CHEMICAL FORMULA

 CHEMICAL FORMULA

When chemistry was a young science and the number of known compounds was small, it was possible to memorize their names. Many of the names were derived from their physical appearance, properties, origin, or application—for example, milk of magnesia, laughing gas, limestone, caustic soda, lye, washing soda, and baking soda. Today the number of known compounds is well over 20 million. Fortunately, it is not necessary to memorize their names. Over the years chemists have devised a clear system for naming chemical substances.  Mastering these rules now will prove beneficial almost immediately as we proceed with our study of chemistry.

 

Writing Chemical Formulas

 A formula represents a relative number of atoms of different elements present in the substance. To        write a formula, you need to analyze a chemical. However, we can take several points for granted,        and     concentrate on writing chemical formulas by a method called the ion-charge method (you        may wish to “brush-up” your knowledge on what an ion is).

 

An ion is an atom or a group of atoms that has a net positive or negative charge. The number of positively charged protons in the nucleus of an atom remains the same during ordinary chemical changes (called chemical reactions), but negatively charged electrons may be lost or gained.

 

The loss of one or more electrons from a neutral atom results in a cation, an ion with a net positive charge. For example, a sodium atom (Na) can readily lose an electron to become a sodium cation, which is represented by Na+:

Sodium atom (Na)	Sodium ion (Na+)
11 protons Þ +11 11 electrons Þ -11 Overall charge = +11 + -11 = 0 charge	11 protons Þ +11 10 electrons Þ -10 (lost an electron) Overall charge = +11 + -10 = +1 charge

On the other hand, an anion is an ion whose net charge is negative due to an increase in the number of electrons. A chlorine atom (Cl), for instance, can gain an electron to become the chloride ion, Cl- :

Chlorine atom (Cl)	Chloride ion (Cl- )
17 protons Þ +17 17 electrons Þ -17 Overall charge = +17 + -17 = 0 charge	17 protons Þ +17 18 electrons Þ -18 (gained an electron) Overall charge = +17 + -18 = -1 charge

For the ion-charge method, we must know the charge of each ion, or the group of ions. Table 1 shows the common ions, their charges and names. Please note, it is very important to learn the names and charges of the common ions listed in the table below, as we will be using them regularly in our study of chemistry

Table 1: Common Ions and their Charges.

COMMON IONS, THEIR CHARGES AND NAMES
Positive ions (CATIONS)
1+ charge	2+ charge	3+ charge
Sodium, Na + Potassium, K + Copper(I), Cu + Silver, Ag + Ammonium, NH4 +	Magnesium, Mg 2+ Barium, Ba 2+ Calcium, Ca 2+ Iron(II), Fe 2 + Nickel(II), Ni 2+ Copper(II), Cu 2 + Zinc, Zn 2 + Lead(II), Pb 2+	Aluminum, Al 3+ Chromium(III), Cr 3+ Iron(III), Fe 3+

        

Negative ions (ANIONS)
1- charge	2- charge	3- charge
Fluoride, F- Chloride, CI- Bromide, Br- Iodide, I- Hydroxide, OH- Nitrate, NO- Nitrite, NO- Chlorate, ClO3 Hydrogen carbonate or bicarbonate,HCO3 Hydrogen sulfate or bisulfate, HSO- Acetate, CH3COO-	Oxide, O2-Peroxide, O2- Carbonate, CO2- Chromate, CrO2- Dichromate, Cr2O2- Sulfide, S2- Sulfate, SO2- Sulfite, SO2-	Phosphate, PO3-

  Example 1: 

To illustrate the use of this method, place each symbol side by side and under each write down the ionic charge, as shown below:

Cross – multiply each symbol with the ionic charges

Sodium Chloride    

                       Na       Cl

                     =    Na1Cl1 = NaCl

                                                   1          1 
                                                                        

                               

Calcium hydroxide

                                                Ca        OH

                                                                      =    Ca1OH2 = Ca(OH)2

                                                  2            1        

Aluminum nitrate

                                                   Al        NO3

                                                                      = Al1(NO3)3 = Al(NO3)3

                                                     3            1            

Video:

 

         Activity:

     Write down the chemical formula for each of the following compounds:

a. sodium carbonate = _____________

 

b. aluminum hydroxide = ___________

 

c. potassium sulfate = ______________

 

d. calcium nitrate = ________________

 

e. lithium oxide = _________________

 

f. magnesium chloride = ___________

 

g. bariumchloride = ______________

 

h. potassium oxide = ______________

 

i. zinc iodide = __________________

 

j. copper(II) nitrate = _____________

k. calcium hydroxide = ____________