the element will be because there is a greater chance of electrons being involved in chemical reactions. We will look at this in more detail in the next section.
Ionisation energy increases as one moves from left to right across a period. As the valence electron shell moves closer to being full, it becomes more diﬃcult to remove electrons.
Ionisation energy decreases when you move down a group in the table because more energy shells are being added. The electrons that are closer to the nucleus ’shield’ the outer electrons from the attractive force of the positive nucleus. Because these electrons are not being held to the nucleus as strongly, it is easier for them to be removed and the ionisation energy decreases.