# CHEMISTRY

posted by MELISSA

Write formulas for the compounds that form from Sr and each of the following polyatomic ion: NO−3, SO2−4, PO3−4.
THE -3, -4, -4 ARE THE CHARGES, CANT TYPE SUBSCRIPT ON HERE.

WHAT I TRIED IS SrNO6,SrSO4,Sr3(PO4)2 AND WRONG

1. DrRebel

OK... When writing formulas of neutral compounds the sum of the charges of the ions (monatomic or polyatomic) must equal zero. NaCl is 1:1 ratio b/c Na => +1 and Cl => -1. (+1)+(-1)=0.

Now, Strontium is a Group II cation which always has a +2 charge in Ionic Compounds;i.e., metal cation + non-metal anion. So,
1. (Sr+2) + (NO3-1) requires 2 NO3- ions so sum of charges = 0.
(Sr+2) + 2(NO3-1)=> Sr(NO3)2 => (+2)+2(-1) = (+2)+(-2) = 0 Strontium Nitrate is a 1 to 2 ion ratio.

2. First, the sulfate ion only carries a -2 charge not a -4. This might be confusing b/c when writing the ionic compound formula to get Sum Chgs = 0, some people write 2(Sr+2) + 2(SO2-2) in order to => sum = 0. This would lead one to assume that the SO4 ion would be -4, but for ionic compounds, the final form of the formula needs to have subscripts reduced to the lowest whole number ratios. It is much simpler to just recognizs (Sr+2)+(SO4-2) => (+2)+(-2)=0 and use a 1 to 1 combination ratio => SrSO4

3.The phosphate ion does not carry a -4 charge but a -3 charge. Combining the ions so as to have sum = 0 => (Sr+2)+(PO4-3) requires 3(Sr+2)+2(PO4-3) => (Sr)3(PO4)2 Strontium Phosphate.

2. DrRebel

Also, when writing formulas of ionic compounds, the 'polyions' such as the nitrate ion (NO3-), sulfate ion (SO4)-2 and phosphate ion (PO4-3) need to be in parenthesis so the subscripts outside the parenthesis indicate that you only have '2' ions of the kind as in using PO-3 => Calcium Phosphate => 3(Ca+2)+2(PO4-3)=> Ca3(PO4)2 ... the symbology indicates there are only two Phosphate ions in a formula molecule of the compound. Leaving 'off' the parenthesis => Ca2PO8 which says there are 2Ca, 1P and 8Oxys ... Such a compound does not exist.

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