Question

What is the formula of the compound formed by Ba and F?

Answer 1

The formula unit is `"BaF"_2"`.

Explanation:

`"Ba"` is the chemical symbol for the metal barium, and `"F"` is the chemical symbol for the nonmetal fluorine. The name of this compound is barium fluoride. Usually metals and nonmetals form ionic compounds when chemically combined.

Since this is an ionic compound, its chemical formula is called a formula unit because there are no discrete molecules. A formula unit represents the lowest whole number ratio of elements in the compound.

Barium is a group 2 element. Group 2 elements form cations with a `2^+` charge. The formula for the barium ion is `"Ba"^(2+)"`. Fluorine is a group 17 element. Group 17 elements form anions with a `1^-` charge. The formula for the fluoride ion is `"F"^(-)"`.

Ionic compounds are neutral, so the charges of the positive and negative ions must equal zero when added. So for every `"Ba"^(2+)"` ion, two `"F"^(-)"` ions are needed. So the formula unit for barium and fluorine compound is `"BaF"_2"`.

`"Ba" + "2F"` `rarr` `["Ba"]^(2+) + 2["F"]^(-)` `rarr` `"BaF"_2"`

You can also use the "crisscross method" to determine the formula for an ionic compound as shown below.