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How to predict a formula of a compound???

I have a big trouble in predicting for example zincsulphide zn+2 s-2
But in the lattice each zinc atom is bonded to 4 sulphur atoms, so which of these informations do I use to predict the foemula?/???

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Answer #1

Most atoms are usually a certain charge, though some can be either of two charges. For example the alkaline metals in the first column of the periodic table tend to be +1, then the next column tends to be +2. The second to last column OTOH tends to be -1, and the 3rd to last column tends to be -2. Those are easy.

Other metals, like the transition metals in the middle left can be +2 or +3 or maybe both, while other minerals to the right could be -2 or -3. For those it’s easiest to just look it up for each element, because it varies.

Then for an ionic compound (a compound with + and -), you just make sure the total + equals the total -. So Ca and Cl would form CaCl2, because Ca is +2 (Ca is in 2nd column) and Cl is -1 (Cl is in 2nd to last column), so you need 2 Cl’s to balance the Ca.

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