Potassium (K) has a higher atomic number than rubidium (Rb) and caesium (Cs) because atomic numbers increase from left to right across periods in the periodic table.
The periodic table is organized based on the atomic structure and the number of protons in an atom's nucleus. Elements are arranged in order of increasing atomic number, and each element in a specific period (horizontal row) corresponds to the number of electron shells or energy levels.
Potassium, rubidium, and caesium belong to Group 1 (or Group IA), which is the alkali metal group. This group contains elements with similar chemical properties due to having one valence electron in their outermost energy level.
The atomic numbers of these elements are as follows:
- Potassium (K): Atomic number 19
- Rubidium (Rb): Atomic number 37
- Caesium (Cs): Atomic number 55
Since atomic numbers increase from left to right across periods, potassium, being positioned on the leftmost side of Group 1, has the lowest atomic number (19). Rubidium, which follows potassium in the same group, has an atomic number of 37. Caesium, located next to rubidium, has the highest atomic number of the three with a value of 55.
Therefore, the higher atomic number of potassium compared to rubidium and caesium is a consequence of their respective positions in the periodic table, which reflect the increasing number of protons and electrons as you move from left to right across a period.