Electrons in Atoms

Chapter 5

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

Later Models

• Bohr – Planetary Model

• Schrodinger – Wave Mechanical Model

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

enti

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nerg

yP

oten

tial

Ene

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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

Max Capacity of Bohr Orbits

2n2n

324

183

82

21

Max # of ElectronsOrbit

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)

When Matter is Heated it Gives Off

Light

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

THE MYSTERY OF EMISSION SPECTRUMS

there are two types:

1.continuous spectrum2.bright line spectrum

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

atoms cannot emit energy continuously, rather they emit energy in precise quantities

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

more examples:

Hydrogen:

Practice Q

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

The characteristic bright-line spectrum of atom is produced by

itsa) electrons absorbing energyb) electrons emitting energyc) protons absorbing energyd) protons emitting energy

answer: b