The correct formula for a potassium ion that has 18 electrons is K+.
To understand this, we need to look at the atomic structure of potassium. Potassium (K) has an atomic number of 19, which means it has 19 protons in its nucleus. In a neutral atom, the number of protons equals the number of electrons, so a neutral potassium atom will also have 19 electrons.
However, in this case, the potassium ion has 18 electrons. This indicates that it has lost one electron. When an atom loses an electron, it becomes positively charged. In the case of potassium, losing one electron results in a K+ ion. Therefore, with 18 electrons, the potassium ion can be represented as K+.
This process is common among alkali metals like potassium, which tend to lose one electron to achieve a stable electron configuration similar to that of noble gases.