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Potassium has an atomic number of 19, which means it has 19 electrons. According to Bohr's theory, electrons are arranged in discrete energy levels around the nucleus. The electron configuration for potassium can be written as K(2, 8, 8, 1) to represent the distribution of electrons in energy levels (shells).

Bohr placed the last (19th) electron of potassium in the fourth energy level (shell). This arrangement is represented by the electron configuration K(2, 8, 8, 1).

Bohr justified the placement of potassium's last electron in the fourth energy level based on his theory of electrons occupying discrete energy levels with increasing energy. The first energy level (closest to the nucleus) can hold a maximum of 2 electrons, the second can hold up to 8 electrons, and the third can hold up to 18 electrons. However, the third energy level is considered "stable" when it holds 8 electrons, which is the case for elements in the first three periods of the periodic table. In the case of potassium, the first three energy levels are filled to their "stable" capacities: 2 electrons in the first level, 8 electrons in the second, and 8 electrons in the third. This leaves the 19th electron with no room in these lower energy levels. Therefore, according to Bohr's theory, the electron occupies the next available energy level, which is the fourth energy level. Bohr's justification is in line with the observed chemical properties and reactivities of potassium, as it behaves similarly to other elements with one electron in their outermost shells, like sodium and lithium.