An atom of potassium has atomic number 19 and mass number 39 - Edexcel - GCSE Chemistry Combined Science - Question 5 - 2020 - Paper 1

For the potassium ion K⁺, the number of protons remains 19 (since protons define the element), and the mass number is 39. Thus, the number of neutrons can be calculated as:

$\text{Neutrons} = \text{Mass number} - \text{Protons} = 39 - 19 = 20$

Therefore, the correct choice is B: 19 protons and 20 neutrons.

Potassium (K) and caesium (Cs) belong to the same group in the periodic table because they both have the same number of electrons in their outermost shell. Specifically, both elements have one electron in their outer shell. This similarity in the arrangement of outer electrons leads to comparable chemical properties, such as their tendency to lose one electron and form +1 ions.

Fluorine boils at -188°C due to the presence of weak intermolecular forces called van der Waals forces (or London dispersion forces). These forces arise from temporary fluctuations in electron density, leading to an induced dipole in neighboring molecules. Since fluorine molecules are small and have low mass, these van der Waals forces are relatively weak, resulting in a low boiling point as less energy is needed to overcome these forces during the phase transition from liquid to gas.