S.MORRIS 2006More free powerpoints at www.worldofteaching.com

HISTORY OF THE ATOMHISTORY OF THE ATOM

460 BC Democritus develops the idea of atoms

he pounded up materials in his pestle and

mortar until he had reduced them to

smaller and smaller particles which he

called

ATOMAATOMA

(greek for indivisible)

HISTORY OF THE ATOMHISTORY OF THE ATOM

1808 John Dalton

suggested that all matter was made up of

tiny spheres that were able to bounce

around with perfect elasticity and called

them

ATOMSATOMS

Dalton’s Atomic TheoryDalton’s Atomic Theory All matter is made of tiny indivisible All matter is made of tiny indivisible

particles called particles called atoms.atoms. Atoms of the same element are identical, Atoms of the same element are identical,

those of different atoms are different.those of different atoms are different. Atoms of different elements combine in Atoms of different elements combine in

whole number ratios to form whole number ratios to form compoundscompounds.. Chemical reactions involve the Chemical reactions involve the

rearrangement of atoms. rearrangement of atoms. 5 Atoms cannot be created or destroyed.5 Atoms cannot be created or destroyed.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1898 Joseph John Thompson

found that atoms could sometimes eject a

far smaller negative particle which he

called an

ELECTRONELECTRON

HISTORY OF THE ATOMHISTORY OF THE ATOM

Thompson develops the idea that an atom was made up of

electrons scattered unevenly within an elastic sphere

surrounded by a soup of positive charge to balance the

electron's charge

1904

like plums surrounded by pudding.

PLUM PUDDING

MODEL

HISTORY OF THE ATOMHISTORY OF THE ATOM

1910 Ernest Rutherford

oversaw Geiger and Marsden carrying out

his famous experiment.

they fired Helium nuclei at a piece of gold

foil which was only a few atoms thick.

they found that although most of them

passed through. About 1 in 10,000 hit

HISTORY OF THE ATOMHISTORY OF THE ATOM

gold foil

helium nuclei

They found that while most of the helium nuclei passed

through the foil, a small number were deflected and, to

their surprise, some helium nuclei bounced straight back.

helium nuclei

HISTORY OF THE ATOMHISTORY OF THE ATOM

Rutherford’s new evidence allowed him to propose a

more detailed model with a central nucleus.

He suggested that the positive charge was all in a

central nucleus. With this holding the electrons in place

by electrical attraction

However, this was not the end of the story.

HISTORY OF THE ATOMHISTORY OF THE ATOM

1913 Niels Bohr

studied under Rutherford at the Victoria

University in Manchester.

Bohr refined Rutherford's idea by

adding that the electrons were in

orbits. Rather like planets orbiting the

sun. With each orbit only able to

contain a set number of electrons.

Bohr Model of the Atom• Proposed by Niels Bohr in 1915;• Not completely correct, but has many

features that are approximately correct.

• Overcame major flaw with Rutherford model, which predicted that an orbiting electron would emit white as it

accelerated towards the nucleus. • Gives us a basic conceptual model of

electrons orbits and energies.• Key success was in explaining the

spectral emission lines of atomic hydrogen

Shortcomings of the Bohr Model

1. Treats each electron as a miniature planet, with definite radius and momentum - in direct violation of the uncertainty principle which dictates that position and momentum cannot be simultaneously determined.

2. It fails to provide any understanding of why certain spectral lines are brighter than others.

Schrodinger’s Cloud Model• Schrödinger developed the ‘probability

function’ for the hydrogen atom (and a few others). 

• This describes a cloud-like region where the electron is likely to be found. 

• It can not say with any certainty, where the electron actually is, yet can describe where it ought to be.  

• ‘Clarity through fuzzines’ is one way to describe the idea. 

• The probable locations of the electron predicted by Schrödinger's equation happen to coincide with the locations specified in Bohr's model.

Modern View - SummaryModern View - Summary

• The atom is mostly The atom is mostly empty space.empty space.

• Two regions.Two regions.• Nucleus- protons andNucleus- protons and

neutrons.neutrons.• Electron cloud- region Electron cloud- region

where you might find where you might find an electron.an electron.

HELIUM ATOM

+N

N

+-

-

proton

electron

neutron

Shell

What do these particles consist of?

ATOMIC STRUCTUREATOMIC STRUCTURE

Particle

proton

neutron

electron

Charge

+ ve charge

-ve charge

No charge

1

1

nil

Mass

ATOMIC STRUCTUREATOMIC STRUCTURE

the number of protons in an atom

the number of protons and neutrons in an atom

HeHe22

44 Atomic mass

Atomic number

number of electrons = number of protons

IsotopesIsotopes

• That is not the end of the story…

• Even atoms of the same element can have different numbers of neutrons.

• Such atoms have the same atomic number but…

• Different atomic mass.• Called isotopes.

X