Electrons in Atoms

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Transcript Electrons in Atoms

Electrons in Atoms
Chapter 5
What were early steps in
development of atomic theory?
• John Dalton – Billiard Ball Theory
– Atom was indivisible
• J.J. Thomson – Plum Pudding Model
– Atom was composed of smaller particles
Rutherford Model
• nucleus contains:
– all the positive charge & most of mass of atom
• nucleus very small:
– only 1/10,000th of atomic diameter
• electrons occupy most of volume
Later Models
• Bohr – Planetary Model
• Schrodinger – Wave Mechanical Model
Problems with the Rutherford
Model
• Why don’t electrons crash into nucleus?
• How are electrons arranged?
• Why do different elements exhibit different
chemical behavior?
• How is atomic emission spectra produced?
Atomic Emission Spectra
• gas in glass tube & apply voltage across ends
– produces light
• color of light depends on gas in tube
• every element produces its own unique color
emission spectrum of
element is set of
frequencies
(or wavelengths)
emitted
Why is emission spectra useful?
• use it to determine if given element is
present in sample
• Neon lights
Emission & Absorption Spectra
of Elements
Bohr Model
• Bohr - electrons in atom can have only specific
amounts of energy
NEW idea!
• each specific amount energy is associated with
specific orbit
– electrons restricted to these orbits
• Bohr assigned quantum number (n) to each orbit
– the smallest orbit (n= 1)
• closest to nucleus
• has lowest energy
– larger the orbit, more energy it has
Bohr Diagram
• Shows all the electrons in orbits or
shells about the nucleus.
n=3
n=2
n=1
E3 n=3
E2 n=2
E1 n=1
Bohr Model
• energy absorbed when electron:
– moves to higher orbit (farther from nucleus)
– endothermic process
• energy released when electron:
– drops to lower orbit (closer to nucleus)
– exothermic process
energy levels get closer
together the farther away
they are from nucleus
Larger orbits can hold more electrons
Max Capacity of Bohr Orbits
Orbit
Max # of Electrons
1
2
2
8
3
18
4
32
n
2n2
Electron Transitions
• If electron gains (absorbs) specific amount
of energy
– it can be excited to move to higher energy level
• If electron loses specific amount of energy
– it drops down to lower energy level
Hydrogen has 1 electron, but it can make many
possible electron transitions
Absorption & Emission
• cannot easily detect absorption of
energy by electron
BUT
• can easily detect emission of energy by
electron
– photons (light) given off as excess energy
is released
Emitted Light
• energy of emitted light (E = h
– matches difference in energy between 2
levels
• don’t know absolute energy of energy
levels, but
– observe light emitted due to energy changes
Potential Energy
ladder often used as
analogy for energy levels
of atom
How is this one different?
Ground State vs. Excited State
• Ground state:
– lowest energy state of atom
– electrons in lowest possible energy levels
• configurations in Reference Tables are ground state
• Excited state:
– many possible excited states for each atom
– one or more electrons excited to higher energy
level
Success of Bohr’s Model
• Bohr’s model could predict frequencies in
emission spectrum of hydrogen
• Predicted correct size of H atom
• Unfortunately, didn’t work for anything with
more than 1 electron
Which principal energy level of an
atom contains electron with the
lowest energy?
a)
b)
c)
d)
n=1
n=2
n=3
n=4
What is total # of occupied
principal energy levels in atom
of neon in ground state?
a)
b)
c)
d)
1
2
3
4
What is total # of fully occupied
principal energy levels in atom of
nitrogen in ground state?
a)
b)
c)
d)
1
2
3
4
What is total # of electrons in
completely filled fourth principal
energy level?
a)
b)
c)
d)
8
10
18
32
Which atom in ground state has
five electrons in its outer level
and 10 electrons in its kernel?
a)
b)
c)
d)
C
Cl
Si
P
Which electron configuration
represents atom in excited state?
a)
a)
b)
c)
2-8-2
2-8-1
2-8
2-7-1
Which electron configuration
represents atom of Li in an
excited state?
a)
b)
c)
d)
1-1
1-2
2-1
2-2
The characteristic bright-line
spectrum of atom is produced by its
a)
b)
c)
d)
Electrons absorbing energy
Electrons emitting energy
Protons absorbing energy
Protons emitting energy