American Chemical Society (ACS) Chemistry Test 2025 – 400 Free Practice Questions to Pass the Exam

The distinction between a sodium ion and a sodium atom primarily lies in their electron counts. A sodium atom, which is neutral, contains 11 protons and 11 electrons. When a sodium atom loses an electron to form a sodium ion (specifically, Na+), it has 11 protons and only 10 electrons. This loss of one electron results in a positively charged ion, which is what defines the sodium ion.

The correct answer illustrates that the sodium ion has fewer electrons compared to the sodium atom. This change in electron count affects the atom's charge and its chemical behavior. The sodium ion, now positively charged, tends to participate in different interactions compared to its neutral parent atom.

The incorrect options highlight misunderstandings regarding the fundamental makeup of atoms and ions. The sodium ion does not have more electrons or more protons than the sodium atom, nor does it have fewer neutrons since the number of neutrons in the sodium nucleus remains unchanged in the transformation from atom to ion.