Here's why:
* Astatine (At) is a halogen, meaning it exists in Group 17 of the periodic table. Halogens typically form -1 anions (negatively charged ions).
* Hydrogen (H) is in Group 1 and typically forms +1 cations (positively charged ions).
To form a neutral compound, the charges need to balance. Therefore, one hydrogen atom (+1 charge) will bond with one astatine atom (-1 charge) to create the compound HAt.
