Relative atomic mass - Ar

Just another way of saying how heavy different atoms are compared with the mass of one atom of carbon – 12 (regular carbon!)

Mass number

Q. What does the atomic mass number represent?

A. Total number of protons and neutrons found in an atoms

nucleus

Atomic number

Number of protons present

Another handy trick you can do with the periodic table

Mass number

Atomic number

You can work out how many neutrons an element has by subtracting the proton number

from the mass number!!

So how many neutrons:

Relative atomic mass - Ar

Element Ar

H 1

C 12

O 16

Mg 24

Cl 35.

Table.1. Different elements and their different Ar

Relative atomic mass is easy!! It’s the same value as the mass number – it just sounds scarier!

So what does this tell us about Mg and H?

Relative formula mass - Mr

• To find the relative formula mass (Mr) of a compound, you just add together the Ar values for all the atoms in its formula.

Example 1: Find the Mr of carbon monoxide (CO).The Ar of carbon is 12 and the Ar of oxygen is 16.So the Mr of carbon monoxide is 12 + 16 = 28.

O16

8

Relative formula mass - Mr

• Example 2:Find the Mr of sodium oxide-Na2O The Ar of sodium is 23 and the Ar of oxygen is 16.

So the Mr of sodium oxide is (23 x 2) + 16 = 62.

Carbon dioxideSulphur dioxide SO2

Calcium carbonate CaCO3

Sodium hydroxide NaOHSulphuric acid H2SO4

Hydrochloric acid HClCopper sulphate CuSO4

Magnesium chloride MgCl2

Sodium carbonate Na2CO3

Find the Mr of these:

ATOMIC MASS AND AVERAGE ATOMIC MASS

Atomic Mass = used to numerically indicate the mass of an atom in its ground state, it is expressed in the non SI unit of u

u = refers to unified atomic mass unit (formerly known as atomic mass unit or amu)

1 amu = 1/12 the mass of carbon-12 atom,

therefore the mass of C-12 atom is made equal to 12 amu

Carbon-12 atom is an isotope of carbon 

1 amu = 1.66 x 10-24 g Note that: Atomic mass of 12C = mass of p + mass of n + mass of e

Mass of e = 1/1800 of mass of p and n so it is negligible making the equation

Atomic mass of 12C = mass of p + mass of n

Atomic mass vs. Average atomic mass • For carbon it is 12 u not 12.01 u

· Used to relate the fact that the numerical value assigned to each element in the periodic table reflects the average abundances of the atoms that compose a naturally occurring element

· Related to isotopes

· For carbon it is 12.01 u

· Chemists often will use the term “atomic mass” when they are actually referring to average atomic mass of an atom.

Average Mass of Isotopes

• Isotopes are naturally occurring.

• The mass # of an element (periodic table) is the weightedavg. of all isotopes that exist in nature.

- abundance of isotope is just as important as mass! • Ex...

Natural copper (Cu) consists of 2 isotopes ...

Copper - 63 (mass = 62 .930 g/mole)

Copper - 65 (mass = 64 .930 g/mole)

69%

31% • To calculate avg. mass...

mass x abundance for each isotopeStep 1 :Step 2 : add the two values from step 1 together

62 .93 x .69 =

64 .93 x .31 =

43.42

20.13

43.4220.13+

63.55 g/mole

• Ex...

Three isotopes of Oxygen:

Oxygen - 18

Oxygen - 16 Oxygen - 17

• The avg. mass (from P.T.) is closest to 16, therefore, Oxygen-16 is the isotope that is most abundant in nature.

99 . 759%0.037%0.204%

• The average mass of an element is closest to the isotopethat is most plentiful in nature.

Working with Weighted Averages&

Calculating Average Atomic Mass

Read page 165 of your text and make short notes.