Is a freshly squeezed grapefruit a solution or a mechanical mixture?

Well it would be a mechanical mixture because you use mechanical energy when you squeeze it and the mixture is the juice and the pulp.

Neither model is able to show the reactivity of potassium.

Order of filling levels and sublevels of electrons: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²3f¹⁴5d¹⁰6p⁶for an element with 5 electrons in the 3d energy level: full configuration: 1s²2s²2p⁶3s²3p³.