Final answer:
Phosphorus will form an ion that is larger than its neutral atom by gaining electrons and becoming an anion, while strontium will form a smaller cation upon losing electrons.
Step-by-step explanation:
The question of which atom will form an ion that is larger than its neutral atom can be answered by understanding how cations and anions form and how their sizes compare to the neutral atoms from which they are derived. When an atom gains electrons to become an anion, it gets larger due to increased electron-electron repulsion and a decrease in effective nuclear charge per electron. Based on this, phosphorus, which forms an anion by gaining electrons, will form an ion that is larger than its neutral atom. Conversely, when an atom loses electrons to form a cation, such as strontium, it becomes smaller because of a reduced electron-electron repulsion and an increased effective nuclear charge felt by the remaining electrons.