Given the actual compounds, predict the chemical formulas of compounds with the elements listed.

Given: Al₂S₃, CaO, BeCl₂, GaBr₃

Predict: if the following two elements formed a compound, what would its formula be?

Elements	Similar Compound	Atom Ratio	Predicted Formula
Ca + S	?	?	?
Ga + O	?	?	?
In + At	?	?	?
Ba + F	?	?	?

Answers:

Given: Al2S3, CaO, BeCl2, GaBr3

Elements	Similar Compound	Atom Ratio	Predicted Formula
Ca + S	CaO	1:1	CaS
Ga + O	Al2S3	2:3	Ga2O3
In + At	GaBr3	1:3	InAt3
Ba + F	BeCl2	1:2	BaF2

Explanations:

1) Ca + S

• Look at each of the “given” compound formulas, and find one that is made up from elements in the same family as Ca and S.
• CaO is a similar compound to what Ca and S would make, because O and S are in the same family.
• Therefore, Ca and S would react in the same ratio as CaO, a 1:1 ratio.
• The predicted formula (the answer) is therefore CaS.

2)Ga + O

• GaBr₃ is not a similar compound to Ga and O, because Br is not in the same family as O.
• Al₂S₃ does match our needs: Al is in the same family as Ga, and
• S is in the same family as O
• Therefore, the Ga + O compound will have a 2:3 ratio like Al₂S₃.
• Our predicted formula is Ga₂O₃.

3)In + At

• In is in the same family as Ga
• At is in the same family as Br
• Therefore, GaBr₃ is a similar compound to In + At.
• We predict that In + At will have a 1:3 ratio like GaBr₃
• Our prediction is InAt₃.

4) Ba +  F

• Ba & Be are in the same family
• F & Cl are in the same family
• Therefore, BeCl₂ is a similar compound
• Ba and F react in a 1:2 ratio similar to BeCl₂
• Our prediction is BaF₂

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