Final answer:
The cation formed when a sodium atom loses an electron is called the sodium ion, denoted as Na+, having 11 protons and 10 electrons to result in a +1 charge.
Step-by-step explanation:
The cation formed when a sodium atom loses an electron is called the sodium ion. When a neutral sodium atom, which has 11 protons and 11 electrons, loses one electron, it becomes positively charged. The loss of an electron means the sodium atom now has one more proton than electrons, resulting in a charge of +1, which we denote as Na+. The formation of a sodium cation involves the loss of its outermost, or valence, electron, resulting in a stable octet configuration in the underlying electron shell.
Therefore, the correct answer is option 4) sodium ion.