To do these, there are two "things" to be aware of. First, making sure each part is "written" correctly. For instance, on the first one, the first part, Sodium, that is "Na+" as an ion, exactly. For "perchlorate", I would assume your teacher gave you a list of specific ions like "perchlorate". "Perchlorate" is ClO4- as an ion. The positive on the Na shows it gives one positive charge. The negative on the ClO4 shows it has one negative charge.
That comes to our second thing to watch out for, that the ions balance out. Each one has one charge of opposite signs. So, it would balance out if we have one of each. Therefore, Sodium Perchlorate is:
(The 4 should be a subscript)
Next is ammonium oxalate. The ammonium ion is NH4+, one positive charge. Oxalate is C2O4-2. So, C2O4 with a -2 charge. That's the first step, making sure we have the exact ions. Next, how many of each do we need. Since the ammonium ion has only 1 positive charge, we would need two of those to balance out the oxalate ion, a negative 2 charge. Therefore, ammonium oxalate is:
(all the numbers are subscripts)
We show two ammonium ions by putting parenthesis around the ammonium ion then putting a 2 outside it.
Good luck, Cindy. I hope this helps.
Sodium perchlorate - `"NaClO_4"`
Sodium's chemical symbol is Na, which you can find on your periodic table (#11). It is located in Group 1, or the alkali metals. This means that sodium has one valence electron and that its ion has a +1 charge. Meanwhile, perchlorate is a polyatomic ion--meaning it is made up of more than one atom. Perchlorate has a -1 charge. There is no easy way to determine its charge, so its best you search it up and memorize it. Sodium's and perchlorate's opposite charges cancel each other out.
Ammonium oxalate - `"(NH_4)_2C_2O_4"`
Both the ions are polyatomic ions, meaning you need to either know or search their charges. Ammonium has a charge of +1, while oxalate has a charge of -2. To neutralize oxalate's -2 charge, you need two +1 charged ammonium ions.