Final answer:
Bond type can be determined by the atoms' positions on the periodic table and their electronegativity differences; two nonmetals usually form a covalent bond while a metal and a nonmetal form an ionic bond.
Step-by-step explanation:
The type of bonding expected between atoms can usually be predicted by considering the types of atoms involved and their positions on the periodic table. Generally, bonds between two nonmetals are covalent, where electrons are shared, and bonds between a metal and a nonmetal are often ionic, involving the transfer of electrons from one atom to another.
If two atoms bonded together have an electronegativity difference of 1.9, the bond type is typically polar covalent. However, it is worth noting that this is on the border with ionic character, generally considered to be when the difference is 2.0 or greater. In the case of BH₂, because boron and hydrogen are both nonmetals, they would likely share electrons, forming a covalent bond.
For the LiF compound, the bond is ionic, not covalent. This is because lithium (Li) is a metal and fluorine (F) is a nonmetal, and they have a significant difference in electronegativity, which leads to the transfer of an electron from lithium to fluorine, forming Li+ and F- ions.