The Structure of the Atom

Honors ChemistrySection 3.2

Demonstrate the Think Tube

The Think Tube

The smallest particle of an element that still retains the properties of that element.

Made of two regionsNucleus

In center of atomExtremely smallMajority of Mass of AtomProtons/Neutrons

Electron CloudContains electrons

Atom

The Atom

Parts of the Atom

Video

Properties of Subatomic Particles

How did we find the electron?J.J. Thomson – late 1800’sUse the Cathode Ray Tube

Thomson's Experiment

Thompson's Experiment

Video

Cathode Rays move away from a negative charge

Cathode Rays move away from a magnetic field in the same manner as a wire carrying a negative charge

Found the Cathode Rays have mass – could move a paddle wheel in their path

Charge to mass ratio of Cathode Rays the same regardless ofType of metal used in electrodesType of gas used in tube

Thomson's Observations

Cathode Rays are negatively chargedCathode Rays must be a fundamental particle

of matterCalculated the charge to mass ratio of the

particlesParticles later named electrons

Thomson's Conculsions

Robert Millikan

Millikan's Experiment

Video

Determined the fundamental charge on matter (the electron)

Used Thomson’s charge to mass ratio to determine the mass of an electron

Verified that electrons are negative

Millikan's Results

Atom is divisibleOne of the basic subatomic particles is the

negatively charged electronAtom is electrically neutral, so it must contain

positive charges to balance out the electronsElectrons have an extremely small mass,

therefore there must be other massive particles in the atom

Results

Thomson's and Millikan's Model

Demo the Cloud Chamber

Radiation Demonstration

Ernest Rutherford - 1911

α particles are positively charged and massiveHe nucleus

Rutherford's Results

Rutherford's Experiment

Video

Nucleus existed at the center of the atomSmall – extremely smallMassiveDensely PackedPositive

Electrons are in orbit around the nucleus.

Rutherford's Conclusions

All nuclei (execpt H) have protons and neutrons

Proton – positiveCharge equal and opposite the electronMass about the same as a neutron1.673 x 10-27 kg (1836 x the mass of an

electron)Neutron – electrically neutral

1.675 x 10-27 kgAtom neutral so # protons = # electronsThe number of protons determines the

identity of the element

Early 1900's Atomic Theory

When two protons are extremely close to each other, there is a strong attraction between them.

A similar attraction exists when neutrons are very close to each other or when protons and neutrons are very close together.

The short-range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together are referred to as nuclear forces.

Forces in the Nucleus

Nuclear Forces

• The radius of an atom is the distance from the center of the nucleus to the outer portion of its electron cloud.

• Because atomic radii are so small, they are expressed using a unit that is more convenient for the sizes of atoms.

• This unit is the picometer, pm.

The Size of an Atom