An atom barely has a weight, so scientists found the weight of an element by comparing it to one carbon-12 atom. This is called the relative molecular mass (Ar), and is not the actual weight of the atom. It can be found in the periodic table. 

Magnesium is 24 is the weight of 2 carbon-12 atoms, 12 x 2 which is 24

 

 

Some examples:

Hydrogen- 1

Carbon- 12

Nitrogen- 14

Oxygen- 16

Sodium- 23

Magnesium- 24

Sulfur- 32

Chlorine- 35.5

Potassium- 39

Calcium- 40

Iron- 56

Copper- 64

Zinc- 65

Iodine- 127

 

 

 

FINDING THE MASSES: MOLECULES & IONS

The mass of a group of molecules can be found using the Ar.

For example, the mass of a sodium atom is 23 and the mass of the sodium ion is 23, since ions are made from atoms.

The Ar is basically the number of atoms, so there is no calculations involved.

 

 

 

 

RELATIVE MOLECULAR MASS

 

If the substance is made of molecules (multiple atoms), it's mass is found of relative molecular mass or Mr.

For example, the Mr for ammonia is 17. This is how you find it.

 

Step 1- Find the formula

Ammonia= NH3 

 

Step 2- Use the formula to find the amount of atoms

1 nitrogen, 3 hydrogen

 

Step 3- Find the Ar of the atoms

Nitrogen= 14

Hydrogen= 1

 

Step 4- amount of atoms x Ar

Nitrogen: 1 x 14 = 14

Hydrogen: 3 x 1 =3

 

Step 5- add the answers and you've found your Mr

Ammonia:  14 + 3 = 17

 

 

Here's another example

Calcium chloride

 

CaCl2 

 

1 calcium, 2 chlorine

 

calcium= 40

chlorine= 35.5

 

Calcium: 1 x 40 = 40

Chlorine: 2 x 35.5 = 71

 

Mr for calcium chloride is 40 + 71 =111

 

 

                                          

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