Chapter #9Atomic Theory

Quantum Model of the Atom

Physics Review• J.J. Thompson determined all mater contains electrons

with cathode ray tube experiments.• Rutherford demonstrated that atoms contain mostly empty

space by bombarding metals with alpha particles (helium nuclei)

• Previously we talked about the plumb pudding model of an atom and Rutherford’s planetary model.

• Then along came Bohr who applied Planck’s the ideas of energy quantization to Rutherford’s planetary model of an atom.

The circular orbits of the Bohr model are characterized by the principle quantum number, n, which has positive integer values. e.g.1, 2, 3….

Evidence for Quantization of Energy

Energy is anything that has the capacity to do work.

Energy Examples:

Food; how much energy for a heart beat?

Gasoline; how many $ goes to heat and how much to work?

Dynamite; what can it move?

Electricity; what can it move?

Apple on a tree; what can it move?

About Energy

Rutherford proposed the electrons were located in orbits around the nucleus similar to planets around a star. This is sometimes called the planetary model, which is not the modern day model of an atom.

Li atom3p, 3n, 3e-

The Bohr Model of an Atom

Neils Bohr developed a mathematical model based on Rutherford's proposal using Planck’s quantized energy levels.

In Bohr’s model electrons could only have fixed Potential energy levels.

The Bohr Model of an Atom

White light is the collection of all of the colors in the rainbow. When white light is passed through a glass prism the waves are bent by the glass. The shortest (most energetic) are bent the most.

Classification of Electromagnetic Radiation

Potential and Kinetic Energy

Potential energy; energy due to position above the ground.

Kinetic energy; energy due to matter in motion.

Electromagnetic energy; energy possessing both particle and wave properties traveling at the speed of light.

Units of energy, joule, or calorie.

Properties of Waves

Wavelength (m)Amplitude (m)Speed 3.0X108

Energy (j)m/s

Frequency (1/s, Hz)

Electromagnetic Radiation

10-12 10-10 10-8 10-7 10-4 10-2 100 102

Gamma rays X- rays Uv-rays Visible rays

Infrared rays

MicroWaves(Radar)

Radio and Television waves

Increasing wave length in meters

Increasing energy

Visible Radiation Passing Through a Prism

Continuous Spectrum

Evidence for Quantization of Energy

The red-orange light from hydrogen gas passes through a prism to form a line spectra. Each different colored light has its own unique energy. This is called an emission spectra

Line Spectrum

Additional Emission Line Spectra

Note: Each element has its own unique emission spectra (Finger print)

Emission vs. Absorption SpectraEarly on physicists theorized that light emitted by the sun should be a continuous spectrum. They were troubled by black lines when observing the suns spectrum through a glass prism. Bunsen (Bunsen burner) and Kirchoff studied emission spectra from emission tubes containing various gaseous elements. Interesting the wavelengths of the black lines match know wave lengths of different elements. This lead physics to conclude that gases surround the sun that absorb the emitted wave lengths of the sun.

Absorption Spectra

A turtle on stairs may only be at specific heights. Its potential energy is quantized.

Energy levels in atoms are quantized.

Quantization means no in between

Anything that comes in units such as: stairs television channels, gears, and bookshelves are quantized.

What is Quantization

Coming to class you increase your potential energy one quanta (step) at a time.

Quantization

Are you quantized right now?

Evidence for Quantization of Energy

The circular orbits of the Bohr model are characterized by the

principle quantum number, n, which has positive integer values.

e.g.1, 2, 3….

Evidence for Quantization of Energy

In terms of the Bohr model absorption and emission looks like

this.

Evidence for Quantization of Energy

Electrons move between energy levels by absorbing and emitting energy in the form of light.

We call the lowest energy level the ground state. The higher energy level is called the excited state.

Evidence for Quantization of Energy

The Bohr model works well for the hydrogen atom which has only one electron but performs poorly for more complex atoms.

This led to the development of the current quantum mechanical model describing the arrangement of electrons in atoms.

Unfortunately for us this model is more complex than that developed by Neils Bohr.

Evidence for Quantization of Energy

The location of an electron in an atom can be described in order of precision by its:

• shell (a positive integer given the symbol n = 1,2,3,….)

• subshell (Orbital) (designated by letters s, p, d or f)

• orientation (orbital symmetrical on x, y, or z axis)

•Spin (clockwise or counter clockwise)

Like an electrons Social Security number: 4px

+1/2

Quantum Numbers

As the value for n of a shell increases its energy and distance from the nucleus increases. This is similar to the Bohr model.

Each shell has a number of subshells equal to its value for n (up to a maximum of 4).

e.g. A shell with n = 1 will have one subshell (s)

A shell with n = 2 will have two subshells (s,p)

A shell with n = 3 will have three subshells (s,p,d)

A shell with n = 4 will have four subshells (s,p,d,f)

A shell with n = 5 will have four subshells (s,p,d,f,g)

Quantum Numbers

Each subshell is designated with the letter s, p, d or f.

The subshells are named by putting the value of n in front of the

symbol for the subshell.

e.g. a p subshell in the second shell is named 2p

The subshells vary in order of energy s < p < d < f.

The difference in energy between subshells is much smaller than the difference in energy between shells. Just like the space between buildings is smaller than the space between streets.

Quantum Numbers

As mentioned earlier each shell has a number of subshells equal

to its value for n.

Therefore for:

• n = 1 there will be one subshell 1s

• n = 2 there will be two subshells 2s and 2p

• n = 3 there will be three subshells 3s, 3p and 3d

• n = 4 there will be four subshells 4s, 4p, 4d and 4f

• n = 5 there will be four subshells 5s, 5p, 5d and 5f

Quantum Numbers

Each subshell is made up of one or more orbitals.

An orbital is a volume of space where an electron is likely to be found.

What is the orbital for books called? Fish? Cars?

It is important not to confuse an orbit (a circular path on which an electron moves in the Bohr model) with an orbital. They are two very different things.

Quantum Numbers

An s subshell has one orbital which is spherically shaped.

If you were to measure where the electron was within an

s subshell many, many times and plot the results on a

graph you would get something like this.

Quantum Numbers

The p-orbital is next in energy after the s-orbital.The p-orbital has a dumbbell shape with electrons located either side of the nucleus in tear drop shaped lobes.

There are three types of p-orbitals all having identical shape and energy directed along the x, y and z axis.

Quantum Numbers

Next in energy are the d-orbitals of which there are five all with the same energy. The different d-orbitals do have different shapes as well orientation. These are followed by the seven f-orbitals.

Quantum Numbers

The way in which electrons are organized into shells, subshells and orbitals in an atom is called the electronic configuration.

The electronic configuration of an atom can be determined using the “Aufbau rule” also known as the “building up principle”.

Aufbau comes from the German meaning construction although it was the Danish physicist Neils Bohr who came up with the idea !!

Quantum Numbers

The Aufbau Principle states that:“The orbitals of lower energy are filled in first with the electrons and only then the orbitals of high energy are filled.”

What is the lowest energy orbital of an atom?

1s orbital

What is the third lowest energy orbital of an atom?

2p orbital

Quantum Numbers

As we have seen previously for p, d and f subshells there are multiple orbitals with the same energy, called degenerate orbitals.

In particular:

• p subshells have three orbitals with the same energy

• d subshells have five orbitals with the same energy

• f subshells have seven orbitals with the same energyEach of these orbitals may accommodate a maximum of two electrons.

Quantum Numbers

If there are multiple orbitals with the same energy how do we decide which orbital to put an electron?

We use Hund’s rule which states:

“Electrons fill degenerate orbitals one at a time before doubling up in the same orbital”

Quantum Numbers

Using Hund’s rule how would we put three electrons in a p subshell ?

px pzpy

p subshell

px pzpy

p subshell

Quantum Numbers

When we do put two electrons in one orbital then they obey the Pauli exclusion principle.

“only electrons with opposite spin can occupy the same orbital”

px pzpy

p subshell

px pzpy

p subshell

Quantum Numbers

2s subshell

1s subshell

How would we use our rules to “build up” the electron configuration of a Li atom?

Li has Z = 3 so has 3 e-.

We can write this in shorthand as 1s22s1

Quantum Numbers

2s subshell

1s subshell

How would we use our rules to “build up” the electron configuration of a N atom?

N has Z = 7 so has 7 e-.

We can write this in shorthand as 1s22s22p3

2p subshell

Quantum Numbers

Beyond the 3p subshell the orbitals don’t fill in an obvious way.

For example the 4s level lies lower in energy than the 3d .

Quantum Numbers

There is an easy way to

remember the sequence of the

energies of the subshells.

Diagonal Rule

So now we have everything we need to determine the electronic configuration of any atom

What is the electronic configuration of a Na atom?

Z = 11, Na has 11 e-

1s2 2s2 2p6 3s1

Quantum Numbers

Start here

The electrons in the highest energy shell of an atom are (those

furthest from the nucleus) are called the valence electrons.

These electrons are very important as when atoms interact with

each other it is through their valence electrons.

Quantum Numbers

Diagonal Rule Within the Periodic Chart

Periodic Chart

PERIODIC TRENDS

1. Atomic size

a. Increases from top to bottom (gaining a new outer shell)

b. Decreases left to right in a period (Increase of protons attracts electrons stronger, thus contracting the element.

2. Ease of ionization (relative ease of losing electrons)

1. The larger the atom, the further electrons are from nucleus, and the less they are held by nucleus.

3. Metals and nonmetals

a. Metallic character (increases going away from metals)

b. Nonmetallic character

Periodic Trends

Metals, Nonmetals, and Mettaloids

Chapter #9Review

How many electrons fill the following sublevels?

a. The 3rd level.b. The 4th level.c. The 5th level

How many electrons fill the following sublevels?

a. The 3rd level. 2(n)2 =2(3)2=18b. The 4th level.c. The 5th level

How many electrons fill the following sublevels?

a. The 3rd level. 2(n)2 =2(3)2=18b. The 4th level. 2(n)2 =2(4)2=32 c. The 5th level

How many electrons fill the following sublevels?

a. The 3rd level. 2(n)2 =2(3)2=18b. The 4th level. 2(n)2 =2(4)2=32 c. The 5th level 2(n)2 =2(5)2=50

How many orbitals are in an atom with n=3?

How many orbitals are in an atom with n=3?

n = 3 refers to the third energy level, which contains the s, p, and d sublevels. There is only one orbital in the s, three in the p, and five in the d sublevels; for a total of 9 orbitals.

Answer:

Identify the element with the following electron configurations, also stating the group and period the where the element where found.

a. 1s22s2p63s23p64s23d104p4

b. [Ar]3s23d7

Identify the element with the following electron configurations, also stating the group and period the where the element where found.

a. 1s22s2p63s23p64s23d104p4

Se, Period 4, Group VI (6)

b. [Ar]3s23d7

Identify the element with the following electron configurations, also stating the group and period the where the element where found.

a. 1s22s2p63s23p64s23d104p4

Se, Period 4, Group VI (6)

b. [Ar]4s23d7 Co, Period 4, Group 9

The End