Atomic Structure

Unit 7 – Physical Science

Atomic Particles

– Proton

– Neutron

– Electron -

+

N

Protons and Neutrons together are called Nucleons

Atomic Number is the number of Protons

Atomic Mass is Protons and Neutrons together

Proton

• Positively charged particle• Resides in the nucleus• Weighs 1 Atomic Mass Unit (AMU)• Repel other protons• The number of protons determines the

element number.

+

Proton

• Positively charged particle• Resides in the nucleus• Weighs 1 Atomic Mass Unit (AMU)• Repel other protons• The number of protons determines the

element number.+

+

+ +++

Which element would this be?

Proton

• Positively charged particle• Resides in the nucleus• Weighs 1 Atomic Mass Unit (AMU)• Repel other protons• The number of protons determines the

element number.+

+

+ +++

Which element would this be?

Carbon

Too Many Protons

• Protons repel each other due to the electromagnetic force

• If an element has too many protons, it will be unstable and will break apart.

++

Too Many Protons

• This is why all elements over #82 Lead are radioactive!

• In addition, none over #94 Plutonium exist naturally.

++

Neutron

• A Neutron has a neutral charge• Neutrons weigh 1.001 AMU• Resides in the nucleus• It does not affect the elemental or chemical

properties of the atom

N

Neutron

• A Neutron has a neutral charge• Neutrons weigh 1.001 AMU• Resides in the nucleus• It does not affect the elemental or chemical

properties of the atom• It does provide additional nuclear force to

hold the nucleus together

N

Isotopes• Isotopes result from different numbers of

neutrons in an element• Isotopes are chemically the same• They only differ in their mass and their

nuclear stability

N+

+ N

+

+

NN

N

N++ N+

+ N

+

+

NN

N

N++

N N

Carbon-12 C12 Carbon-14 C14

Strong Nuclear Force• The strong nuclear force is the force that holds

the nucleus together.• All Nucleons are attracted to each other.• Because + protons repel each other

(electromagnetic force), the presence of neutrons helps bind the nucleus together.

++

Electromagnetic Force

Strong Nuclear Force

+N

Strong Nuclear Force

NN

Strong Nuclear Force

Atomic Stability• The more protons there are, the more neutrons

are necessary to stabilize the nucleus.• Note that Helium has 2p/2n ratio• Lead has 82p/125n ratio

He2

4Helium

Pb82

207Lead

Atomic Mass• Atomic mass is the average of all isotopes on

Earth together.• To figure out the number of neutrons, just go

Atomic Mass-Atomic # = Neutrons• He 4-2=2 neutrons Pb 207-82 = 125 neutrons

He2

4Helium

Pb82

207Lead

Atomic Stability• The more protons there are, the more neutrons are

necessary to stabilize the nucleus.

Electrons• Electrons are negative particles• Electrons weight 0.001 AMU• They reside outside the nucleus in the electron

cloud• They usually match the number of protons to

make a neutral atom

-

+ -

This is the relative scale

Except the proton should be 10x bigger!

+ -

This is Hydrogen

H1

1Hydrogen

What is a Neutron Anyways?

• OK, so you’ve heard that Neutrons are neutral and weigh 1.001 AMU

• Do you notice any pattern here?

What is a Neutron Anyways?

• Think of a Neutron as a Proton and Electron stuck together.

What is a Neutron Anyways?

• Think of a Neutron as a Proton and Electron stuck together.

N

Beta Decay?• When an isotope has too many neutrons, a

neutron will emit a β-particle

(which is basically an electron)

• This causes the neutron to change from neutral to positive (i.e. a proton).

• This actually changes the element it is!

Beta Decay

K19

40Potassium

N

Beta DecayPotassuim-40 with 19 protons becomes

Calcium-40 with 20 protons!

K19

40Potassium

Ca20

40Calcium

β-

+

What about Electrons?

• Electrons reside outside the nucleus in the electron cloud

Models of ElectronsRutherford Model

Models of ElectronsBohr Model

Energy Levels

Energy Levels• When energy is added, electrons can jump to higher energy

shells.• When that electron returns to its normal state, that energy is

released (often as light). • That is how a fluorescent or neon light works.

Electricity is exciting air molecules

Models of ElectronsClouds

Heisenburg Uncertainty Principal

• Heinsenburg realized it is impossible to knkow both an electrons position and velocity at the same time.

• That is because the moment you detect its position, you have changed its velocity – and vice-versa.

Why the uncertainty?

Why the uncertainty?

you have changed its direction and speed.

You know where it was, but now

Quantum Mechanics• Probability densities• The electrons do not appear

to move in patterns, but simply seem to flash on and off in different locations.

• It is described as being almost like a instantaneous transporter

Models of ElectronsOrbitals