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### 2A.23 7th edition (Chemical Formula)

Posted: Wed Oct 31, 2018 12:42 pm
On the basis of the expected charges on the monatomic ions, give the chemical formula of each of the following compounds;

I just need to know why the chemical formula for (c) aluminum hydride is = AlH3

I thought aluminum has a charge of 3+ and Hydrogen has a charge of 1+?

### Re: 2A.23 7th edition (Chemical Formula)

Posted: Wed Oct 31, 2018 2:25 pm
Youre right in that aluminum has a charge of +3 but Hydrogen actually has a charge of -1, remember that hydrogen is willing to gain not lose an electron because it is forming a duplet not octet therefore the -1 charge is meaning it needs to gain one while aluminum wants to give away 3 to fill its shell. Since the charges in the molecule have to balance you get 3 hydrogens (-1*3=-3) to balance the aluminum +3. hope this helps!

### Re: 2A.23 7th edition (Chemical Formula)

Posted: Thu Nov 01, 2018 5:20 pm
Hydrogen is actually willing to gain or lose an electron which is shown in this example; hydride refers to the hydrogen anion while the hydrogen in hydrogen telluride is a cation. When naming ionic compounds it is common convention to name them with the cation first and the anion second. Also, for this problem specifically, it would be more logical for the hydrogen to lose its electron since an aluminum ion with a -3 charge would not make sense (not an octet). Also, anything ending in -ide is typically an anion (e.g. chloride, fluoride, oxide)!

### Re: 2A.23 7th edition (Chemical Formula)

Posted: Thu Nov 01, 2018 6:41 pm
^^ To add on, Hydrogen is willing to give away its one electron to become more stable and Aluminum wants electrons in order to get closer to the noble gas configuration and more stability, so Aluminum will take 3 Hydrogens that each provide 1 electron.