Final answer:
The largest atom among the given elements and ions is potassium (K), due to its position in the fourth period and being farthest to the left on the periodic table, indicating it has the largest atomic radius.
Step-by-step explanation:
The largest atom among Si4-, Al3+, Mg2+, Na, Ne, Ar, Ca2+, K, N3-, O2-, P3- is potassium (K). When considering atomic radius, anions (negative ions) are larger than their parent atoms because they gain electrons, which increases electron-electron repulsion and overall size.
Cations (positive ions) are smaller than their parent atoms because they lose electrons, which decreases repulsion and size. Noble gases are typically not involved in ionic size comparisons as they are neutrally charged and do not form ions readily.
Looking at the periodic table, as we move down a group, atomic size increases due to the addition of electron shells. Moving left to right across a period, atomic size typically decreases as increasing nuclear charge pulls electrons closer. Since potassium (K) is located in the fourth period and is the farthest to the left among the listed elements, it is the largest atom.