Calcium reacts with a polyatomic ion to form a compound with the general formula Ca3(X)2. What would be the most likely formula for the compound formed between lithium and the polyatomic ion X?

Question

BlackTom AI · Accepted Answer

The problem asks us to determine the formula of a compound formed between lithium (Li) and a polyatomic ion X that has a 2− charge, given that calcium forms Ca3(X)2 with X as a 2− anion.
First, recognize the charges: calcium is 2+, and the formula Ca3(X)2 indicates two X ions balance the 3 Ca2+ ions (3×(+2) = +6, 2×(−3)? No, the idea is to infer X carries a 2− charge so that 3×(+2) balances 2×(−2). This leads us to conclude X has a 2− charge.
Now consider lithium, which typically forms Li+ ions with a +1 charge. To balance the 2− charge of X, you need two Li+ ions for every X2− ion, giving the neutral formula Li2X.
Option-by-option analysis:
- Li2X: This matches the expected 2+ charges from two Li+ ions balancing the 2− charge of X2−, giving a neutral compound. This is the correct charge-neutral combination based on simple ionic charge balance.
- LiX: This would pair Li+ (1+) with X2− (2−), resulting in a net charge of −1, which cannot be neutral. Thus this option is incorrect.
- Li2X2: Although Li2X2 also has a 2:2 ratio, the simplest whole-number ratio reduces to LiX, so Li2X2 is not the smallest whole-number formula. In standard chemical notation, we use the lowest terms, making Li2X2 an unnecessary repetition.
- Li3X2: This would produce 3(+1) = +3 total positive charge and 2(−2) = −4 total negative charge, which does not balance to zero. Hence it cannot be the formula of a neutral compound.
- Li3X: Here we have 3(+1) = +3 and 1(−2) = −2, giving a net +1 charge, so the compound would not be neutral. Therefore this option is also incorrect.
In summary, only Li2X provides the proper 2+ to 2− charge balance in the smallest whole-number ratio, yielding a neutral compound.

Calcium reacts with a polyatomic ion to form a compound with the general formula Ca3(X)2. What would be the most likely formula for the compound formed between lithium and the polyatomic ion X? 单项选择题

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