Chapter 4 the structure of the atom
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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 4The 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

    and…


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


Vocabulary

*atom

*atomic mass

*atomic mass unit (amu)

*atomic number

*electron

*isotope

*mass number

*neutron

*nucleus

*proton


Early Theories of Matter

  • Democritus

    ( 460-370 BC)


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

  • 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 (give matter its properties)

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


Aristotle

  • One of most influential Greek philosophers


Aristotle

  • 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


Aristotle

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


John Dalton(1766-1844)

  • Revived the ideas of Democritus

  • First atomic model

  • A teacher in England


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…

  • 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

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


Relative size of an atom


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

  • The electron

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

Tube filled with a gas,

+ and a – end,


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)

  • 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

  • Identified the first subatomic particle – the electron

  • Atoms were divisible into smaller particles


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)

  • American physicist

  • Determined the charge of an electron

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


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


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


Radiation can be shielded by..


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..


Used alpha particles for “bullets”

  • Alpha particles =

    Helium nucleus

    2 protons + 2 neutrons


What happened in the experiment


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?

  • 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

  • Proton

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

  • Neutron

    • Discovered by James Chadwick in 1932


Atomic number

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


Henry Moseley (1887-1915)

  • Discovered atomic numbers

  • Died at a young age in WWI


The atomic number

Mass number=

Protons + neutrons

12

C

6

Symbol for element

Atomic number =

number of protons

in the nucleus


Practice writing atomic notation

23

Na

11

What does this tell us?


Write atomic notation

  • For an atom of Fe

  • For an atom of U

And ……


56 238

Fe U

26 92


Isotopes

  • Same atomic number but a different atomic mass

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


Isotopes of hydrogen

1 2 3

H H H

  • 1 1

    Protium deuterium tritium


Calculating Atomic Mass(Turn to pg. 102)

  • 1. mass x %abundance (for each

    isotope)

  • 2. Add them together to get the atomic mass

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

    VIOLA!


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

  • Let’s do # 15 and #16 together.

  • Now, you do #17 on your own.


Pg. 104, #15

(10.013)(0.198) = 1.982 amu

(11.009)(0.802) = 8.829 amu

10.811 amu


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

  • (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

  • The nucleus of the atoms actually change

  • Nuclear stability is based on neutron/proton ratio

  • Experience nuclear decay to become more stable


Alpha decay


Nuclear equation becomes

238 234 4

U  Th + He

92 90 2


Beta decay


Nuclear equation

14 0 15

N  e + O

7 -1 8


Another example of Beta decay


Gamma radiation γ

  • Electromagentic radiation

  • Has no charge

  • Often found along with Beta or alpha radiation


Gamma radiation


Fission of atoms


The end – for now!


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