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This question already has an answer here:

Groups say the valence number. For example Hydrogen is on group 1, has 1 electron on it's last and only level. So we can know the valence number on that are on the groups 1-2 and 13-18. But how am I supposed to know the valence number on transition metals(3-12)?

PD: You could tell me to use the oxidation number but that is completely wrong since they are not the same.

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marked as duplicate by bon, M.A.R., Wildcat, orthocresol, Geoff Hutchison Dec 4 '15 at 16:23

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