An element X has the electronic configuration 2, 8, 7. When X reacts with sodium to form a compound, the type of bonding in the compound and the formula of the compound are respectively

Element X with configuration 2, 8, 7 has 7 valence electrons (a halogen, like chlorine) and gains one electron to achieve a stable octet, forming X⁻. Sodium loses one electron to form Na⁺. The resulting compound NaX (e.g., NaCl) is held together by ionic bonding with a 1:1 ratio of Na⁺ to X⁻.