Write the empirical formula of at least four binary ionic compounds that could be formed from the following ions:
a. Sodium (Na^+) and Chloride (Cl⁻)
b. Potassium (K^+) and Bromide (Br⁻)
c. Magnesium (Mg²+) and Fluoride (F⁻)
d. Calcium (Ca²+) and Oxide (O²-)