General trends in oxidation number
Point your cursor over the main group elements to see their common oxidation number. Across a period, in general, the common oxidation numbers are MORE POSITIVE. From +1 for Group 1 an +2 to Group 2, , the maximum positive oxidation number increases steadily to +7 from Group 7 across the main group elements. The maximum positive oxidation state for any main group element is equal to the group number, because an atom can lose up to however many electrons in its outermost principal energy level (equal to the group number). The maximum negative oxidation state for the more electronegative elements (group 4A to group 7A) is the group number - 8 (corresopnding to how an atom will gain however many electrons it takes to gain a full octet). Consequently, elements towards the right with more valence electrons have higher maximum oxidation states, and have more variable oxidation states. Down a group (for non-metal groups), MORE POSITIVE oxidation numbers tend to be available. For these elements, electrons may be lost from lower principal energy levels. One characteristic of the transition elements is that they have 'variable valency'; they have multiple common oxidation states. In higher oxidation states, they commonly form covalently bonded compounds with oxygen. In lower oxidation states, they lose one, two or three electrons to form atomic ions. Common oxidation numbers for groups beginning with these elements are as follows:
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