Writing Formulas

Chemistry Mrs Stoops

Finding the charge

Na has a Dot diagram of:

Na

So to become more stable, and form an ion it will LOSE 1 electron

Na+

1

Mg has a Dot diagram of:

Mg

So to become more stable, and form an ion it will LOSE 2 electrons

Mg+2

SO,

Cl has a Dot diagram of:

Cl

So to become more stable, and form an ion it will GAIN 1 electron

Cl-1

S has a Dot diagram of:

So to become more stable, and form an ion it will GAIN 2 electrons

S-2

S

There is a PREDICATBLE pattern

+1

+2 +

3+/-4 -3 -2 -1

0

+1

+2

You need to memorize this

POLYATOMIC IONS• Groups of atoms with a charge• NEED to memorize name, symbol, and charge they have• There are 12

1. Acetate = C2H3O2 -1

2. Chlorate = ClO3-1

3. Cyanide = CN-1

4. Hydroxide = OH-1

5. Nitrate = NO3-1

6. Nitrite = NO2-1

7. Carbonate = CO3-2

8. Chromate = CrO4-2

9. Sulfate = SO4-2

10.Sulfite = SO3-2

11.Phosphate = PO4-3

12.Ammonium = NH4+1

Rules for Writing Formulas

• Formulas always have a positive and negative part

• Formulas always have the positive part first• Chemists don’t write the number 1• Formulas use whole numbers only• Formulas are simplified and reduced when

possible• Add parentheses when we need more than one

polyatomic ion (When a polyatomic has an extra subscript)

• Formulas will have NO OVERALL charge.

How to write formulas “Criss Cross” method

Steps:1. Write the positive part first with the charge and

the negative part second (w/ charge)2. Cross charge number only – Take the charge

number from the “attic” and put it in the “basement” of the OTHER element.

3. Erase the original charges4. Simplify.

Example Ca & Cl

“what are the charges?” Ca +2 Cl-1

“cross the charges” Ca +2 Cl-1

Cl-1 2Ca +2 1

“erase charges” Ca 1 Cl 2

“reduce simplify” CaCl2

ExampleAl OH

“what are the charges?”

“cross the charges”

“reduce simplify”

Al+3 OH-1

OH3Al

What is wrong with this?

Al(OH)3

#2

Sn+4 O-2

Sn2O4

SnO2

#3

Mg PO4

Mg3(PO4)2

#4

NH4 CO3

(NH4)2CO3