Cambridge Secondary (7-9). Stage 9. MCE Science. Student Book - Unit 4: The Structure of the Atom and the Periodic Table - Question 19

Does the atomic number and mass number increase or decrease? Context: compare lithium, sodium and potassium, whose atomic numbers are 3, 11, 19 and whose isotope-model mass numbers in the book are 7, 23, 39. In pairs, discuss your answer.