Download atomic number Protons, Neutrons, and Electrons

Atomic Structure
(P. 544-553 and P.258-267)
A. Atomic Structure
» Atomic number
» Mass Number
» Protons, Neutrons, and
Electrons
B. Isotopes
» Average atomic mass
» Radioactive Decay
A. Structure of an Atom
The atom consists of
-protons and neutrons concentrated in the
nucleus
-electrons constantly moving in a “cloud”
around the nucleus
Sub-atomic Particles
Particle Mass(amu) Charge
Proton
1.0
+1
Neutron
1.0
0
Electron
1/1836
-1
Location in Atom
Nucleus
Nucleus
Cloud surrounding nucleus
1 atomic mass unit (amu) =1.673x10-24 g
Defined to be 1/12 of the mass of a carbon atom containing
6 protons and 6 neutrons.
Atomic Number
• Atomic number = # protons in an atom
• Uniquely identifies the element
Atomic number
Symbol
Atomic mass
14
Si
28.086
Silicon
Mass Number
• Mass Number = # protons + # neutrons
• Mass Number = atomic mass rounded
to nearest integer
• # neutrons =
mass number – atomic number
Protons, Neutrons, and Electrons
6
C
12.011
Carbon: 6 protons and 6 neutrons
+ + +
+ +
8
O
15.999
Oxygen: 8 protons and 8 neutrons
29
Cu
63.546
Copper: 29 protons and 35 neutrons
Protons, Neutrons, and Electrons
• In order to have neutrally charged
atoms
# protons = # electrons
Bohr Model
• Number of protons and neutrons in
nucleus
• Electrons in cloud
Sulfur
Energy
level
Maximum
# electrons
1
2
2
8
3
18
Lewis Dot Diagram
• Valence electrons: electrons in the outermost
energy level
• Use one dot to represent each valence electron
Atomic Structure Summary
• A=P=E M=A+N
• Atomic # = # Protons = # Electrons
• Mass # = # Protons + # Neutrons
= Atomic # + # Neutrons
B. Isotopes
• Atoms of the same element with same
number of protons but different
numbers of neutrons.
• Isotope symbol:
Mass #
Atomic #
“Carbon-12”
12
6
C
B. Isotopes
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B. Isotopes
Hydrogen Isotopes
B. Isotopes
• How many protons, neutrons, and
electrons are there in the Uranium
isotopes U-235 and U-238?
B. Isotopes
• Average Atomic Mass
– Reported on the Periodic Table
– Weighted average of all naturally
occurring isotopes of that element
Avg.
(mass # )(# of atoms)  (mass # )(# of atoms)
Atomic 
total # of atoms
Mass
B. Isotopes
• About 8 out of 10 chlorine atoms are
chlorine-35. Two out of 10 are chlorine-37.
Avg.
(35 amu)(8 atoms)  (37 amu)(2 atoms)
Atomic 

10 atoms
Mass
Average atomic mass = 35.4 amu
How many isotopes can an
element have?
Can an atom have any number of
neutrons?
• There are “preferred” combinations of
neutrons and protons, at which the
forces holding nuclei together seem to
balance best.
• Light elements tend to have about as
many neutrons as protons.
• Heavy elements need more neutrons
than protons in order to stick together.
B. Radioactive Decay
• Atoms with too few or too many
neutrons can sometimes exist for a
while, but they are unstable.
• Unstable atoms are radioactive: their
nuclei change or decay by spitting out
radiation.
http://www.brainpop.com/science/energy/radioactivity/
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• Nova Science Now – Island of Stability
• http://www.pbs.org/wgbh/nova/physics/sta
bility-elements.html