Final answer:
When an element becomes an ion, its size can either increase or decrease depending on whether it gains or loses electrons. Aluminum would decrease in size when it becomes an ion.
Step-by-step explanation:
When an element becomes an ion, its size can either increase or decrease depending on whether it gains or loses electrons. In general, when an element gains electrons to become a negatively charged ion (an anion), it will increase in size. Conversely, when an element loses electrons to become a positively charged ion (a cation), it will decrease in size.
Out of the options provided, the element that would decrease in size when it becomes an ion is J Aluminum. Aluminum tends to lose three electrons to form an ion with a 3+ charge, resulting in a smaller size compared to its neutral atom.
Learn more about Effects of ion formation on atomic size