4.2

ATOMIC NUMBER

Elements are different because they contain different numbers of protons.

Atomic number = # protons = # electrons in a neutral atom

Atomic number

Chemical symbol

Chemical nameAverage atomic mass

82 82

O 8 8

30 30Zn

Atomic Mass Unitsamu: atomic mass units

Unit used to measure the mass of VERY SMALL particles (atoms)

Standard is the carbon-12 atom

1 carbon-12 atom = 12 amu

1 amu = 1/12 carbon-12 atom mass

1 proton = 1 amu

1 neutron = 1 amu

1 atomic mass unit =1.66053892 × 10-24 grams

Mass Number

Mass Number – Mass of a specific atom

Mass number = # protons + # neutrons

REVIEW:

Atomic Number:

The total number of protons

Atomic number = # protons = # electrons

Mass Number:

The total number of protons and neutrons

Mass number = # protons + # neutrons

Isotopes

• Each element may have different numbers of neutrons => ISOTOPES

• The mass number is different for different isotopes.

Isotopes – Naming (Two ways)

• Element name – mass number

• Ex. carbon-12 or neon-22

• Using the element symbol:

Isotopes - Naming

• How would you write Uranium-235 using the symbol? (atomic number = 92)

Atomic MassThe weighted average mass of the isotopes of that element.

*different than mass number

Calculating Average Atomic Mass All isotopes of an element do not exist in equal abundance in the world

Percent abundance in world:

90.48% 0.27% 9.25%

Example #1

Calculate the average atomic mass of NeonAtomic Mass = (% abundance A)(mass A) +

(% abundance B)(mass B) + ….

Isotope Mass (amu) (M)

Percent abundance (P)

M x P÷100

Neon-20 20.0 90.48%

Neon-21 21.0 0.27%

Neon-22 22.0 9.25%

Average atomic mass (amu)

18.1

0.057

2.03

20.2

Atomic MassWhich isotope is most abundant?

If Neon has 3 isotopes:

Neon-20, Neon-21, Neon-22,

Weighted average atomic mass is 20.2 amu.

Neon-20

Example #1

Calculate the average atomic mass of NeonAtomic Mass = (% abundance A)(mass A) +

(% abundance B)(mass B) + ….

Another way to write it out:

Atomic Mass = (90.48/100)(20.0) +

(0.27/100)(21.0) +

(9.25/100)(22.0)

= 20.2 amu

Example #1

Calculate the average atomic mass of LithiumAtomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….

Isotope Mass (amu) (M)

Percent abundance (P)

M x P÷100

Lithium-6 6.01 7.5%

Lithium-7 7.01 92.5%

Average atomic mass (amu)

0.45

6.49

6.94

OR

Atomic Mass = (7.5/100)(6.01) + (92.5/100)(7.01) = 6.94 amu

Example #3

Calculate the average atomic mass of CesiumAtomic Mass = (% abundance A)(mass A) + (% abundance B)(mass B) + ….

Isotope Mass (amu) (M)

Percent abundance (P)

M x P÷100

Cesium-132 132.0 75.0%

Cesium-133 133.0 20.0%

Cesium-134 134.0 5.0%

Average atomic mass (amu)

99.0

26.4

6.7

132.1

OR Atomic Mass = (75.0/100)(132.0)

+ (20.0/100)(133.0) + (5.0/100)(134.0) = 132.1 amu

Questions

1) What particles make up an atom?

Protons, Neutrons and Electrons

2) What are the charges on these particles?

Protons (+), Neutrons (none), Electrons (-)

3) What particles make up the nucleus?

Protons and Neutrons

More Questions:

Atomic Number tells us:

The identity of the element; the number of protons and electrons

Atomic Mass tells us:

The average mass in amu of all naturally occuring isotopes

Mass

The Development of Atomic Models

The timeline shoes the development of atomic models from 1803 to 1911.

End Show

Slide 20 of 18

© Copyright Pearson Prentice Hall

Defining the Atom > The Development of Atomic Models

The timeline shows the development of atomic models from 1913 to 1932.

5.1