Early Greek Theory

All matter was composed of four elements: fire, air, water and earth.

Stone = mostly earth Rabbit = mostly water and fire Theory that certain substances are

made up of a combination of “elements”

Greek Model

Greek philosopher 460-371 b.c. Idea of ‘atomos’

Atomos = ‘indivisible’ ‘Atom’ is derived

No experiments to support idea

Democritus’s model of atom

No protons, electrons, or neutrons

Solid and INDESTRUCTABLE

Democritus“To understand the very large,

we must understand the very small.”

Early Ideas on Elements

Robert Boyle stated... A substance was an element unless

it could be broken down to two or more simpler substances.

Air therefore could not be an element because it could be broken down in to many pure substances.

Subatomic particles

Electron

Proton

Neutron

Name Symbol ChargeRelative mass

Actual mass (g)

e-

p+

no

-1

+1

0

1/1840

1

1

9.11 x 10-28

1.67 x 10-24

1.67 x 10-24

Structure of the Atom

There are two regions1. The Nucleus

With protons and neutrons Positive charge Almost all the mass

2. Electron Cloud Almost all the volume Where electrons are found

Single Atom

Water Molecule

The Sad State of Chemistry Humor

Two atoms are walking down the street.One atom says to the other, “Hey! I think I lost an electron!”The other says, “Are you sure??”“Yes, I’m positive!”

A neutron walks into a restaurant and orders a couple of drinks. As she is about to leave, she asks the waiter how much she owes. The waiter replies, “For you, No Charge!!!”

Size of an atom

Atoms are incredibly tiny.Measured in picometers (10-12 meters)

Hydrogen atom, 32 pm radius

Nucleus tiny compared to atomRadius of the nucleus near 10-15 m.Density near 1014 g/cm3

IF the atom was the size of a stadium, the nucleus would be the size of a marble.

California WEB

The Atomic Theory of Matter In 1803, Dalton proposed that elements

consist of individual particles called atoms.

His atomic theory of matter contains five hypotheses:

Elements are made of tiny particles called atoms. All atoms of a given element are identical.

The atoms of a given element are different from those of any other element

Atoms of one element can combine with atoms of another element to form compounds. A given compound always has the same relative number and types of atoms.

Atoms are indivisible in chemical processes. Atoms are not created or destroyed in chemical reactions. A chemical reaction simply changes the way the atoms are grouped together.

Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.

Legos are Similar to Atoms

Lego's can be taken apart and built into many different things.

H

H

O

O

O

O

H

H

H

H

H

HH2

H2

O2

H2O

H2O

+

Atoms can be rearranged to form different substances (compounds).

Chemical Formulas

A compound contains atoms from two or more elements

Always contains the same relative numbers of atoms of each element

The types and number of atoms is expressed in a chemical formula

Carbon & Oxygen

1 Carbon atom 2 Oxygen

atoms

CO2

CO2

Practice

Try These: NH3 2H2O

Mg(NO2)2

NH3 : 1 Nitrogen, 3 Hydrogen atoms

2H2O : (2 water molecules)

4 H, 2 O atomsMg(NO2)2 : 1 Mg, 2 N, 4 O

atoms

More Practice

P4O10

12 H atoms

4 Au, 6 C, and 18 O atoms

Write the chemical formula for four phosphorus atoms and ten oxygen atoms

Write the number of hydrogen atoms in (NH4)2C8H4O2

Write the number and types of atoms in 2Au2(CO3)3

J.J. Thompson’s A Cathode Ray Tube

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 58

Cathode RayExperiment

Thompson’s 1897 Experiment Using a cathode ray tube,

Thomson was able to deflect cathode rays with an electrical field.

The rays bent towards the positive pole, indicating that they are negatively charged.

Cathode Ray Experiment

Deflectionregion

Drift region

Displacement

+

-Anodes / collimators

Cathode

Volts

Thomson Model of the Atom

J.J. Thomson discovered the electron and knew that electrons could be emitted from matter (1897).

William Thomson proposed that atoms consist of small, negative electrons embedded in a massive, positive sphere.

The electrons were like raisins in a plum pudding, called the ‘plum pudding’ model of the atom.

Feeling overwhelmed?

Read Chapter 4!

"Teacher, may I be excused? My brain is full."

Chemistry

Ernest Rutherford (1871-1937)

Learned physics in J.J. Thomson’ lab.

Noticed that ‘alpha’ particles were sometime deflected by something in the air.

Gold-foil experimentAnimation by Raymond Chang – All rights reserved.

Rutherford’s Apparatus

beam of alpha particles

radioactive substance

gold foil

circular ZnS - coatedfluorescent screen

Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 120

Rutherford Scattering In 1909 Rutherford fired (alpha) particles at

a very thin sample of gold foil. According to the Thomson model the

particles would only be slightly deflected. Rutherford discovered that they were

deflected by large angles and could even be reflected straight back to the source.

particlesource

Lead collimator Gold foil

What he expected…

Because he thought the mass was evenly distributed in the atom, as described by the plum pudding model

-

-

-

-

-

+

+

+

+

+-

+

What he got…richochetingalpha particles

The Predicted Result:

expected path

expected marks on screen

mark onscreen

likely alphaparticle path

Observed Result:

Density and the Atom

Since most of the particles went through, the atom was mostly empty.

Because the alpha rays were deflected so much, the positive pieces it was striking were heavy.

Small volume and big mass = big density

This small dense positively charged area is the nucleus

California WEB

Actual Results of Gold-Leaf Experiment

Zumdahl, Zumdahl, DeCoste, World of Chemistry 2002, page 57

n +

Electrons are in constant motion around the nucleus, protons and neutrons jiggle within the nucleus, and quarks jiggle within the protons and neutrons. This picture is quite distorted. If we drew the atom to scale and made protons and neutrons a centimeter in diameter, then the electrons and quarks would be less than the diameter of a hair and the entire atom's diameter would be greater than the length of thirty football fields! 99.999999999999% of an atom's volume is just empty space!

Website “The Particle Adventure”

The Modern Atom Model

Bohr’s Model

Nucleus

Electron

Orbit

Energy Levels

The Planetary Model

+

Nucleus

Electron

Orbit

The Bohr model of the atom, was built upon the incorrect idea that electrons orbit the nucleus like planets around the sun.

Bohr Model of Atom

The Bohr model of the atom, like many ideas in the history of science, was at first prompted by and later partially disproved by experimentation.

Increasing energyof orbits

n = 1

n = 2

n = 3

A photon is emittedwith energy E = hf

e-e-

e-

e-

e-

e-

e-

e-

e-

e-

e-

An unsatisfactory model for the hydrogen atom

According to classical physics, lightshould be emitted as the electron circles the nucleus. A loss of energywould cause the electron to be drawncloser to the nucleus and eventuallyspiral into it.

Hill, Petrucci, General Chemistry An Integrated Approach 2nd Edition, page 294

Cartoon courtesy of NearingZero.net

Subatomic Particles

Quarks component of

protons & neutrons

6 types (flavors)3 quarks = 1 proton or 1 neutron

He

Courtesy Christy Johannesson www.nisd.net/communicationsarts/pages/chem