Atomic Structure

The Periodic Table, Isotopes, and Average

Atomic Mass

Atomic Number

• Number of protons in an atom

• Periodic table is arranged by increasing atomic number

• The number of electrons in a neutral atom equals the number of protons in that atom

Reading the Periodic Table

H

Hydrogen

1

1.008

Element Name

Atomic Number

Symbol

Atomic Mass (amu)

Mass Number

• Sum of protons and neutrons in an atom

NOT THE SAME AS ATOMIC MASS!

H

Hydrogen

1

1.008

Cannot be found on periodic table

NO NO NO NO NO!!!!

6

CCarbon12.011

Atomic Number

ElementSymbol

ElementName

Atomic Massround this off

to get Mass Number

Reading the Periodic Table

Reading the Periodic Table

Number of:

Protons = Atomic Number

Neutrons = Mass Number – Atomic Number

Electrons = Atomic Number

Wrap-Up 1

• Which of the following can be found on a periodic table?

1) Atomic number

2) Atomic mass

3) Mass number

What’s in a cheeseburger?

Isotope• Atoms with same number of

protons, but different number of neutrons

HYDROGEN - 1 HYDROGEN - 2 HYDROGEN - 3

= neutrons

= protons

= electrons

3 Different Hydrogen Isotopes

HYDROGEN – 1(protium)

1 proton0 neutrons1 electron

HYDROGEN – 2(deuterium)

HYDROGEN – 3(tritium)

1 proton1 neutron1 electron

1 proton2 neutrons1 electron

Ways to Write Isotopes

1) Hyphen notation (2 parts)

name of element – # OR symbol – #

ex: carbon-14 OR C-14 chlorine-35OR Cl-35 hydrogen-3 OR H-3

MASSNUMBER

Ways to Write Isotopes

2) Nuclear symbol (3 parts)

C14

6

elementsymbol

mass number

atomic number

Ex problem

K

Potassium

19

39.10

# protons

# neutrons

# electrons

Nuclearsymbol

Potassium-39 Potassium-40 Potassium-41

19

19

1919

19

2220 21

19

K KK39

19

41

19

40

19

Wrap Up 2

• There are three common isotopes of carbon. The most prevalent has a mass of 12 amu. The other two have masses of 13 amu or 14 amu. Write the nuclear symbols for all three of these carbon isotopes.

CCC 146

136

126

Mass of Individual Atoms

• Atoms have extremely small masses (~1×10-27 kg)

• Instead we use a relative mass – atomic mass unit (amu)

• 1 amu ≈ mass of 1 proton or 1 neutron

Comparing Subatomic Particles

particle Mass (amu)

e-

p+

n0

0.000549

1.007276

1.008665

0

1

1

Atomic Mass

• Weighted average mass of all isotopes of an element

• Where can it be found found on periodic table?

K

Potassium

19

39.10

Example: calculate atomic mass of chlorine

Isotope mass (amu) percent abundance

35Cl 35 75.770%37Cl 37 24.230%

amu

amuamu

amuamu

485.35

965.8520.26

24230.03775770.035

decimal as %isotopeisotope of mass mass Atomic

Which isotope is most abundant?Ar-38, Ar-40, or Ar-41?

Ar

Argon

18

39.948

Ar-40