What is the percent composition of chromium in crcl3?

The oxidation state of Chromium chloride (III) is +3

How can we find the answer?

First of all write down what you know about the molecule:

1: The molecule hasn't got an electric charge (is not an ione), this means that either positive and negative charges of its atoms are balanced (we have the same number of positve and negative charges)

2: Since it's a salt, where the metal is chromium and the non metal is the alogen Chlorine, we know that the negative charge belogns to the non metal element because of its elettronegativity, therefore the positive charge belongs to the metale element (chromium).

3: when chlorin forms binary salts its oxidation state is always -1 (you can find out this info in a periodic table)

In CrCl3 we have 3 chlorine atoms where each of them carrys 1 negative charge, so the total amount of negative charges is -3

Since the charges are balanced, the question is: Which is the positive charge that Chromium must carry in order to balance 3 negative charges?

The answer comes out to +3

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.