How to write the formula for ionic compounds

How do I write formulas for ionic compounds such as Beryllium Chloride, Copper(I)Bromide, Tin(II)Bromide ?

Step #1:

Identify the charge of the cation and anion. 

Step #2:

Write the chemical symbol for the cation follow by the chemical formula for the anion.

Step #3:

Balance the charge for the formula. Use subscripts to indicate the number of ions of each element necessary to balance the charge.

Ex #1: Beryllium chloride

Be,  alkaline earth metal: charge =2+ ;  Cl, halogen: charge = 1-

To balance the charge, there must be two Cl- ions for every one Be2+ ion, hence the formula, BeCl2

Ex #2: Copper (I) bromide

The roman numeral indicates the charge of the cation (Cu+). Bromine is a halogen and has a charge of 1-.

One Cu+ ion is necessary to balance the charge for every one Cl- ion, hence the formula, CuCl.

Ex #3: Tin (II) bromide

Again, the roman numeral indicates the charge of the cation (Sn2+). Bromine is a halogen and has a charge of 1-.

There are two Br- ions necessary to balance the Sn2+ ion, hence the formula SnBr2.

 

 

  1. Identify the cation.
    • Cation is written first in the name of the compound.
  2. Write the correct formula and charge for the cation.
  3. Identify the anion.
    • Anion is written last in the name of the compound.
  4. Write the correct formula and charge for the anion.
  5. Combine the cation and anion to produce an electrically neutral compound.
    • If the charges on the cation and anion are equal in magnitude (i.e. +1/-1, +2/-2, +3/-3), combine the cation and anion in a 1:1 ratio.
    • If the charges on the cation and anion are NOT equal in magnitude, use the charge on the cation as the subscript for the anion. Use the charge on the anion (omitting the negative sign) as the subscript for the cation.
    • Place parentheses around a polyatomic ion if you need more than one of them in the final formula.
    • Do not show the charges of the ions when you write the final formula for the compound.
  6. Make sure that the subscripts for the cation and anion are the smallest whole number ratio.
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