Sodium and fluorine are very reactive because they have low ionization energies and high electronegativities, respectively. This means that sodium atoms can easily lose their outermost electron to form a positively charged ion (Na+), while fluorine atoms can easily accept an electron to form a negatively charged ion (F-). These ions are then attracted to each other to form a stable compound, such as sodium fluoride (NaF).

In contrast, neon has a high ionization energy and a low electronegativity, which means that it is difficult to remove an electron from a neon atom and difficult for a neon atom to accept an electron. This makes neon very unreactive, and it does not form stable compounds with other elements.

Here is a table summarizing the key differences between sodium, fluorine, and neon:

| Element | Ionization Energy (eV) | Electronegativity | Reactivity |

|---|---|---|---|

| Sodium | 5.14 | 0.93 | Very reactive |

| Fluorine | 17.42 | 3.98 | Very reactive |

| Neon | 21.56 | 0.0 | Almost no reactivity |