AtomAtom

Definition: the Definition: the smallest particle smallest particle of any element of any element that retains the that retains the properties of properties of that element.that element.

Practice quiz question:Practice quiz question:

What is the smallest particle of What is the smallest particle of magnesium that retains the magnesium that retains the properties of magnesium?properties of magnesium?

Interesting side note…Interesting side note…

Atoms are fantastically durable. Atoms are fantastically durable. Because they last so long, atoms Because they last so long, atoms really get around!really get around!

Every atom you possess has Every atom you possess has almost certainly passed through almost certainly passed through several stars and been part of several stars and been part of millions of organisms on its way millions of organisms on its way to becoming you.to becoming you.

Interesting side note…Interesting side note…

It has been suggested It has been suggested that up to a billion of that up to a billion of each person’s atoms each person’s atoms once belonged to once belonged to Shakespeare. Another Shakespeare. Another billion came from billion came from Beethoven. You Beethoven. You probably have some probably have some atoms from any atoms from any historical figure you historical figure you can think of!can think of!

Nowadays, with the use of tunneling Nowadays, with the use of tunneling electron microscopes, we can sort of electron microscopes, we can sort of

“see” atoms…“see” atoms…

image of copper atoms

This is an image of silicon atoms arranged This is an image of silicon atoms arranged on a face of a crystal. It is impossible to on a face of a crystal. It is impossible to "see" atoms this way using ordinary light. "see" atoms this way using ordinary light. The image was made by a Scanning The image was made by a Scanning Tunneling Microscope, a device that "feels" Tunneling Microscope, a device that "feels" the cloud of electrons that form the outer the cloud of electrons that form the outer surface of atoms, like a phonograph needle surface of atoms, like a phonograph needle feels the grooves in a record.feels the grooves in a record.

These electron microscopes These electron microscopes were invented in the mid-1900’s. were invented in the mid-1900’s.

How did scientists figure out How did scientists figure out atomic structure without being atomic structure without being

able to look at any atoms?able to look at any atoms?

The History of Atomic TheoryThe History of Atomic Theory

But first…But first…

What is a What is a theorytheory in science? in science?

A theory is a well-tested A theory is a well-tested explanationexplanation of of what happens in nature.what happens in nature.

In layman’s terms, if something is said In layman’s terms, if something is said to be “just a theory,” it usually means to be “just a theory,” it usually means that it is a mere guess, or is that it is a mere guess, or is unproven. But in scientific terms, a unproven. But in scientific terms, a theory theory implies that something has implies that something has been well-tested and verified many been well-tested and verified many times by multiple groups of scientists. times by multiple groups of scientists.

And while we’re at it…And while we’re at it…

A scientific A scientific lawlaw is a statement of is a statement of something that seems to be true in something that seems to be true in the natural world.the natural world.

Example: The Example: The LawLaw of Gravity. Newton of Gravity. Newton could use this law to predict the could use this law to predict the behavior of a dropped object, but he behavior of a dropped object, but he couldn't explain why it happened. couldn't explain why it happened.

Einstein’s Einstein’s TheoryTheory of Relativity of Relativityexplains gravity as a curve in the explains gravity as a curve in the

fabric of space-time:fabric of space-time:

Remember!Remember!

A law states or describes what A law states or describes what happens in nature.happens in nature.

A theory explains what happens.A theory explains what happens.

Both are Both are well-testedwell-tested by experiments. by experiments.

Memory hint: “explanation starts Memory hint: “explanation starts with an “e” and “theory” has an with an “e” and “theory” has an “e”.“e”.

True or False?True or False?

The difference between a The difference between a scientific law and a scientific scientific law and a scientific theory is that a law has been theory is that a law has been proven, but a theory has not proven, but a theory has not been proven.been proven.

The History of Atomic TheoryThe History of Atomic Theory

DemocritusDemocritus

ancient Greek philosopher, not scientistancient Greek philosopher, not scientist

first to suggest that matter is made of tiny particles first to suggest that matter is made of tiny particles called “atomos”called “atomos”

““atomos” = Greek for “indivisible”atomos” = Greek for “indivisible”

ideas were rejected by Aristotle, who was very ideas were rejected by Aristotle, who was very influential, and therefore forgotten for two thousand influential, and therefore forgotten for two thousand years.years.

Democritus’ atomic model:Democritus’ atomic model:atoms were small, hard particles that atoms were small, hard particles that

were different shapes and sizeswere different shapes and sizes

John Dalton:John Dalton:

First to propose an atomic theoryFirst to propose an atomic theory based on science: based on science:1. Each element is composed of small 1. Each element is composed of small particles called atoms.particles called atoms.2. All atoms of a given element are identical*; 2. All atoms of a given element are identical*; the atoms of different elements are different.the atoms of different elements are different.3. Atoms are not created or destroyed in 3. Atoms are not created or destroyed in chemical reactions.chemical reactions.4. Compounds are formed when atoms of 4. Compounds are formed when atoms of more than one element combine.more than one element combine.

John Dalton:John Dalton:First to propose an atomic theoryFirst to propose an atomic theory based on science: based on science:1. Each element is composed of small particles called 1. Each element is composed of small particles called atoms.atoms.2. All atoms of a given element are identical*; the atoms of 2. All atoms of a given element are identical*; the atoms of different elements are different.different elements are different.3. Atoms are not created or destroyed in chemical reactions.3. Atoms are not created or destroyed in chemical reactions.4. Compounds are formed when atoms of more than one 4. Compounds are formed when atoms of more than one element combine.element combine.

* We now know this part is not true!* We now know this part is not true!

Dalton’s atomic model:Dalton’s atomic model:a tiny sphere that is indivisiblea tiny sphere that is indivisible

1.1. All matter is composed of All matter is composed of extremely small particles extremely small particles called atomscalled atoms

2.2. Atoms of a given Atoms of a given element are identical in element are identical in size, mass, and other size, mass, and other properties; atoms of properties; atoms of different elements differ different elements differ in size, mass, & other in size, mass, & other propertiesproperties

ELEMENT2

ELEMENT2

ELEMENT 3

ELEMENT 3

ELEMENT 1

ELEMENT 1

ELEMENT4

ELEMENT4

Atomic TheoryAtomic Theory

3.3. Atoms cannot be Atoms cannot be created or created or destroyeddestroyed

4.4. Atoms of different Atoms of different elements combine elements combine to form compoundsto form compounds

5.5. In chemical rxns, In chemical rxns, atoms are atoms are combined, combined, separated, or separated, or rearrangedrearranged

++++ ++++

Atomic TheoryAtomic Theory

J.J. Thomson:J.J. Thomson:

First to suggest that there were particles smaller First to suggest that there were particles smaller than the atom.than the atom.

Discovered the electron.Discovered the electron.

Developed the “plum pudding model.”Developed the “plum pudding model.”

Thomson’s “plum-pudding” model:Thomson’s “plum-pudding” model:the atom is composed of electrons surrounded by a the atom is composed of electrons surrounded by a

cloud of positive charge to balance the electron's cloud of positive charge to balance the electron's

negative chargenegative charge

Ernest RutherfordErnest Rutherford

gold foil experimentgold foil experiment

discovered the nucleus, and realized discovered the nucleus, and realized that it was very dense and positively that it was very dense and positively chargedcharged

suggested that most of the atom is suggested that most of the atom is empty spaceempty space

Gold Foil ExperimentGold Foil Experiment::Rutherford shot tiny positively-charged Rutherford shot tiny positively-charged alpha particles through a thin sheet of alpha particles through a thin sheet of

gold foilgold foil

What Rutherford expected:

What really happened:

Gold Foil ExperimentGold Foil Experiment::Some of the alpha particles bounced Some of the alpha particles bounced back. “It was as if you fired a 15-inch back. “It was as if you fired a 15-inch cannon ball at a piece of tissue paper cannon ball at a piece of tissue paper

and it came back and hit you.”and it came back and hit you.”

What Rutherford expected:

What really happened:

Such huge deflections could Such huge deflections could mean only one thing: some of mean only one thing: some of

the alpha particles had run into the alpha particles had run into massive concentrations of massive concentrations of

positive charge and, since like positive charge and, since like charges repel, had been hurled charges repel, had been hurled

straight back by them. straight back by them.

Rutherford’s atomic model:Rutherford’s atomic model:All of the positive charge is crammed inside a tiny, All of the positive charge is crammed inside a tiny, massive massive nucleusnucleus about ten thousand times smaller about ten thousand times smaller

than the atom as a whole. The atom is mostly empty than the atom as a whole. The atom is mostly empty space. space.

Animation of Rutherford’s Animation of Rutherford’s experimentexperiment

http://www.mhhe.com/physsci/http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/chemistry/essentialchemistry/flash/ruther14.swfruther14.swf

James ChadwickJames Chadwick

Scientists were looking forScientists were looking for

missing mass – the protonsmissing mass – the protons

and electrons in the atom didn’tand electrons in the atom didn’t

weigh enough to account for the weigh enough to account for the mass of the atommass of the atom

proved the existence ofproved the existence of

neutronsneutrons

Chadwick’s model of the atom:Chadwick’s model of the atom:

Protons and neutrons are in the nucleus, which only takes up one millionth of a billionth of the volume of the atom. The electrons arein the electron cloud, which constitutes almost all of the volume of the atom.

In fact, if an atom were expanded In fact, if an atom were expanded to the size of a cathedral,to the size of a cathedral,

the nucleus would be about the size of a housefly!

Subatomic Particles to KnowSubatomic Particles to Know

SubatomiSubatomicc

ParticleParticle

LocationLocation MassMass ChargeCharge

ProtonProton NucleusNucleus 1 amu1 amu +1+1

NeutronNeutron NucleusNucleus 1 amu1 amu 00

ElectronElectron ElectronElectron

CloudCloudPracticallyPractically

zerozero-1-1