Bohr’s Model of the Atom. Bohr’s Model Why don’t the electrons fall into the nucleus? e- move like planets around the sun. They move in circular.

Bohr’s Model of the Atom

Bohr’s Model

Why don’t the electrons fall into the nucleus? e- move like planets around the

sun. They move in circular orbits at

different levels. Amounts of energy separate

one level from another.

Bohr postulated that:

Fixed energy related to the orbit Electrons cannot exist between orbits The higher the energy level, the

further it is away from the nucleus An atom with maximum number of

electrons in the outermost orbital energy level is stable (unreactive)

How did he develop his theory?

He used mathematics to explain the visible spectrum of hydrogen gas

Radiowaves

Microwaves

Infrared .

Ultra-violet

X-Rays

GammaRays

Low energy

High energy

Low Frequency

High Frequency

Long Wavelength

Short Wavelength

Visible Light

Electromagnetic Spectrum

The line spectrum

Electricity passed through a gaseous element emits light at a certain wavelength

The colors can be seen when passed through a prism

Every gas has a unique pattern (color)

Further away from the nucleus means more energy.

There is no “in between” energy

First

Second

Third

Fourth

Fifth

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Line spectrum of various elements

Electrons orbiting closest to the nucleus are said to be in the lowest energy state called the ground state

Atoms can absorb an amount of energy This promotes an electron to a higher energy

level called the excited state This energy level is unstable and so the

electron will fall back to its ground state When it does this, the excess energy will be

emitted as light

When the e- falls from one energy level to another, an amount of energy is emitted as light

This light emitted at specific wavelengths, which corresponds to our atomic spectra

Each atom will have different electron “jumps” therefore emitting different amounts of energy as light

This creates different line spectra for various elements

Let’s watch this video

http://www.mhhe.com/physsci/chemistry/chang7/esp/folder_structure/pe/m1/s3/

More videos (view OYO)

http://www.mhhe.com/physsci/astronomy/applets/Bohr/applet_files/Bohr.html

http://highered.mcgraw-hill.com/sites/0072482621/student_view0/interactives.html#

Bohr’s Triumph

His theory helped to explain periodic law

Halogens are so reactive because it has one e- less than a full outer orbital

Alkali metals are also reactive because they have only one e- in outer orbital

Drawback

Bohr’s theory did not explain or show the shape or the path traveled by the e-.

His theory could only explain hydrogen and not the more complex atoms

The Quantum Mechanical Model

Energy is quantized – meaning it comes in chunks.

A quanta is the amount of energy needed to move from one energy level to another.

Since the energy of an atom is never “in between” there must be a quantum leap in energy.

An equation has been developed that described the energy and position of the electrons in an atom

Atomic Orbitals

Principal Quantum Number (n) = the energy level of the electron.

Within each energy level the complex math equation describes several shapes.

These are called atomic orbitals Orbitals are regions where there is a

high probability of finding an e-

S sublevel

1 s orbital for every energy level 1s 2s 3s

Spherical shaped Each s orbital can hold 2 electrons Called the 1s, 2s, 3s, etc.. orbitals

P sublevel

Start at the second energy level 3 different directions 3 different shapes Each orbital can hold 2 electrons The p Sublevel has 3 p orbitals

The D sublevel contains 5 D orbitals

The D sublevel starts in the 3rd energy level 5 different shapes (orbitals) Each orbital can hold 2 electrons

The F sublevel has 7 F orbitals

The F sublevel starts in the fourth energy level has seven different shapes (orbitals)

2 electrons per orbital

Summary

Sublevel # of Orbitals

# e- in sublevel

Starts in what energy level

s 1 2 1

p 3 6 2

d 5 10 3

f 7 14 4

Electron Configurations

The way electrons are arranged in atoms.

Aufbau principle- e- enter the lowest energy first.

This causes difficulties because of the overlap of orbitals of different energies.

Pauli Exclusion Principle- at most 2 e-per orbital with different spins

Hund’s Rule- When e- occupy orbitals of equal energy they don’t pair up until they have to .

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

Orbitals fill in order

Lowest energy to higher energy.

Adding electrons can change the energy of the orbital.

Half filled orbitals have a lower energy.

Makes them more stable.

Changes the filling order

Electron Configuration for phosphorus

The first two electrons go into the 1s orbital

Notice the opposite spins

only 13 more

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

The next electrons go into the 2s orbital

only 11 more

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next electrons go into the 2p orbital

• only 5 more

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The next electrons go into the 3s orbital

• only 3 more

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

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1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5f

• The last three electrons go into the 3p orbitals.

• They each go into separate shapes

• 3 unpaired electrons

• 1s22s22p63s23p3

Write these electron onfigurations

Titanium

Vanadium

Chromium

Copper

Electron Configurations

Titanium - 22 electrons 1s2

2s2

2p6

3s2

3p6

4s2

3d2

Vanadium - 23 electrons 1s2

2s2

2p6

3s2

3p6

4s2

3d3

Chromium - 24 electrons 1s2

2s2

2p6

3s2

3p6

4s2

3d4

Expected. But, this is wrong!!

1s2

2s2

2p6

3s2

3p6

4s1

3d5

Copper – 29 electrons 1s2

2s2

2p6

3s2

3p6

4s2

3d9

Actual configuration is

1s2

2s2

2p6

3s2

3p6

4s1

3d10

Why are chromium and copper configurations different? This gives two half filled orbitals for chromium and one

completely full and one half filled orbital for copper.

Slightly lower in energy. Remember these exceptions

Practice

Element 1s 2s 2px 2py 2pz 3s Electron ConfigurationH

HeLiCNOF

NeNa

Electron Configuration for Some Selected ElementsOrbital filling