If a certain compound has a formula XCl₃ what is the valency of X?

The general form of a compound can be written in the form of:

[X (a) Y (b) ] ^ c

Where a and b are subscripts, and c is the superscript of the whole formula.

The relationship that we can derived here between the constants is:

(valence of X) * a + (valence of Y) * b = c

Since the formula obviously has no superscript, therefore c = 0. We also know in chemistry class that the valence of Cl is - 1, therefore:

valence of X * 1 + (-1) * 3 = 0

valence of X - 3 = 0

valence of X = 3