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Dot and Cross diagram help

The question i was asked was ......

'Draw a dot and cross diagrams for the following ions.

a.The Hydroxide ion OH-
b.The Carbonate ion CO3^2-
This is similar in structure to the nitrate ion but carbon has only four outer shell electrons.'

Can someone give me an answer and an explaination. This confuses me as i get basic covalent bonds between something like H2O for example. Its the minuses that confuse me

Reply 1

Kyri

The minus simply means that an extra electron has been added to one of the atoms in the ion i.e. there are more electrons than protons in the overall ion.

For OH- there is 1 more electron than there is protons. Without the minus, OH would be of course a dot and a cross between the O and the H and 5 more dots around the O (5 because 1 of its 6 is shared between the O and the H), giving 7 electrons around the O atom. According to the octet rule, atoms are most stable with 8 electrons around them, so we put an extra dot around the O to make 8 electrons overall. By doing this, there is 1 more electron than there are protons in the overall OH group so it becomes OH-.

For CO3^2-, neutral CO3 would have two dots and two crosses between one O and the central C atom (a double bond), and one dot and one cross between the other two O atoms and the C. This means there are 8 electrons around the C and the first O, which is fine, but the other two O atoms only have 7 electrons around them, so we add one extra electron to each of them, completing their octets. The two extra electrons make the overall ion CO3^2-.

Sorry I didn't draw them out out as I'm not sure how to on here, but hopefully with that info you'll be able to draw out the structures yourself, including NO3- :smile: