Question

What is the chemical formula for Y and O?

I know that the answer is `Y_2O_3`, but why?

Answer 1

Well, ytrrium is a rare earth, and commonly forms `Y^(3+)` ions....(it comes from Group 3 on the Periodic Table!).

Explanation:

And of course, oxygen, a Group 16 element, commonly forms `O^(2-)` ions....

When yttrium metal, and oxygen gas combine, the simple oxide is of course neutral with respect to charge. And so we cross-multiply in the usual way: `2xxY^(3+) + 3xxO^(2-)rarrY_2O_3`...aka `"yttria"`...

(This reminds me of an old joke of a colleague of mine who worked in our lab. Anyway, we worked with a lot of rare earths, and when we ran low, he told us to grab a spade, go over to Ytterby in Sweden, and dig up some ytterbium, yttrium, terbium, and erbium, and some scandium and holmium and thulium.)

Answer 2

Because yttrium tends to form 3+ ions and oxygen forms 2- ions

Explanation:

Yttrium is a metal and oxygen a non-metal - because non-metals tend to gain and metals tend to lose electrons, metals typically react with non-metals by transferring electrons between them and forming ionic compounds ('salts').

Yttrium has the electronic configuration Kr 5s2 4d1. This means that it tends to react by losing 3 electrons and forming a 3+ ion. Oxygen forms 2- ions by gaining 2 electrons. Therefore, 2 yttrium atoms transfer 6 electrons to 3 oxygen atoms, making two yttrium 3+ ions and three oxide (2-) ions and thus the formula of yttrium oxide is `Y_2O_3`.