Ionic Formula Writing

Video Notes

In this lesson, you will:

• Learn to write formulas for a variety of ionic compounds.

Chemical Formula• Tells the relative number of atoms of each element in a

compound.

• Molecular (covalent bonding) formula represents the number of atoms of each element in a single molecule.

• Ionic (ionic bonding) formula represent the simplest ratio of positive and negative ions in one formula unit. (remember only covalent bonding produces molecules)

Ionic Bond• The attraction that binds oppositely charged ions together• One or more electrons are transferred

Metals• give away electrons• Positively charged• Cations

Nonmetals• Take electrons• Negatively charged• Anions• Tries to get a noble gas formulation to increase stability.

How to interpret ions

Ex: AlCl3 Aluminum chloride• With no subscript, the assumption is there is there

is one ion of that element.• With a subscript, there are the number of ions

present as the subscript represents.• 1 aluminum cation• 3 chloride anions (notice it is chloride, not

chlorine- this is because IDE is added to the end of anions)

Aluminum Chloride How do we know?Oxidation number- speaks of whether an element

tends to give up, share, or gain electrons.Metals- positive oxidation numbersGasses or non metals- negative oxidation numberIonic compounds- no net charge as the positive and

negative charges in the compound balance each other out.

If there is a charge, the compound is not stable and will continue to react until it is stable (no charge)

Rules for Ionic Compounds

Writing Ionic Formulas1. Write the symbols for the elements.

• The cation is written first.• The anion is written second.

1. Determine the charge on each ion (use the periodic table to determine charge)

2. Select subscripts that will make the total positive charge equal to the total negative charge. (overall charge of our ionic formula must be zero)

Example- Sodium Chloride

• NaCl• Since Na is in group 1 of the periodic table it

needs to lose or give away one electron (gives it a positive charge)

• Since Cl is in group 17 of the periodic table, it needs to gain one electron (has a negative charge.

• (1+) + (1-) = 0• Notice the number precedes the charge.

Magnesium Chloride

• Mg is in group 2. Needs to lose 2 electrons• Cl is in group 17, needs to gain 1 electron

• MgCl2

• (2+) + (2 x 1+) = 0• There needed to be 2 chlorine atoms to

make the net charge zero.

Criss-Cross MethodBarium and Iodine1. Write the symbols for your cation and anion

(how do you know?– Barium is a metal and gives up electrons. Iodine is a non-metal and gains electrons)

2. Ba is in group 2 so it has a 2+ charge, I is in group 17, so it has a 1- charge. Write it as a superscript

3. Criss-cross the charges and make it the subscripts for the ions.

Criss-Cross Method Continued

Ba2+I1-

Ba1-

I2+

You now don’t need the charges or the 1 in the subscript, it is understood. Take out the top charges as well. New formula- BaI2

Calcium Sulfide

Ca 2+ S 2-

Ca2S2

But the ionic formula is the simplest ratio possible, so the formula is CaS.

Criss-Cross Method for Formula Writing

• Write symbol of cation followed by symbol of anion along with their ionic charges.

• Use the absolute value of the charge of each ion as the subscript for the other.

• If a subscript is one omit it.• If the subscripts are the same omit them.

Polyatomic Ions

• In the polyatomic ion, you still write the cation first, then write the anion

• Ex. Mg(NO3)2 (the 2 outside the parenthesis denotes that there are 2 nitrates for every magnesium ion)

NO3-1 Mg+2 NO3

-1

What is the formula?

• Calcium Nitrate- Ca2+NO31- (don’t forget to

put the polyatomic nitrate ion in parenthesis before criss-crossing the charges!)

• Ca(NO3)2

You Try

1. Ca2+ O2-

2. K1+ ClO31-

3. Al3+ SO42-

How did you do?

1. CaO2. KClO3

3. Al2(SO4)3

Chemistry Quiz

CR 1. The orbital shaped like a dumb-bell is: a. sb. pc. dd. f

Chemistry Quiz

CR2. What is the volume of a cube if each side measures 2.0 cm?

a. 8.0 cmb. 8.0 cm3

c. 2.0 cmd. 6.0 cm3

Chemistry Quiz

1. In an ionic compound, the chemical formula represents____________.

a. One molecule of the compound.b. One formula unit of the compound.c. The simplest ration of positive to negative

ions in the compound.d. Both B and C are correct.

Chemistry Quiz

2. Are electrons lost or gained during the formation of a negatively charged ion?

a. Lostb. Gained

Chemistry Quiz

3. When writing the formula of an ionic compound:

a. The net charge must be zero.b. The total positive charge equals the total

negative charge.c. The cation is written first followed by the

anion.d. All of the above are true.

Chemistry Quiz

4. Choose the formula that correctly represents the compound formed from the ions Sr2+ and Se2-.

a. SrSeb. Sr2Se2

c. Se2Sr2

d. SeSr

Chemistry Quiz5. When writing the formula of a compound

containing at least one polyatomic ion, parenthesis ___________.

a. Must always be placed around the polyatomic ion.

b. Must be placed around the polyatomic ion only if a subscript is to be placed outside the parentheses in order to balance charge.

c. May or may not be used according to the discretion of the chemist.