Unit 8A: Modern Physics - Atomic Theory & Structure

Atoms are the building blocks of most matter.

atom:smallest particle of matter with all the same properties

Elements

element :simplest form of matter made of only one kind of atom

hydrogen (H) helium (He) ~90% ~10% of universe of universe

Elements

Fe

heavyelementsmade instars

hydrogen (H) helium (He) ~90% ~10% of universe of universe

Elements

FeFe

heavierelementsmade fromsupernovae

You are stardust!

heavyelementsmade instars

That’s more atoms than drops of water in all lakes and rivers.

Atoms Are Small

100,000,000,000,000,000,000,000

There are about 1023 atoms in 1 gram of water (thumb size).

100 sextillion

There are more atoms in one breath of air than breathfuls of air in the whole atmosphere.

The passing waves reveal information about the ship, but nothing about the chain.

There is no diffraction for waves that pass the anchor chain.

Atoms Are Small

Atoms are smaller than the wavelengths of visible light (can’t see).

think!Does your brain contain atoms that were once part of Albert Einstein? Explain.

Answer:

Maybe!

But, they’re combined differently than before.

Atoms Are Small

think!World population grows eachYear; so, does the mass of Earth increase each year?

Answer:

Yes, by about 40,000 tons/year of space dust.

But more people does not increase the mass of the Earth b/c the atoms in our bodies are the same atoms that were here before we were born.

Atoms Are Recyclable

John Dalton experiments with chemical reactions to provide evidence for his atomic theory.

Early Atomic Theory

400 B.C. Greece

Democritus thinks of “atomos” (indivisible)

1800 England

More direct evidence for the existence of atoms is available today.

Atoms can be seen with an electron beam b/c electrons have a wavelength more than a thousand times smaller than visible light.

Early Atomic Theory

With a scanning tunneling microscope, you can see individual atoms.

1. Most of the matter in the universe is made up of what element?

A. iron

B. helium

C. carbon

D. hydrogen

Quick Quiz!

2. Compared with the wavelength of visible light, atoms are …

A. about the same size.

B. smaller.

C. a little larger.

D. fuzzier.

Quick Quiz.

3. Which of these statements is correct?

A. Atoms that make up your body were formed in ancient stars.

B. Atoms that make up your body were previously a part of other bodies.

C. Atoms that make up your body are in motion at all times.

D. ALL of the above.

Quick Quiz.

4. Ancient philosophers thought about atoms, but John Dalton developed a scientific theory of atoms by ________.

A. reading new articles

B. performing experiments

C. thinking harder and guessing

D. dancing a jig

Quick Quiz.

The Atomic Nucleus

1803 Dalton

1904 Thomson

1911 Rutherford

The Atomic Nucleus

– Rutherford’s gold foil experiment:1911

The Atomic Nucleus

Conclusions:

•atoms are mostly empty space

•nucleus: tiny, positive, massive center

(1 trillionth of the atom)(1 trillionth of the atom)

The Atomic Nucleus

nucleons: particles that make up the nucleus

• neutrons: neutral with mass = 1

• protons: positive with mass = 1

The Atomic Nucleus

atomic number: number of protons in the nucleus that identifies atoms

isotopes: atoms of the same element with differentdifferent numbersnumbers of of neutronsneutrons

The Atomic Nucleus

3 subatomic particles:

1. Most of the mass of an atom is in its …

A. isotopes

B. nucleus

C. protons

D. electron cloud

Quick Quiz!

2. Elements are identified by the …

A. mass number

B. number of protons

C. atomic number

D. electric charge

Quick Quiz.

3. Isotopes are atoms of the same element with different numbers of …

A. protons

B. electrons

C. nucleons

D. neutrons

Quick Quiz.

1803 Dalton

1904 Thomson

1911 Rutherford (nucleus)

Models visualize the subatomic world we can’t see.

Electrons in the Atom

Electrons in the Atom

Rutherford’s nuclear model couldn’t explain chemical properties and light.

So…electrons must be involved.

Hot iron changes color, but is still iron.

Electrons in the Atom

Shell model (1913, Niels Bohr)

electrons orbit the nucleus in specific energy levels (shells)

Electrons in the Atom

H (1) He (2)

Li (3)Al (13)

Electrons as Waves

Wave model (1924, Louis de Broglie)

all matter has wave properties

All particles (electrons, protons, atoms, marbles, and even humans) have a wavelength:

(mv)

(constant)

Electrons as Waves

larger mass, smaller wavelength

A tiny particle (electron) has a detectable wavelength like light.

(mv)

(constant)

(undetectable wavelength)

Recall:

The passing waves reveal information about the ship, but nothing about the chain.

There is no diffraction for waves that pass the anchor chain.

Atoms Are Small

Atoms are smaller than the wavelengths of visible light (can’t see).

Electron microscopes use the wave nature of electrons.

• The wavelength of electron beams is thousands of times shorter than those of visible light.

Electrons as Waves

So very small things (like flies) can be seen with great detail using tiny electron wavelengths.

Electrons as Waves standing wave:reinforced constructively in phase like a plucked guitar string.

standing wave:reinforced constructively in phase like a plucked guitar string.

Electrons as Waves

electrons in atoms orbit like standing waves(only specific, whole wavelengths are possible)

Electrons as Waves

1st shell

(energy level)nucleus

electrons in atoms orbit like standing waves(only specific, whole wavelengths are possible)

Electrons as Waves

The electron is visualized not as a particle at some location in the atom, but its mass and charge are spread as a standing wave around the nucleus.

energy levels 1,2,3,4 straightened out

a. Bohr’s model shows electrons orbit the nucleus like planets going around the sun.

b. de Broglie’s idea shows a wave along an orbit.

c. Quantum Mechanical model:electrons are in a 3D “cloud” of orbits of wave equations (math) and probability.

Electrons as Waves

1. The shell model of the atom views electrons as occupying…

A. energy levels.

B. three-dimensional orbitals.

C.circular or elliptical orbits.

D.standing waves.

Quick Quiz!

2. The wavelength of matter is larger when the matter has _________.

A. more momentum

B. more mass (larger)

C. less mass (smaller)

D. less intelligence

Quick Quiz.

3. The orbits of electrons around the atomic nucleus is understood by thinking of the electrons as ______.

A. standing waves

B. single particles

C. resonating vibrations

D. reflections

Quick Quiz.

4. The quantum mechanical model of the atom shows electrons orbiting in ______.

A. rings of specific energy

B. chaotic areas of chance

C.unknown locations

D. the most probableregions of space

Quick Quiz.

A useful atomic model must be consistent with a model for light.

Most of what we know about atoms we learn from the light and other radiations they emit.

Electrons and Light

Electrons and Light

Most light comes from the motion of electrons.

What are photons of light?

Are photons particles or waves of light?

There have been two primary models of light: the particle model and the wave model.

• Newton believed light was tiny particles. • Huygens believed that light was waves.

Electrons and Light

wave model reinforced by observing interference patterns of light, and…

light observed to be an electromagnetic wave with frequencies (Hertz) like radio waves.

But in 1905, Albert Einstein renewed the particle theory of light with a key observation…

Energy from the light shining on a metal plate gives electrons in the metal enough energy to escape.

The Photoelectric Effect

(animation)

High freq. light (blue/violet) CAN eject electrons

Low freq. light (red/orange) NOT eject electrons

Max Planck believed that emission and absorption of light occurred in little packets or chunks.

quantum:a packet or a basicamount of energy

Einstein proposed that light is composed of quanta.

photon:one quantum of light energy

Light Quanta

Energy from the light shining on a metal plate gives electrons in the metal enough energy to escape.• Bright light has more energy than dim light, but

dim blue light can dislodge electrons while bright red light cannot. WHY?

The Photoelectric Effect

High freq. light (blue/violet) CAN eject electrons

Low freq. light (red/orange) NOT eject electrons

Energy from the light shining on a metal plate gives electrons in the metal enough energy to escape.•brightness of light is from the number of photons•frequency of light gives the energy of each photon

The Photoelectric Effect

(many low-energy red photons will not eject an electron)

(even one high-energy blue photon will eject an electron)

Waves as Particles

Light travels like waves, but interacts with matter like particles (photons).

Classical Mechanics:One can measure and calculate exactly the momentum and position of a car or a baseball.

Quantum Mechanics

Quantum Mechanics:

subatomic motion is governed by laws of probability, not certainty.

Heisenberg Uncertainty Principle:cannot know exactly bothposition and momentumof subatomic particles

Quantum Mechanics

Classical Mechanics:

motion of large objects

governed by Newton’s laws

(macroworld)

Quantum Mechanics

Quantum Mechanics:

motion of tiny objects

governed by probability

(microworld)

1. Which of these photons is more likely to initiate the photoelectric effect?

A. red

B. green

C.blue

D.violet

Quick Quiz!

2. Which of these best illustrates the dual nature of light?

A. Light travels as a wave and interacts with solid matter like a particle.

B. Light travels as a particle and interacts with solid matter like a wave.

C.Light can interact in empty spaces as do particles, and travel around solid matter as do waves.

D.Light is wishy-washy.

Quick Quiz.

3. Subatomic interactions described by quantum mechanics are governed by…

A. Newton’s laws of motion.

B. laws of certainty.

C. laws of probability.

D.exact measurements.

E. ALL of the above

Quick Quiz.