Wrapping up Chapter 2: Ionic Compounds with Polyatomic Ions (Lecture 15)

What is the ONLY positive polyatomic ion?

Ammonium, NH₄. This polyatomic ion is the only positive polyatomic ion with a +1 charge.

How do we write the Chemical Formula of Mg and PO₄ (magnesium and phosphate)?

Mg₃(PO₄)₂. Magnesium’s charge is +2 , Phosphate’s charge is -3. When writing the chemical formula of ionic compounds, you switch the charges of both ions and convert it to the subscript of each ion (think of criss-cross) to make the overall charge 0. Remember that all ionic compounds charge must be 0. However, because Phosphate already has a subscript, you put it in parenthesis. Also remember that the subscript of a polyatomic ion never changes.

Superscript

(Super-above) this is where you would write the charge of an ion.

Subscript

(Sub-below) where you write the number of compounds of that an ion.

Ammonium and metals are…

Cations  (+ charge).

Polyatomic means…

More than one ion.

Compounds must have a charge of…

0

Polyatomic ion and Ionic Compounds…

Form completely new charges. You also cannot find it’s charge on the Periodic Table. For example, phosphate PO4 charge is -3, you cannot find that on the periodic table as the periodic table is based on each of 1 element. Ionic compounds are made up of 2 or more elements.

When writing the Chemical Formula of an Ionic compound and a transition metal is involved…

You must find the charge of the transition metal, as a transition metals charge is not known (for the most part).

You cannot simplify….

Polyatomic ions (the way you would with binary compounds to the smallest whole number ratio).

How do we find the charge of transition metal Ti in 𝑇𝑖2𝑆𝑒3?

Solve for X, we have 3 of Se which its charge is -2. We have 2 of Ti which its charge is X. Remember that the overall charge of an ionic compound must equal 0. So, 3(-2) + 2(x) = 0 —> -6 + 2x = 0 ?(add 6 on both sides, since it is negative, you add (do the opposite)) Since 6 cancelled itself out on the left, and we added 6 to 0 —> 2x = 6, now divide by 2, you get +3. Therefore, Transition metal Ti’s charge is +3

Ammonium

NH₄ The only positive polyatomic ion with a +1 charge.

Sulfate

SO₄²⁻

Phosphate

PO₄³⁻

Carbonate

CO₃^2–

Nitrate

NO⁻ ₃

Hydroxide

OH¯

Solving for the subscript of an elements unknown charge

Your subscript in your answer from finding the unknown charge is usually whatever you multiplied that number by for the charge of the compound to be 0. EX: Potassium and Phosphate. Potassium (K) charge is +1. PO₄ charge is -3. To make these compounds equal zero, you multiply K’s charge by 3, and PO₄^-3 by 1, that would make the charges +3 and -3 which balance each other out to 0. The chemical formula would now be K₃PO₄ notice how potassium’s subscript is 3, which is what we multiplied its charge by to balance out the formula, and phosphate remained the same as we only multiplied it by 1. (Keep in mind a polyatomic ions subscript can never change. Unless it is in parenthesis and the subscript is outside of the parenthesis).

Determine the charge of Copper in Cu₂CO₃^-2

Cu = +1. This is because CO₃ charge is -2, to balance it out, you need 2 +1 charges to make the compound equal 0.

When writing the simplified version of the chemical formula of a compound

It cannot be simplified if not all subscripts share a common multiple

If ammonium (Positive polyatomic ion) and another metal are present, how is the compound written?

The metal always goes first, then the positive polyatomic ion can follow.

The subscript next to a compound…

Only applies to the element its next to. EX: NO₃ the 3 only applies to oxygen. Unless it is (NO₃)₄ then there are 4 N’s and 12 O’s