Here are some more rules:
Rule 8: The sum of all the ONs must add up to the overall charge on the particle (The overall charge on all the above particles is zero).
Rule 9: The ONs of the alkaline earth metals (Group 2) are always +2.
Rule 10: The ON of #"Al"# is usually +3.
#bb"Na"_3bb"N"#
The ON of #"Na"# is #"+1"# (Rule 1):
#stackrelcolor(blue)(+1)"Na"_3"N"#
The total ON for 3 #"Na"# atoms is #"+3"#:
#underbrace(stackrelcolor(blue)(+1)("Na"_3))_color(red)(+3)"N"#
∴ The ON of #"N"# must be #"-3"# (Rule 8):
#underbrace(stackrelcolor(blue)(+1)("Na"_3))_color(red)(+3)underbrace(stackrelcolor(blue)("-3")("N"))_color(red)("-3")#
#bb"NH"_4bb"Cl"#
The ON of #"H"# is #"+1"# (Rule 4). The ON of #"Cl"# is #"-1"# (Rule 3):
#"N"stackrelcolor(blue)(+1)("H"_4)stackrelcolor(blue)("-1")("Cl")#
The total ON for #"4H + 1Cl" = "4(+1) + (-1)" = "+3"#:
#"N"underbrace(stackrelcolor(blue)(+1)("H"_4))_color(red)("+4")underbrace(stackrelcolor(blue)("-1")("Cl"))_color(red)("-1")#
∴ The ON of #"N = -3"#:
#underbrace(stackrelcolor(blue)("-3")("N"))_color(red)("-3")underbrace(stackrelcolor(blue)(+1)("H"_4))_color(red)("+4")underbrace(stackrelcolor(blue)("-1")("Cl"))_color(red)("-1")#
#bb"MgSO"_3#
The ON of #"Mg"# is #"+2"# (Rule 9). The ON of #"O"# is #"-2"# (Rule 4):
#stackrelcolor(blue)("+2")("Mg")"S"stackrelcolor(blue)("-2")("O"_3)#
The total ON for #"1Mg + 3O" = "+2 + 3(-2)" = "-4"#:
#underbrace(stackrelcolor(blue)("+2")("Mg"))_color(red)("+2")"S"underbrace(stackrelcolor(blue)("-2")("O"_3))_color(red)("-6")#
∴ The ON of #"S"# is #"+4"#:
#underbrace(stackrelcolor(blue)("+2")("Mg"))_color(red)("+2")underbrace(stackrelcolor(blue)("+4")("S"))_color(red)("+4")underbrace(stackrelcolor(blue)("-2")("O"_3))_color(red)("-6")#
#bb"Al"(bb"NO"_2)_3#
The ON of #"Al"# is #"+3"# (Rule 10). The ON of #"O"# is #"-2"# (Rule 4):
#stackrelcolor(blue)("+3")("Al")("N"stackrelcolor(blue)("-2")("O")_2)_3#
The total ON for #"1Al + 6O" = "+3 + 6(-2)" = "-9"#:
#underbrace(stackrelcolor(blue)("+3")("Al"))_color(red)("+3")("N"underbrace(stackrelcolor(blue)("-2")("O")_2)_color(red)("-4"))_3#
Thus the total ON for the 3 #"N"# atoms is #"+9"#.
The ON for 1 #"N"# atom is #"+3"#:
#underbrace(stackrelcolor(blue)("+3")("Al"))_color(red)("+3")(underbrace((underbrace(stackrelcolor(blue)("+3")("N"))_color(red)("+3")underbrace(stackrelcolor(blue)("-2")("O")_2)_color(red)("-4")))_color(red)("-1"))_3#
#"MgSO"_3# has the atom with the highest ON (#"S = +4"#).
"N"#
