History of Atomic Theory

Essential Question

• How did the atomic theory evolve from Dalton's Postulates to the current modern atomic theory?

Ancient Philosophy• Who: Aristotle, Democritus• When: More than 2000 years ago• Where: Greece• What: Aristotle believed in 4 elements: Earth,

Air, Fire, and Water. Democritus believed that matter was made of small particles he named “atoms”.

• Why: Aristotle and Democritus used observation and inference to explain the existence of everything.

Democritus• Matter is composed of empty space through which

atoms move.• Atoms are solid, homogeneous, indestructible,

and indivisible.• Different kinds of atoms have different sizes and

shapes.• The different properties of matter are due to the

size, shape, and movement of atoms.• Apparent changes in matter result from changes

in the groupings of atoms and not from changes in the atoms themselves.

Aristotle

Democritus

Alchemists• Who: European Scientists

• When: 800 – 900 years ago

• Where: Europe

• What: Their work developed into what is now modern chemistry.

• Why: Trying to change ordinary materials into gold.

Alchemic Symbols

Particle Theory• Who: John Dalton

• When: 1808

• Where: England

• What: Described atoms as tiny particles that could not be divided. Thought each element was made of its own kind of atom.

• Why: Building on the ideas of Democritus in ancient Greece.

John Dalton• Matter composed of small particles (atoms)• Atoms of an element have the same size, mass, and

chemical properties.• Atoms cannot be created, divided into smaller particles,

or destroyed.• Atoms of different elements combine in whole number

ratios to form compounds (H2O, CO2, MgO, etc.).

• In a chemical reaction, atoms are separated, combined or rearranged.

John Dalton

Discovery of Electrons• Who: J. J. Thomson

• When: 1897

• Where: England

• What: Thomson discovered that electrons were smaller particles of an atom and were negatively charged.

• Why: Thomson knew atoms were neutrally charged, but couldn’t find the positive particle.

Cathode Ray Tube- Cathode rays are a stream

of charged particles

- Particles carry a negative charge (electrons)

- Thomson concluded that electrons were smaller than a hydrogen atom (lightest known atom)

- Conclusion: Electrons are smaller than atoms

- Dalton was wrong: Atoms were divisible into smaller subatomic particles

The Atom, circa 1900:

• “Plum pudding” model, put forward by Thomson.

• Positive sphere of matter with negative electrons imbedded in it.

Plum

Pudding

J. J. Thomson

Atomic Structure I• Who: Ernest Rutherford

• When: 1911

• Where: England

• What: Conducted an experiment to isolate the positive particles in an atom. Decided that the atoms were mostly empty space, but had a dense central core.

• Why: He knew that atoms had positive and negative particles, but could not decide how they were arranged.

Discovery of the Nucleus:Gold Foil Experiment

Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles.

(Zinc sulfide)

The Nuclear Atom

Since some particles were deflected at large angles, Thomson’s model could not be correct.

THE NUCLEAR MODELAccording to Rutherford,

most of the atom consists of electrons moving rapidly through empty space and a very dense, tiny, positively charged nucleus.

Ernest Rutherford

Other Subatomic Particles• By 1920 , Rutherford had refined his

concept of the nucleus: He concluded that the very dense nucleus contained positively particles called protons.

• James Chadwick (1932), a coworker, showed that the nucleus also contained a neutral particle. This was the neutron – a particle with nearly equal mass as a proton.

Atomic Structure II• Who: Niels Bohr

• When: 1913

• Where: England

• What: Proposed that electrons traveled in fixed paths around the nucleus. Scientists still use the Bohr model to show the number of electrons in each orbit around the nucleus.

• Why: Bohr was trying to show why the negative electrons were not sucked into the nucleus of the atom.

Bohr Model

THE BOHR MODEL

Niels Bohr

Electron Cloud Model• Electrons travel around the nucleus in

random orbits.

• Scientists cannot predict where they will be at any given moment.

• Electrons travel so fast, they appear to form a “cloud” around the nucleus.

Electron Cloud Model

                                 

Properties of Subatomic Particles

Particle Symbol Location ChargeRelative

massActual mass

(g)

Electron e- Around nucleus

1– 1

18409.11×10-28

Proton p+ Nucleus 1+ 1 1.673×10-24

Neutron n0 nucleus 0 1 1.675×10-24

Atomic Structure TimelineRequirements:• Must include a title.• Must include a spot for each of the 8 items.• Must include at least the Who, When, and

What.• Must have a picture (include color) for each

item.Options:1. Comic book format2. Standard timeline

Atomic Structure and the Periodic Table

14

Si

28.086

Silicon

Atomic number = # protons

Symbol

NameAtomic Mass: Round to a whole #:

28 = Relative mass of nucleus

-14 = # protons

14 = # neutrons

Practice From the Periodic Table

Complete the following chart:

Name SymbolRelative

mass#

protons#

neutrons#

electrons

Lithium

Mg

26

16