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ALEVEL chemistry chemical formulae

hi, i have just started alevels and i have been given some homework. there are about 40 questions and i have been able to do 33 of them however i am a bit stuck on 2 questions. it says:

write the chemical formula for these covalent compounds:
nitrogen fluoride
silicon oxide

i know how to work out the chemical formulas for ionic compounds as the charges would be + and -
here however, the charges are just -
there were other questions about covalent molecules such as writing the formula for dinitrogen trioxide but i was able to do that as it says in the name, so i put N203

how would i write nitrogen fluoride and silicon oxide? would it simply be NF and SiO?
i have tried researching it but no real answers come up.

thank you for the help :smile:
Nitrogen makes 3 bonds (needs 3 more electrons for a full shell) so bonds with 3 fluorides (which only makes one bond)

Silicon needs 4 electrons for a full shell and since oxygen makes a double bond, silicon bonds with two oxygen atoms.
Original post by PixiePresents
Nitrogen makes 3 bonds (needs 3 more electrons for a full shell) so bonds with 3 fluorides (which only makes one bond)

Silicon needs 4 electrons for a full shell and since oxygen makes a double bond, silicon bonds with two oxygen atoms.

Couldn’t have put it better myself.

@harlz_chalamet how familiar are you with the octet rule from GCSE? If you aren’t aware of it, I’d quickly read up on it.
Original post by harlz_chalamet
hi, i have just started alevels and i have been given some homework. there are about 40 questions and i have been able to do 33 of them however i am a bit stuck on 2 questions. it says:

write the chemical formula for these covalent compounds:
nitrogen fluoride
silicon oxide

i know how to work out the chemical formulas for ionic compounds as the charges would be + and -
here however, the charges are just -
there were other questions about covalent molecules such as writing the formula for dinitrogen trioxide but i was able to do that as it says in the name, so i put N203

how would i write nitrogen fluoride and silicon oxide? would it simply be NF and SiO?
i have tried researching it but no real answers come up.

thank you for the help :smile:

Above is all correct.

Drawing a line diagram definitely helps at first:smile:
Original post by PixiePresents
Nitrogen makes 3 bonds (needs 3 more electrons for a full shell) so bonds with 3 fluorides (which only makes one bond)

Silicon needs 4 electrons for a full shell and since oxygen makes a double bond, silicon bonds with two oxygen atoms.


i'm still a bit confused as i've actually only been taught ionic bonding at GCSE level and my first lessons of alevels.

for example, when you have potassium chloride, potassium has a charge of 1+ and chlorine has a charge of 1-. together, they make an overall charge of 0 so the formula would be KCl.

however, nitrogen has a charge of 3- and fluorine has a charge of 1-. together, they don't make a charge of 0 ?

i don't think i have learned this concept with non-metals before

Original post by TypicalNerd
Couldn’t have put it better myself.

@harlz_chalamet how familiar are you with the octet rule from GCSE? If you aren’t aware of it, I’d quickly read up on it.


hi, i am very much aware of the rule and i understand it.
i am just confused on how to write chemical formulas for non-metal molecules.
i understand completely how to write ionic compound formulas
Original post by harlz_chalamet
i'm still a bit confused as i've actually only been taught ionic bonding at GCSE level and my first lessons of alevels.

for example, when you have potassium chloride, potassium has a charge of 1+ and chlorine has a charge of 1-. together, they make an overall charge of 0 so the formula would be KCl.

however, nitrogen has a charge of 3- and fluorine has a charge of 1-. together, they don't make a charge of 0 ?

i don't think i have learned this concept with non-metals before



hi, i am very much aware of the rule and i understand it.
i am just confused on how to write chemical formulas for non-metal molecules.
i understand completely how to write ionic compound formulas

With covalent compounds, you use the groups of the elements to work out how many electrons they need to get an octet.

Group 4 elements need 4 bonds. This can be done 4 ways: 4 single bonds; 2 single bonds and 1 double bond; 2 double bonds; 1 triple bond and 1 single bond.

Group 5 elements need 3 bonds. This can be done 3 ways: 3 single bonds; 1 single bond and 1 double bond; 1 triple bond.

Group 6 elements need 2 bonds. Either 2 single bonds or 1 double bond does the trick.

Group 7 elements only need 1 single bond.

Learn these requirements for each group and you’ll hopefully be able to work out the formula of any simple covalent compound.

Remember how with ionic compounds you’d use the ‘swapping the charges’ method to find the ratio of the cation to the anion. You can do something similar here. Instead of charges, it’s the number of bonds that get ‘swapped’.

For example, let’s take phosphorus selenide.

Phosphorus is in group 5, so it needs 3 bonds.
Selenium is in group 6, so it needs 2 bonds.
This suggests we should have 2 phosphorus atoms and 3 selenium atoms in a molecule of phosphorus selenide. Hence, it’s P2Se3.

At A level, you will come across some exceptions to this rule (group 3 elements not forming a full octet and ‘hypervalent molecules’ having more than 8 outer electrons), but you won’t have to worry too much about them at the moment.


EDIT: You may want to simplify the results down to the empirical formula. If you try this method with carbon and oxygen, it would suggest the formula should be C2O4. Of course, carbon dioxide is CO2.
(edited 1 year ago)

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