Pudding and Clouds

Pictures and Notes

Use textbook as second source of information

• For info on atom: Pages 105-108

• For info on theories and models: pg 8-10

WHAT IS A MODEL?

A model (theory)

• The most accurate visual and written description/explanation of a concept based on all the known facts at the time.

• A model (theory) must be changed/modified if any new evidence is discovered

HISTORY OF THE ATOMIC MODEL

First recorded use of the term

• Democritus (4th century BC)– Universe is made of invisible units called

atoms– “unable to be divided”– Movement of atoms caused changes in

matter

Democritus

5 REQUIRED MODELS OF ATOMIC STRUCTURE

1st model (Billiard Ball)

• John Dalton (1808)– English school teacher– All atoms of a given atom are exactly alike– Solid spheres (no parts of atoms)– Revived ancient Greek notion of atom– Atoms of different elements are not exactly

the same– Compounds are made up of atoms of 2 or

more elements

Billiard Ball model

• Dalton

• Atoms are solid spheres (no parts)

• Neutral

• Different elements have different atoms

John Dalton

2nd Model (Plum Pudding)

• JJ Thompson

• Discovered the existence of electrons

• “Plum Pudding” model – “Pudding” solid, positively charged mass– “Raisins” small pieces imbedded within

pudding, negatively charged (electrons)

Plum Pudding

• J.J. Thompson

• Electrons stuck in solid material

JJ Thompson

3rd model (Nuclear)

• Ernst Rutherford

• His experiments determined that an atom is composed mainly of empty space

• At the center of an atom is a small, dense, negatively charged nucleus

• Electrons move around the nucleus

• Nucleus is still a blob, electrons move

Nuclear model

• Rutherford

• Atoms have a nucleus

• Nucleus is small,dense, and positively charged

• Electrons move around nucleus

Rutherford

4th model (solar system)

• Niels Bohr• Electrons can only be found at certain

distances from the nucleus• These distances are called shells or orbitals• Shells are similar to orbits of planets, but on

a much smaller scale• Motion of electrons is predictable

• Nucleus is still a single blob

Solar System Model

• Nucleus is same as before

• Electrons move in orbits at different distances from nucleus

Niels Bohr

5th and Current model

• Nucleus is composed of protons and neutrons• Neutrons are the same mass as protons , but

have no charge• The motion of electrons is generally random, but

always a certain distance from the nucleus.• One can never know exactly where a specific

electron is at a specific point in time.• However electrons move within a cloud. Since

the boundaries of the cloud are defined, it is known that the electron is somewhere within

Current model

• Nucleus is composed of neutrons and protons

• Electrons move about the nucleus in random paths (but keeping the same distance away from nucleus)

Schrödinger

ATOMIC PARTS

What is in an atom?

WHAT ARE THE 3 REGIONS OF ANY ATOMS?

Regions

Nucleus:Location of all protons and neutronsHas nearly all of atom’s mass Located at center of atom

Electron Cloud:Region where the electrons travel around the nucleus

Void: The space between the electron cloud and the nucleus

WHAT’S COMPOSES THE NUCLEUS AND CLOUD?

AnswerProtons:

Identical in size and mass to a neutronHas a positive electric chargeLocated in the nucleus

Neutrons:Identical in size and mass to a protonHas a no electrical chargeLocated in nucleus

Electrons:Much smaller in size and mass than other componentsHas a negative chargeOrbits the nucleus within a cloud

ELECTRON CLOUD

Electrons and shells

• Position in a shell is defined by the amount of energy an electron possesses

• Electrons seek to occupy the shell that requires the least amount of energy

Electron cloud

• Region where all electrons are found in atom

• Electrons moving too fast to individually identify, can only define space

• Electrons are like the moving blades of a fan. You can not really tell the exact position of the blade at any moment of time, but you know the blade is moving within its casing(safety bars)

Electron cloud

ELECTRIC CHARGE

Charge

• The overall charge of the normal atom is neutral

• The charge of protons is positive• The charge of electrons is negative

• 1 electron will neutralize 1 proton• The number of neutrons in a nucleus does

NOT change its charge

Number of electrons

• For a neutral atom

• The number of electrons = the number of protons

PARTICLE COMPARISON

Size comparisons of the particles

• If the nucleus were the size of a mini-marshmallow, the electrons would be moving in a cloud about 100 yards away

• The neutrons and protons are over 1000 times larger than an electron

Mass comparisonIf you were an electron weighing 150 lbs, a

proton would be about 187,000 lbs, about the weight of a commercial jet

Mass comparison between electron and proton

UNITS OF ATOMIC MASS

Why don’t we use the regular units of mass?

• The mass of an atom or its parts is incredibly small compared to our scale.

• Scientific notation (6 x 10 -23lbs) would have to be extensively used, confusion would result

• Solution was to invent a new unit on the scale of subatomic particles for mass

Atomic Mass Unit

• The unit of measurement for atomic mass

• 1 atomic mass unit (amu or au) = the mass of one neutron or proton

• 1 amu = 1/12th of the mass of the standard carbon atom

WHAT ARE PROTONS, NEUTRONS AND ELECTRONS MADE UP OF?

Parts of Parts

• Neutrons and protons are made up of smaller pieces, called quarks

• There are six types of quarks called:up, down, charm, strange, truth and beauty

Electrons are considered fundamental particles

How do we draw an atom?

• It would be very confusing to represent the electrons of an atom as a cloud or shell.

• Easier to draw electrons in orbits.

• Several electrons can occupy the same orbit or different ones

• Protons and neutrons are drawn the same except one type will be shaded or colored

Electrons and Protons

• For the atom to have an overall neutral charge, what must be true about the number of protons and electrons it has?

The Bohr model drawing of an atom

• Helium

Nitrogen

Draw a Bohr model of

a) Beryllium (4 protons, 5 neutrons, 4 electrons)

b) Boron (5 protons, 6 neutrons, 5 electrons)

c) Neon (10 protons, 10 neutrons, 10 electrons)