Cambridge Secondary (7-9). Stage 9. Science. Learner's Book - Unit 2: Properties of materials - Question 18

Data: lithium (Li): atomic number 3, mass number 7, melting point 180 °C, boiling point 1360 °C; sodium (Na): 11, 23, 98 °C, 900 °C; potassium (K): 19, 39, 63 °C, 777 °C.

How many more electrons than lithium does sodium have?