Quantum Theory and the Atom

Bohr’s Model of the Atom

Atomic Emission SpectrumWhen atoms of different elements emit

electromagnetic waves of light, they emit only a certain set of frequencies of light that we call the

element’s atomic emission spectrum.

Identifying ElementsEach element’s atomic emission spectrum is unique and can be used to identify various elements within

compounds or even within stars.

Non-Continuous LinesNotice that the emission lines are individual lines, not a

continuous spectrum like in a rainbow.

Non-Continuous Continuous

Niels BohrIn 1913, Niels Bohr built upon Planck’s and Einstein's

concepts of quantized energy to explain the discontinuous nature of Hydrogen’s atomic emission

spectrum by proposing a quantum model for the Hydrogen atom.

Ephotons = hv

Energy States of HydrogenBohr proposed that hydrogen’s electron orbits the nucleus in circular orbits related to the amount of

energy that the electron contains.

When an atom gains energy, it is said to be in an excited state.

The lowest energy state is called its ground state

Nucleus

Ground State

Excited State

Bohr called the orbitsenergy states

Multiple Excited States

Ground State

1st Excited State

Although hydrogen has only one electron, it is capable of having many different excited states.

2nd Excited State 3rd Excited

State

4th Excited State

Energy States of HydrogenBohr assigned a quantum number, n, to each orbit and calculated the orbit’s radius. The smaller orbits were

closer to the nucleus.

Bohr’s Description of the Hydrogen Atom

Bohr Atomic Orbit Quantum Number Orbit Radius (nm) Energy Level Relative Energy

First n = 1 0.0529 1 E1

Second n = 2 0.212 2 E2 = 4E1

Third n = 3 0.476 3 E3 = 9E1

Fourth n = 4 0.846 4 E4 = 16E1

Fifth n = 5 1.32 5 E5 = 25E1

Sixth n = 6 1.90 6 E6 = 36E1

Seventh n = 7 2.59 7 E7 = 49E1

Bohr’s Explanation of Hydrogen’s Line SpectrumAccording to Bohr, when the electron is in the ground state, also called the ground state or n = 1 orbit, the

atom does not radiate energy.

When energy is added from an outside source, the electron

moves to a higher energy orbit, such as n=2.

This raises the atom to an excited state.

Bohr’s Explanation of Hydrogen’s Line SpectrumWhen the atom is in an excited state, the electron

can drop from a higher energy orbit down to a lower energy orbit.

As the electron drops down, the atom emits a photon of energy.

Bohr’s Explanation of Hydrogen’s Line SpectrumThe energy of the released photon is equal to the

difference between the energy levels associated with the two orbits.

∆E = Ehigher energy level – Elower energy level = Ephoton = hv

Bohr’s Explanation of Hydrogen’s Line SpectrumBecause only certain atomic energies are possible,

only certain frequencies of electromagnetic radiation can be emitted.

∆E = Ehigher energy level – Elower energy level = Ephoton = hv

Hydrogen’s Atomic Emission Spectrum

Wavelength = Wave Speed Frequency

Hydrogen’s Spectral LinesThe hydrogen atom not only emits photons in the

visible portion of the electromagnetic spectrum, but also in the infrared and ultraviolet portions.

Balmer Series

Lyman Series Paschen Series

Balmer SeriesThe photon emissions in the visible light range, known as

the Balmer Series, occur when electrons drop from higher energy levels into the 2nd energy level.

Balmer Series

Paschen SeriesThe photon emissions in the infrared range, known as the Paschen Series, occur when electrons drop from

higher energy levels into the 3rd energy level.

Paschen Series

Lyman SeriesThe photon emissions in the ultraviolet range, known as

the Lyman Series, occur when electrons drop from higher energy levels into the 1st energy level.

Lyman Series

Reference Table

Nanometer = billionth meter = 10-9

0.000 000 001

434 nm = 0.000 000 434 m

4.34 x 10-7 m

Bohr Model of AtomsWhile hydrogen has only one electron and thus one

ground state energy level, atoms with multiple electrons will have many energy levels when the electrons are at

ground state.

Each energy level can hold a certain amount of electrons.

Always work from the nucleus outwards.

Work clockwise, NSEW, before pairing.

X 1 2 3

4

Nucleus

1 1st Energy level

2 2nd Energy level

3 3rd Energy level

4

4th Energy level

X

Bohr Model of Atoms

X 2 3 4

The 1st Energy Level can Hold Up To 2 Electrons

Bohr Model of Atoms

The 2nd Energy Level Can Hold Up To 8 Electrons

X 1

23

4

Bohr Model of Atoms

The 3rd Energy Level Can Hold Up To 18 Electrons

X 1

23

4

Bohr Model of Atoms

The 4th Energy Level Can Hold Up To 32 Electrons

X 1

23

4

Bohr Model of Atoms

Valence ElectronsDuring chemical reactions, only the outer electrons are involved.

Therefore, these outer electrons are given a special name and are

called valence electrons.

Sodium has 1

valence electronMagnesium has 2

valence electrons

Periodic Patterns

Groups

Valence Electrons

Periods

Energy Levels