Argon (Ar) and potassium (K) have different atomic numbers and mass numbers because they belong to different elements with distinct numbers of protons and neutrons.
Atomic number: The atomic number of an element represents the number of protons in the nucleus of its atoms. It determines the element's identity. Argon has an atomic number of 18, indicating that each argon atom contains 18 protons in its nucleus. Potassium, on the other hand, has an atomic number of 19, meaning that each potassium atom has 19 protons in its nucleus. Since protons are responsible for the element's identity, argon has a lower atomic number than potassium.
Mass number: The mass number of an atom is the sum of its protons and neutrons. Protons have a mass of approximately 1 atomic mass unit (amu), while neutrons have a slightly greater mass, typically around 1 amu. Argon's mass number is 40, which means it has 18 protons and 22 neutrons (40 - 18 = 22). Potassium, on the other hand, has a mass number of 39, indicating that it has 19 protons and 20 neutrons (39 - 19 = 20). Therefore, argon has a greater mass number than potassium.
In summary, while argon has a lower atomic number than potassium, it has a greater mass number because it has more neutrons in its nucleus. The atomic number determines the element's identity, while the mass number represents the total number of protons and neutrons in an atom.