Atomic Theory

ATOMIC THEORY

DEVELOPMENT OF ATOMIC THEORY

VARIOUS ATOMS AND COMPOUNDS AS DEPICTED IN JOHN DALTON’S A NEW SYSTEM

OF CHEMICAL PHILOSOPHY

THOMSON’S CROOKE’S TUBE IN WHICH HE OBSERVED THE DEFLECTION OF CATHODE

RAYS BY AN ELECTRIC FIELD

PLUM PUDDING’S ATOMIC MODEL

THE RUTHERFORD’S GOLD FOIL EXPERIMENT

NEIL BOHR’S ATOMIC MODEL

Electron cloud Model or Quantum Mechanical Model

Scientist & approximate

Date

Name of Model, Sketchand main idea

of theory

Importance and

Improvement on previous

model

Shortcomings - Problems

or why was it changed

Democritus   c.300 BC

Atom the indivisible particle

Atomos (in ancient Greek)

means "that which cannot

be further broken down into smaller

pieces".

Atom as the smallest

particle of matter.

Explains certain natural occurrences such as the existence of elements `

Does not give a scientific view of the atom

only a conceptual definition

Does not talk about

subatomic particles

(Electrons, Protons,

Neutrons)  

Dalton  c.1800

The solid sphere model

Atoms are seen as solid,

indestructible spheres (like billiard balls)

 Explains how atoms combine

to form molecules

Explains chemical

change better than the 

Particle Theory

Confirms the basic Laws of

Chemistry: Conservation of Mass & definite

Proportions

 Does not include the

existence of the nucleus

Does not explain the existence of

ions or isotopes

Does not talk about

subatomic particles

(Electrons, Protons,

Neutrons)

J.J. Thomson c.1850

The raisin bun Model or

thechocolate chip cookie model :

Atoms are solid spheres made-up of a solid positive

mass (or core) with tiny negative particles

embedded in the positive

core.

 Infers on the existence of electrons and protons

Introduces the concept of the nucleus

 Does not explain the existence of electrons outside the nucleus

Does not talk about neutrons therefore can't explain radioactivity and the existence of isotopes

E.

Rutherfordc. 1905

Famous Gold Leaf Experiment

proves that the nucleus is

positive and the electrons are outside the

nucleus.

First real modern view of the atom

Proposes that the atom is really mostly empty space

 Does not place

electrons in definite

energy levels around the nucleus

Doesn't include

neutrons in the nucleus

(Neils Bohr) Bohr-

Rutherfordc. 1920

Electrons in Definite energy Levels

around  the nucleus

Used atomic spectra to prove that

electrons are placed in definite orbitals

around the nucleus.

 Explains the role of valence

electrons in bonding

Fully explains ionic and

covalent bonding

Places electrons in definite energy

levels

2 e- in the first

8 e- in the second

8 e- in the third

 It does not explain the shapes of

molecules or other

abnormalities that result

form unevenly shared pairs of electrons

Modern TheoryMany Scientists Contributed.  Sosome of the more famous  are:Schroedinger Einstein Luis De Broglie Max Planck Frank Hertz Maxwell Fermi

Quantum Mechanical Model

or Electron

Cloud Model

The analogy here is that of a "beehive" where the bees are the

electrons moving around the nucleus in a

"cloud" of energy levels.

Advanced Theories will explain bonding and other facts about the behaviour of atoms and their chemical and physical properties in forming new compounds.  

Selected properties of the three basic Subatomic Particles

Name Charge Mas (amu) Mass (g)

Electrons -1 5.4 x 10 -4 9.1095 x 10 -28

Proton +1 1.00 1.6725 x 10 -24

Neutron 0 1.00 1.6750 x 10 -24

Calculate the number of protons, neutrons, and electrons in each of the ff:

a. 32S16

b. 23Na11

c. 19F9

d. 35Cl17

Isotopes and Atomic Mass

Mass no.

Isotopic Mass

% Abundance

Ave. At. Mass

1. Chlorine

35

37

34.969 amu

36.966 amu75.53

24.47 35.458

2. Oxygen

16

17

18

15.9949 amu

16.9991 amu

17.9992 amu

99.76

0.04

0.20 15.9993

PERIOD SCIENTIST ATOMIC THEORY 5th Cent. DEMOCRITUS MATTER IS

COMPOSED OF ATOMOS

1803 JOHN DALTON ATOMIC THEORY BASED ON THE

NATURE OF MATTER

1830 MICHAEL FARADAY

EFFECTS OF ELECTRICITY ON

MATTER RESULTED TO THE DISCOVERY

OF SUB ATOMIC PARTICLES

PERIOD SCIENTIST ATOMIC THEORY

1880 WILLIAM CROOKES

STUDIED ATOMIC STRUCTURE USING THE ELECTRIC DISCHARGE

THROUGH GASES

1896 EUGEN GOLDSTEIN

DISCOVERED NEW PARTICLES CALLED CANAL RAYS NOW

CALLED IONS

1897 JOSEPH JOHN THOMSON

AN ATOM COULD BE THOUGHT OF A

UNIFORM, POSITIVE SPHERE OF MATTER IN WHICH ELECTRON ARE

EMBEDED.

PERIOD SCIENTIST ATOMIC THEORY

1909 ERNEST RUTHERFORD

ALPHA SCATERRING EXPERIMENT DISCOVERED

NUCLEUS AS A CENTRAL PART OF ATOM &

DISCOVERED FUNDAMENTAL PARTICLE CALLED PROTON.

1913 NEILS BOHR PROPOSED THAT AN ATOM IS A MINIATURE SOLAR SYSTEM

IN WHICH PLANETARY ELECTRONS REVOLVE

AROUND THE SUN WHICH IS NUCLEUS AND ELECTRON

HAS A DEFINITE ORBIT KNOWN AS THE ENERGY

LEVELS

PERIOD SCIENTIST ATOMIC THEORY

1916 ARNOLD SOMMERFIELD

LOWER ENERGY ELECTRON REVOLVED IN

CIRCULAR PATH AND HIGHER ENERGY

ELECTRON FOLLOW AN ELLIPTICAL PATH

AROUND THE NUCLEUS.

1900 MAX PLANK INTRODUCE THE QUANTUM THEORY.HE

POSTULATED THAT RADIANT ENERGY IS

EMMITTED BY ATOMS & MOLECULES IN SMALL DISCRETE AMOUNTS

(QUANTA)

PERIOD SCIENTIST ATOMIC THEORY

1924 LOIUS DE BROGLIE

ELECTRON HAS DUAL PROPERTIES THAT OF A PARTICLE AND A WAVE.

1924 ERWIN SCHRODINGER

MECHANICAL CALCULATION OF THE

ENERGY OF EACH ELECTRON.THIS MODEL SHOWS THAT ELECTRON

OCCUPIES VARIOUS ENERGY LEVELS AND

SUB LEVELS AND ORBITALS, THE HISHEST

PROBABILITY OF LOCATING AN ELECTRON

IS FOUND.

PERIOD SCIENTIST ATOMIC THEORY

1926 HEISENBERG UNCERTAINTY PRINCIPLE STATES THAT IT IS

IMPOSSIBLE TO DETERMINE SIMULTANEOUSLY THE

EXACT POSITION AND THE EXACT MOMENTUM OF A BODY AS SMALL AS AN

ELECTRON.

1932 JAMES CHADWICK

HE PROVIDED EXPERIMENTAL EVIDENCE

ON THE PRESENCE OF NEUTRONS IN THE

NUCLEUS.BOMBARDMENT OF Be WITH ALPHA PARTICLES.