What is the net charge on an aluminium ion? 

+3

The correct answer is option 3. +3.

Here's an explanation:

1. Aluminum Atom (Al): A neutral aluminum atom has 13 electrons, arranged in its electron cloud with 2 electrons in its inner shell and 11 in its outer shell. Its electron configuration is \(1s^2 2s^2 2p^6 3s^2 3p^1\).

2. Formation of Aluminum Ion \((Al^{3+})\):  Aluminum, like many other metals, tends to lose electrons to achieve a stable electron configuration. In the case of aluminum, it loses its three outer electrons from the 3s and 3p orbitals to attain the same electron configuration as the noble gas neon \((1s^2 2s^2 2p^6)\), which is stable.

3. Loss of Electrons: When aluminum loses three electrons, it loses three negative charges. Each electron lost contributes a +1 charge, so collectively losing three electrons results in a net positive charge of +3 for the aluminum ion \((Al^{3+})\).

So, the process can be summarized as: \[ \text{Aluminum Atom (Al)} - 3\text{Electrons} \rightarrow \text{Aluminum Ion (}Al^{3+}\text{)} \]

This transformation allows aluminum to achieve a more stable configuration by forming an ion with a net positive charge of +3.