Cambridge Secondary (7-9). Stage 9. MCE Science. Workbook - Unit 4: The Structure of the Atom and the Periodic Table - Question 14

| Element | Symbol | Atomic number | Mass number | Density (g/cm³) | Melting point (°C) | Boiling point (°C) |
|---|---|---:|---:|---:|---:|---:|
| lithium | Li | 3 | 7 | 0.53 | 180 | 1342 |
| sodium | Na | 11 | 23 | 0.97 | 98 | 883 |
| potassium | K | 19 | 39 | 0.89 | 63 | 759 |

Look at the table. Explain why all these elements are placed in the same group of the Periodic Table.