Please read the following table in Markdown format and then answer the question according to the table. Table cells in one row are seperated by '|', and different rows are seperated by '
'.

Table:
| Lithium | Sodium | Potassium | Rubidium | Caesium |
| --- | --- | --- | --- | --- |
| Lithium | Sodium | Potassium | Rubidium | Caesium |
| H a l o g e n s | Fluorine | LiF (3.0) | NaF (3.1) | KF (3.2) |
| H a l o g e n s | Chlorine | LiCl (2.0) | NaCl (2.1) | KCl (2.2) |
| H a l o g e n s | Bromine | LiBr (1.8) | NaBr (1.9) | KBr (2.0) |
| H a l o g e n s | Iodine | LiI (1.5) | NaI (1.6) | KI (1.7) |
Question:
what is the properties of sodium when rubidium is nacl (2.1)?
Answer:
Chlorine