Lecture, Wednesday, August 28, 2002

Cathode Ray tube

Millikan Oil Drop

Radioactive elements

Rutherford exp

Rutherford Model

 

Particle

Charge

Mass

electron

-1.6022 x 10-19 Coulombs

9.109 x 10-31 kilograms

proton

-1.6022 x 10-19 Coulombs

1.673 x 10-27 kilograms

neutron

0

1.675 x 10-27 kilograms

 

 

Particle

Size (meters)

Atom

10-10

Nucleus

10-14

Proton

10-15

Electron

10-18

Atomic mass of hydrogen from the periodic table is

1.00797 amu (u)

The mass in grams of a single hydrogen atom is

1.6736 x 10-24 g

An atomic mass unit is a 'relative' mass, in this case relative to 1/12 of the mass of a carbon atom with 6 protons, 6 neutrons and 6 electrons. The mass of such an atom of carbon is,

1.99268 x 10-23 g

1/12 of that mass is,

 

1.66057 x 10-24 g

So

1 amu (u) = 1.66057 x 10-24 g

Converting

Isotopes

Isotopes of an element are atoms with the same number of protons, but a different number of neutrons.

Relative, weighted average atomic mass

Symbol

Atomic Mass (u)

% Abundance

1H

1.0078250

99.985%

2H

2.0141017

0.015%

weighted average = (atomic mass x fractional abundance)1 + (atomic mass x fractional abundance)2 +....+(atomic mass x fractional abundance)n

Let's go ahead and use this approach to calculate the weight average of the element hydrogen.

The weighted average atomic mass of hydrogen is;

weighted average = (atomic mass x fractional abundance)1 + (atomic mass x fractional abundance)2

substituting,

weighted average = (1.0078250 x 0.99985)1 + (2.0141017 x 0.00015)2 = 1.00794 u

 

 

Determine the relative weighted average atomic mass of the element neon, given the following information,

Isotope

Relative Mass (u)

% Abundance

20Ne

19.992440

90.48

21Ne

20.993846

0.27

22Ne

21.991385

9.25

Answer:

To calculate the relative, weighted average atomic mass we use the mathematical equation,

weighted average = (atomic mass x fractional abundance)1 + (atomic mass x fractional abundance)2 + (atomic mass x fractional abundance)3

substituting,

weighted average = (19.992440 x 0.9048)1 + (20.993846 x 0.0027)2 + (21.991385 x 0.0925)3 = 20.18 u

 

Complete the following table;

Isotope

Protons

Electrons

Neutrons

109Ag

 

 

 

24Al