Do you remember the 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 atom’s volume
Problems Rutherford’s Model Didn’t Address
• 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?
Bohr Model• Bohr - electrons in atom can have only specific
amounts of energy NEW idea!
• each specific orbit is associated with specific amount energy – 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
(shells) around the nucleus
n=1n=1
n=2n=2n=3n=3
E1
E2
E3 n=3n=2
n=1
Bohr Model
• energy absorbed when electron:– moves from lower to higher energy orbit
(goes farther from nucleus)– endothermic process
• energy released when electron:– drops from higher to lower energy orbit
(gets closer to nucleus)– exothermic process
Pot
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ladder often used as ladder often used as analogy for energy levels analogy for energy levels of atomof atom
How is this one different? How is this one different?
nucleus
energy levels get closer together the farther away they are from nucleus – not uniformly spaced
larger orbits can hold more electrons
Electron Transitions
• If electron gains (absorbs) specific amount of energy– it becomes excited & can move to higher
energy level
• If electron loses specific amount of energy– it drops down to lower energy level– it gives off photon of light (color depends on
wavelength of light given off)
Emitted Light
• energy of emitted light (E = h– matches differencedifference in energy between 2
electron levels
• don’t know absolute energy of energy levels, but – observe light emitted due to energy
changes
Example: fire works, pyrotechnics, flame test
–heat energy absorbed by the metal ions excites the atoms’ electrons–absorbed energy is eventually released in the form of light
Example: light bulb– electrical energy absorbed
by the filament excites the atoms’ electrons
– absorbed energy is eventually released in the form of light
• How this works:
– electrons absorb energy, get EXCITED, and “jump” to a higher energy level
– after a brief time, they “fall” back to a lower energy level, giving off a specific amount of energy (a quantum amount) in the form of a photon (colored light)
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
Absorption & Emission
• cannot easily detect absorption of energy by electron
BUT
• can easily detect emission of energy by electron– SEE: photons of light given off as excess
energy is released
Continuous Spectrum
Solids, liquids, and dense gases emit light of all wavelengths, without any gaps
thin gases emit light of only a few wavelengths so see lines of color separated with gaps between them
Bright Line Spectrum
Atomic Emission Spectra(AKA: bright line spectra)
• apply voltage across ends of glass tube containing gas
- light is produced
• color of light depends on gas in tube
• every element produces its own unique color
our eyes see ONE color in the gas spectrum tube, however, if we use a prism we can see that each “color” is really multiple wavelengths of different colors
Hydrogen has 1 electron, but it can make many possible electron transitions
Which principal energy level of an atom contains electron(s)
with the lowest energy?
a) n=1b) n=2c) n=3d) n=4
answer: a
What is total # of occupied principal energy levels in atom of neon in ground
state?
a) 1b) 2c) 3d) 4
neon has 10 electrons: 1st shell: 22nd shell: 8
answer: b
What is total # of fully occupied principal energy levels in atom
of nitrogen in ground state?
a) 1b) 2c) 3d) 4
nitrogen has 7 electrons:1st shell: 22nd shell: 5
answer: a
What is total # of electrons in completely filled fourth principal energy level?
a) 8b) 10c) 18d) 32
2n2
2(42) = 32
answer: d
Which atom in ground state has five electrons in its outer level and 10 electrons in its
kernel?
a) Cb) Clc) Sid) P
15 electrons total:
phosphorus
answer: d
Which electron configuration represents atom in excited
state?
a) 2-8-2b) 2-8-1c) 2-8d) 2-7-1
answer: d
Which electron configuration represents atom of Li in an
excited state?
a) 1-1b) 1-2c) 2-1d) 2-2
Li has 3 electrons
answer: b