Forming Compounds

What Forms a Compound?

Binary compounds, which are made of two parts, are typically categorized as either ionic or molecular. Hint: Notice the colors (ie red and yellow make orange) to help you. 

  • Ionic compounds are composed of a metal and a nonmetal, whereby the metal is positively charged and the nonmetal is negatively charged.

Ionic Compounds

  • Molecular compounds are composed of two nonmetals, one acting as a cation and the other an anion.

Molecular Compounds

The driving force behind the formation of a compound is the attraction of unlike charges: a + charge attracting a – charge. Therefore, each compound, ionic or molecular must contain a + charge and a – charge.

How Is a Compound Written?

Always list the cation first followed by the anion.


Therefore in an ionic compound, the (+) charge metal is listed first in the formula followed by the nonmetal, (-) charge. In the molecular compound, there are two nonmetals, so again, the (+) charged nonmetal is written first followed by the (-) charged nonmetal. Recall that polyatomic ions consist mainly of nonmetals and are treated as such in a binary compound.

When writing a chemical formula, you should first determine if you have ionic or molecular compound, then assign the proper charges to (+) and (-) half.

According to Dalton, compounds are formed in set ratios based on charge equivalencies of the metals and nonmetals present. So knowing the right charge is necessary to write the correct formula. In a neutral compound, the total (+) and (-) charges should always be equal one another!

So you must know the charge of each element or polyatomic ion to be able to form the compound using the correct ratios.

You can use common denominators to help you determine how many of each atom are required to equal one another in a binary formula. These are written as subscripts. Or you can use the shortcut of switching the charges to reflect the subscripts, as in aluminum oxide, below.


Double check the charges to ensure the cation halves and anion halves are EQUAL.


Here’s another example:


New CaCl2

So the + charges must equal the – charges.

What about a molecular compound such as water, H2O?

Hydrogen is written first: it is the cation
Oxygen is written second: it serves as the anion

H is in group I so it should have a +1 charge despite being a nonmetal NOT ALL NONMETALS ARE (-) !!

O is in group VI, so it will have a -2 charge.


So you must know the charge of each element or polyatomic ion to be able to form the compound using the correct ratios.

What About Polyatomic Ions?

Remember they are the same any other ion—just treat them like a single unit.

  • Since some polyatomic ions contain subscripts, you should put them in parentheses to avoid the subscripts of the polyatomic ion and the ratio from running together. This applies only if you have more than 1 of the polyatomic ion





Even if the polyatomic ion does not have a subscript, you should put it in parentheses when there is more than one unit of it so that the ratio truly indicates how many of them you have.





What About Transition Metals?

You should know what version or charge of that transition metal you have so that you can write the correct ratio. Typically the charge is given in parentheses after the name of the metal to indicate this.

Tin (IV) chloride SnCl4
Tin (II) chloride SnCl2

Examples (Answers Below):


Ca + PO= _________________ Ba + Cl = ________________
Al + SO= __________________ Fe(II) + O = ______________
Sn (IV) + Cl = ___________________ Na + C2H3O2= ____________


P + Cl = ____________________ H + O = __________________
H + ClO= __________________ C + O = _________________

Hint: Always classify the compound as ionic or molecular and determine the elements’ charges before writing the formula.


Ca3(PO4)2 BaCl2
Al2(SO4)3 FeO
SnCl4 NaC2H3O2
PCl5 H2O

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