Chapter 12

Electrons in Atoms

Greek Idea

Democritus and Leucippus

Matter is made up of indivisible particles

Dalton - one type of atom for each element

Thomson’s ModelDiscovered electrons

Atoms were made of positive stuff

Negative electron floating around

“Plum-Pudding” model

Rutherford’s ModelDiscovered dense positive piece at the center of the atom

Nucleus

Electrons moved around

Mostly empty space

Bohr’s Model

Why don’t the electrons fall into the nucleus?

Move like planets around the sun.

In circular orbits at different levels.

Amounts of energy separate one level from another.

Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

Bohr’s ModelIn

crea

sing

ene

rgy

Nucleus

First

Second

Third

Fourth

Fifth

} Further away from the nucleus means more energy. There is no “in between” energy Energy Levels

The Quantum Mechanical ModelEnergy is quantized. 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.

Schrödinger derived an equation that described the energy and position of the electrons in an atom

Things that are very small behave differently from things big enough to see.

The quantum mechanical model is a mathematical solution.

It is not like anything you can see.

The Quantum Mechanical Model

Has energy levels for electrons.

Orbits are not circular.

It can only tell us the probability of finding an electron at a certain distance from the nucleus.

The Quantum Mechanical Model

The atom is found inside a blurry “electron cloud”

A area where there is a chance of finding an electron.

Draw a line at 90 %

The Quantum Mechanical Model

Atomic Orbitals

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

Within each energy level the complex math of Schrödinger's equation describes several shapes.

These are called atomic orbitals

Regions where there is a high probability of finding an electron.

There is 1 s orbital for every energy level Spherical shaped

Each s orbital can hold 2 electrons.

Called the 1s, 2s, 3s, etc.. orbitals.

S orbitals

P orbitalsStart at the second energy level

3 different directions 3 different shapes

Each can hold 2 electrons

P Orbitals

D orbitals Start at the third energy level 5 different shapes Each can hold 2 electrons

F orbitals Start at the fourth energy level Have seven different shapes 2 electrons per shape

By Energy Level First Energy

Level only s orbital only 2 electrons 1s2

Second Energy Level

s and p orbitals are available

2 in s, 6 in p 2s22p6

8 total electrons

By Energy Level Third energy level s, p, and d

orbitals 2 in s, 6 in p, and

10 in d 3s23p63d10

18 total electrons

Fourth energy level

s,p,d, and f orbitals

2 in s, 6 in p, 10 in d, and 14 in f

4s24p64d104f14

32 total electrons

By Energy Level Any more than

the fourth and not all the orbitals will fill up.

You simply run out of electrons

The orbitals do not fill up in a neat order.

The energy levels overlap

Lowest energy fill first.

1s

2s

3s

4s

5s6s

7s

2p

3p

4p

5p

6p

3d

4d

5d

7p 6d

4f

5fIn

crea

sing

Ene

rgy

Electron Configurations The way electrons are arranged in

atoms. Aufbau principle- electrons enter the

lowest energy first. This causes difficulties because of the

overlap of orbitals of different energies. Pauli Exclusion Principle- at most 2

electrons per orbital - different spins

Electron Configuration Hund’s Rule- When electrons occupy

orbitals of equal energy they don’t pair up until they have to .

Let’s determine the electron configuration for Phosphorus

Need to account for 15 electrons

1s2s 2p3s 3p 3d4s 4p 4d 4f5s 5p 5d 5f6s 6p 6d 6f7s 7p 7d 7f

1s2 2s2 2p6 3s2 3p6 4s23d104p4

What is it?

Selenium

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

Write these electron configurations Titanium - 22 electrons 1s22s22p63s23p64s23d2

•Vanadium - 23 electrons• 1s22s22p63s23p64s23d3

Chromium - 24 electrons 1s22s22p63s23p64s23d4 is expectedBut this is wrong!! Why??

Light The study of light led to the

development of the quantum mechanical model.

Light is a kind of electromagnetic radiation.

Electromagnetic radiation includes many kinds of waves

All move at 3.00 x 108 m/s = C

Parts of a wave

Wavelength

AmplitudeOrigin

Crest

Trough

Parts of Wave Origin - the base line of the energy. Crest - high point on a wave Trough - Low point on a wave Amplitude - distance from origin to crest Wavelength - distance from crest to crest Wavelength - is abbreviated Greek

letter lambda.

Frequency The number of waves that pass a

given point per second. Units are cycles/sec or hertz (hz) Abbreviated the Greek letter nu

c =

Frequency and wavelength Are inversely related As one goes up the other goes down. Different frequencies of light is

different colors of light. There is a wide variety of frequencies The whole range is called a spectrum

Radio waves

Microwaves

Infrared

Ultra-violet

X-Rays

Gamma Rays

Low energy

High energy

Low Frequency

High Frequency

Long Wavelength

Short WavelengthVisible Light

Atomic Spectrum

How color tells us about atoms

Prism White light is

made up of all the colors of the visible spectrum.

Passing it through a prism separates it.

If the light is not white By heating a gas

with electricity we can get it to give off colors.

Passing this light through a prism does something different.

Wave-Particle DualityJJ Thomson won the Nobel prize for describing the electron as a particle.

His son, George Thomson won the Nobel prize for describing the wave-like nature of the electron.

The electron

is a particle!

The electron is an energy

wave!

Confused??? You’ve Got Company!

“No familiar conceptions can be woven around the

electron; something unknown is doing we

don’t know what.”

Physicist Sir Arthur Eddington

The Nature of the Physical World

1934

The Wave-like Electron

Louis deBroglie

The electron propagates through space as an energy

wave. To understand the atom, one must

understand the behavior of

electromagnetic waves.

c = C = speed of light, a constant (3.00 x 108 m/s) = frequency, in units of hertz (hz, sec-1)

= wavelength, in meters

Electromagnetic radiation propagates through space as a wave moving at the speed of light.

Types of electromagnetic radiation:

E = h

EE = Energy, in units of Joules = Energy, in units of Joules (kg·m(kg·m22/s/s22))hh = Planck’s constant (6.626 x 10 = Planck’s constant (6.626 x 10-34-34 J·s)J·s)

= frequency, in units of hertz (hz, sec= frequency, in units of hertz (hz, sec-1-1))

The energy (E ) of electromagnetic radiation is directly proportional to the frequency () of the radiation.

Long Wavelength

=Low Frequency

=Low ENERGY

Short Wavelength

=High Frequency

=High ENERGY

Wavelength Table

…produces all of the colors in a continuous spectrum

Spectroscopic analysis of the visible spectrum…

…produces a “bright line” spectrum

Spectroscopic analysis of the hydrogen spectrum…

This produces bandsof light with definitewavelengths.

Electron transitionsinvolve jumps of

definite amounts ofenergy.