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Chapter 4 The structure of the atom. AL-COS Objectives 1, 2,3,4,7, 10, 15, 20, 21, 22, 27and 28. You’ll learn to…. Identify the experiments that led to the development of the nuclear model of atomic structure Describe the structure of the atom and the subatomic particles that comprise it

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Chapter 4 The structure of the atom

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Chapter 4 the structure of the atom

Chapter 4The structure of the atom

AL-COS Objectives 1, 2,3,4,7, 10, 15, 20, 21, 22, 27and 28

You ll learn to

You’ll learn to…

  • Identify the experiments that led to the development of the nuclear model of atomic structure

  • Describe the structure of the atom and the subatomic particles that comprise it


You ll also learn

You’ll also learn…

  • Explain the relationship between nuclear stability and radioactivity

  • Write equations representing nuclear decay

  • Discuss the atomic models of Democritus, Thomson, Rutherford, and Bohr




*atomic mass

*atomic mass unit (amu)

*atomic number



*mass number




Early theories of matter

Early Theories of Matter

  • Democritus

    ( 460-370 BC)

A man ahead of his time

A man ahead of his time…

  • First to believe matter was not infintely divisible

  • Believed matter to be made of small particles he called “atomos”

  • Believed these particles could not be created or destroyed

Democritus ideas

Democritus’ Ideas

  • Matter is composed of empty

    space through which atoms


  • Atoms are solid, homogeneous, indestructible and indivisible

  • Different kinds of atoms have different sizes and shapes (give matter its properties)

  • Changes in matter are due to changes in grouping of atoms



  • One of most influential Greek philosophers



  • Believed matter was formed of air, water, earth and fire

  • Did not believe in the “nothingness” of space

  • All of his ideas were NOT based on empirical (experimental) science – just ideas



Given credit for setting the field of science back nearly 2000 years!

John dalton 1766 1844

John Dalton(1766-1844)

  • Revived the ideas of Democritus

  • First atomic model

  • A teacher in England

Dalton s atomic theory

Dalton’s atomic theory

  • Matter is made of small particles called atoms

  • All atoms of an element are identical **** (isotopes?)

  • Atoms of different elements are different

As well as

As well as…

  • Atoms cannot be created or destroyed or divided into smaller particles ****(nuclear fission?)

  • Different atoms combine in simple whole number ratios to form compounds

  • In chemical reactions, atoms are separated, combined or rearranged.

The atom

The atom

  • Smallest particle of an element that retains the properties of that element

Relative size of an atom

Relative size of an atom

How many atoms

How many atoms?

  • World population =

    6 000 000 000

  • Number of atoms in a penny=

    29 000 000 000 000 000 000 000

Subatomic particles and the nuclear atom

Subatomic particles and the nuclear atom

  • The electron

    - Sir William Crookes noticed the flash of light in a tube

Tube filled with a gas,

+ and a – end,

The cathode ray

The cathode ray

Scientists were convinced this cathode ray was really a stream of charged particles -> electrons!

Further cathode tube experiments were conducted by j j thomson 1856 1940

Further cathode tube experiments were conducted by J.J. Thomson (1856-1940)

  • Was trying to determine the charge/mass ratio of these electrons

  • Found the mass of this particles was less than hydrogen’s

J j thomson

J.J. Thomson

  • Identified the first subatomic particle – the electron

  • Atoms were divisible into smaller particles

J j thomson s atomic model

J.J. Thomson’s atomic model

  • A spherical atom composed of evenly distributed positive charges within which the negatively charged electrons were also found

Robert millikan 1868 1953

Robert Millikan (1868-1953)

  • American physicist

  • Determined the charge of an electron

  • Calculated the mass of a single electron (e-)

Sir ernest rutherford

Sir Ernest Rutherford

  • Originally from New Zealand

  • Won a scholarship competition to study at Cambridge Univ. in England

  • Won the Nobel prize in 1908 for discovering radioactivity



  • 3 types

    • Alpha particles

      • Actually the nucleus of a helium atom

    • Beta particle

      • A high speed electron

    • Gamma particle

      • A photon of electromagnetic


Radiation can be shielded by

Radiation can be shielded by..

Designed an experiment to study the atom

Designed an experiment to study the atom

  • Experiment was actually conducted by his two graduate students, Geiger (later developed the Geiger counter) and Marsden

  • Rutherford was their supervising professor

The gold foil experiment

The gold foil experiment..

Used alpha particles for bullets

Used alpha particles for “bullets”

  • Alpha particles =

    Helium nucleus

    2 protons + 2 neutrons

What happened in the experiment

What happened in the experiment

Observed that

Observed that..

  • Most of the alpha particles went straight through

  • Some were deflected away from the foil

  • Some hit something solid and “bounced back”

What was learned

What was learned?

  • Most of an atom is empty space

  • The nucleus has a positive charge (He nucleus was also positive, which caused the deflection)

  • The nucleus, when hit straight on, is dense and solid – not a plum pudding!

Other subatomic particles

Other subatomic particles

  • Proton

    • Discovered by Goldstein using a cathode ray tube (1920’s)

  • Neutron

    • Discovered by James Chadwick in 1932

Atomic number

Atomic number

  • Henry Moseley discovered that atoms of different elements each have a unique positive charge in their nucleus

Henry moseley 1887 1915

Henry Moseley (1887-1915)

  • Discovered atomic numbers

  • Died at a young age in WWI

The atomic number

The atomic number

Mass number=

Protons + neutrons




Symbol for element

Atomic number =

number of protons

in the nucleus

Practice writing atomic notation

Practice writing atomic notation




What does this tell us?

Write atomic notation

Write atomic notation

  • For an atom of Fe

  • For an atom of U

And ……

Chapter 4 the structure of the atom

56 238

Fe U

26 92



  • Same atomic number but a different atomic mass

  • The number of protons remains the same, but the number of neutrons vary.

Isotopes of hydrogen

Isotopes of hydrogen

1 2 3


  • 1 1

    Protium deuterium tritium

Calculating atomic mass turn to pg 102

Calculating Atomic Mass(Turn to pg. 102)

  • 1. mass x %abundance (for each


  • 2. Add them together to get the atomic mass

  • 3. find that atomic mass on the periodic table and that is the element


Example problem pg 103

Example Problem: pg. 103

  • 1.) 6X =(6.015)(0.075)= 0.451 amu

    7X =(7.016)(0.925)= 6.490 amu

  • 2.) 0.451 + 6.490 = 6.941 amu

  • 3.) The element with an atomic mass of 6.941 amu is Lithium, Li.

  • COOL!!!!!!!!!!!!

Look on pg 104

Look on pg. 104

  • Let’s do # 15 and #16 together.

  • Now, you do #17 on your own.

Pg 104 15

Pg. 104, #15

(10.013)(0.198) = 1.982 amu

(11.009)(0.802) = 8.829 amu

10.811 amu

Pg 104 16

Pg. 104, #16

Helium-4 is more abundant in nature because the atomic mass of naturally occurring helium is closer to the mass of helium-4 than to the mass of helium –3.

Pg 104 17

Pg. 104, #17

  • (23.985)(0.7899) = 18.946 amu

  • (24.986)(0.1000) = 2.499 amu

  • (25.982)(0.1101) = 2.861 amu

    24.306 amu

    The element with that atomic mass is magnesium, Mg.

Nuclear reactions

Nuclear reactions

  • The nucleus of the atoms actually change

  • Nuclear stability is based on neutron/proton ratio

  • Experience nuclear decay to become more stable

Alpha decay

Alpha decay

Nuclear equation becomes

Nuclear equation becomes

238 234 4

U  Th + He

92 90 2

Beta decay

Beta decay

Nuclear equation

Nuclear equation

14 0 15

N  e + O

7 -1 8

Another example of beta decay

Another example of Beta decay

Gamma radiation

Gamma radiation γ

  • Electromagentic radiation

  • Has no charge

  • Often found along with Beta or alpha radiation

Gamma radiation1

Gamma radiation

Fission of atoms

Fission of atoms

Chapter 4 the structure of the atom

The end – for now!

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