SOL CH.2a-c

Outline pages 101-119; 172; 799-806 in textbook.

Complete workbook pages workbook pages 33-42; 267-269; and 274.

Atomic Structure and Nuclear Stability

h"p://science.widener.edu/svb/tutorial/protons.html.  

ANTICIPATED  LEARNING  Virginia  Standard:  CH.2  a-­‐c  

The  student  will  inves9gate  and  understand  that  the  placement  of  elements  on  the  periodic  table  is  a  func9on  of  their  atomic  structure.  The  periodic  table  is  a  tool  used  for  the  inves9ga9ons.  Key  Concepts:  

             a.  average  atomic  mass,  mass  number,  and  atomic  number;                b.  isotopes,  half  lives,  and  radioac9ve  decay;  and                c.  mass  and  charge  characteris9cs  of  subatomic  par9cles      EssenCal  Understandings:  •  Loss  of  electrons  from  neutral  atoms  results  in  the  forma9on  of  an  ion  with  a  posi9ve  charge  (ca9on).    Gain  of  

electrons  by  a  neutral  atom  results  in  the  forma9on  of  an  ion  with  a  nega9ve  charge  (anion).  •  The  periodic  table  is  arranged  by  increasing  atomic  numbers.  •  The  atomic  number  of  an  element  is  the  same  as  the  number  of  protons.    In  a  neutral  atom,  the  number  of  

electrons  is  the  same  as  the  number  of  protons.    All  atoms  of  an  element  have  the  same  number  of  protons.  •  The  average  atomic  mass  for  each  element  is  the  weighted  average  of  that  element’s  naturally  occurring  isotopes.  •  The  mass  number  of  an  element  is  the  sum  of  the  number  of  protons  and  neutrons.    It  is  different  for  each  

element’s  isotopes.  •  An  isotope  is  an  atom  that  has  the  same  number  of  protons  as  another  atom  of  the  same  element  but  has  a  

different  number  of  neutrons.    Some  isotopes  are  radioac9ve;  many  are  not.  •  Half-­‐life  is  the  length  of  9me  required  for  half  of  a  given  sample  of  a  radioac9ve  isotope  to  decay.  •  Electrons  have  li"le  mass  and  a  nega9ve  (-­‐)  charge.    They  are  located  in  the  electron  clouds  or  probability  clouds  

outside  the  nucleus.  •  Protons  have  a  posi9ve  (+)  charge.    Neutrons  have  no  charge.    Protons  and  neutrons  are  located  in  the  nucleus  of  

the  atom  and  comprise  most  of  its  mass.    Quarks  are  also  located  in  the  nucleus  of  the  atom.  

Atomic Structure Brief History

Atoms- smallest particle of an element that maintains the chemical identity of that element.

Democritus •  A Greek philosopher, believed

there was a point when it would be impossible to break matter in half. He called this smallest bit matter, ATOMS.

–  Also believed spiritual soul is made of “soul atoms”

•  His views were not based on experimentation.

•  His ideas were not accepted by the church and established Greek philospher, Aristotle.

•  Democritus views were not accepted until the 19th century.

John Dalton •  In the 1800's an English

chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). �

•  Although he did not know about their structure, he knew that the evidence pointed to something fundamental.�

•  He developed the ATOMIC THEORY �

Dalton’s Atomic Theory of Matter 1803

1. Matter consists of tiny particles called atoms which are indivisible and indestructible.

2. All atoms of a particular element are identical. 3. Atoms of different elements differ in mass and properties. 4.   Atoms combine in whole number ratios to form compound atoms. 5.   In chemical reactions, atoms are combined, separated, or rearranged but are never created, destroyed, or changed.

Why were Dalton’s views accepted?

•  Law of Conservation of Mass – mass is neither created nor destroyed in a chemical reaction.

•  Law of Definite Proportions - (chemistry) law stating that every pure substance always contains the same elements combined in the same proportions by weight.

•  Law of Multiple Proportions - two elements can combine to form more than one compound the amounts of one of them that combines with a fixed amount of the other will exhibit a simple multiple relation.

•  The scientific method is now the proper way to “do science.”

•  Dalton’s theory was based on experimental observations: the law of Conservation of Mass and the law of Definite Proportions.

•  Dalton’s theory correctly predicted the outcome of future experiments. These predictions became the law of Multiple Proportions.

The following statements match principles of Dalton’s theory.

Write the principle that explains each statement.

1.  When methane burns, it combines with oxygen in the air to form molecules of water and carbon dioxide.

2.  Matter can never really be thrown away. That is one reason that recycling is important.

3.  The formula for ethanol is C2H6O, and the formula for acetic acid in vinegar is C2H4O2.

4.  Zinc is a softer metal than iron, and it reacts more readily with acid than iron does.

5.  There is no difference between copper found in an ancient Mayan necklace and copper wire freshly made from copper ore.

Discovering Atomic Structure

Like charges -repelUnilike charges- attract

Electrical ChargesPositive or Negative

Benjamin Franklin

Electrons

Cathode Ray Tube

Sir Joseph John (JJ) Thomsonlate 1800's

Mass of an electron9.11 x 10-28 g

Oil Drop Experiment1909

Robert Milikan

Accidently discovered Uraniumradioactive

1896

Henri Becquerel

Protons1919

Henry Moseleystudent of Rutherford

Neutrons1932

James C. Chadwick

Nucleus

Alpha scattering experiment

Ernest Rutherford

Discovery of Atomic Structure

Sir Joseph John (J.J.) Thomson •  In  1897,  the  English  physicist  J.J.  

Thomson  discovered  the  electron  and  proposed  a  model  for  the  structure  of  the  atom.    

 •  Thomson  knew  that  electrons  had  

a  nega9ve  charge  and  thought  that  ma"er  must  have  a  posi9ve  charge.  

 •   His  model  looked  like  raisins  stuck  

on  the  surface  of  a  lump  of  pudding.  

Alpha Scattering Experiment Ernst Rutherford

•  Only small fraction of particles were deflected from foil.

•  Other particles scattered all over the place.

•  Most of the atom is empty space. •  Nucleus -positively charged small

core of the atom that contains most of the atoms mass.

Illustration of Alpha Scattering Experiment

Each atom has a very tiny nucleus. The nucleus is positively charged. The electrons travel around the nucleus.

Particle Location Relative Electrical Charge

Mass (g) Mass (amu)

proton Inside nucleus

+1 1.673 x 10 -24 1.0073 =1

neutron Inside nucleus

0 1.675 x 10 -24 1.0087 =1

electron Outside nucleus

-1 9.11 x 10 -28 0.0006 =0

Subatomic Particle Information

Atomic Number

•  Have the same number of protons and electrons. –  Electrically neutral (there is no charge)

•  Atomic number – number of protons in the nuclei of an atom of that element. –  All helium atoms have 2 protons, therefore the atomic

number for He is 2. –  All sulfur (S) atoms have 16 protons, therefore the atomic

number for sulfur is 16. –  each element has a unique number of protons.

–  # of protons used to determine the element’s identity.

Mass Number

•  Majority  of  the  atom’s  mass  is  in  the  nucleus.  – Protons  and  neutrons  are  inside  the  nucleus.  

•  The  total  number  of  protons  and  neutrons  in  an  atom  is  the  mass  number.  

•  The  number  of  neutrons  can  be  determined  if  you  know  the  atomic  number  and  mass  number.  

Number  of  neutrons  =  mass  number  –  atomic  number    

Shorthand notation of Atom

1H Atomic #

Element Symbol

Mass #

OR Hydrogen-2

Mass #

2

Element Name

Isotopes  

Isotopes -atoms that have the same number of protons but a different number of neutrons.

–  This also means they have a different mass number . –  Isotopes of the same element are chemically alike

because they have the same number of protons and electrons (they determine chemical behavior)

Atomic Mass (amu)

•  Mass spectrometers have been used since 1920’s to determine the masses of elements. –  Fluorine has a mass of

3.155 x 10 -23 g. –  Oxygen has a mass of

2.657X10-23 g. •  The mass of these atoms in

gram is useful information, but are inconveniently small and impractical to work with.

•  It is better to compare the relative masses of atoms using a reference isotope as the standard. –  The chosen isotope is

carbon-12. –  It was assigned a mass of

exactly 12 atomic mass units. •  Atomic Mass unit (amu) - is one

twelfth of the mass of a carbon-12 atom. –  Using this unit, Fluorine has a

mass of 18 amu and Oxygen has a mass of 15.999

Atomic mass

•  In  nature,  most  elements  occur  as  a  mixture  of  two  or  more  isotopes.  –  Hydrogen  has  3  isotope.  –  Each  isotope  has  a  fixed  mass  and  natural  percent  abundance.  

•  The  atomic  mass  of  an  element  is  the  weighted  average  mass  of  the  atoms  in  a  naturally  occurring  sample  of  the  element.  

 •  A  weighted  average  reflects  mass  and  natural  abundance  for  each  

element.    

Calculating the Atomic mass of Chlorine element

To calculate atomic mass (weighted average) of an element- multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.

Example: Chlorine element has two isotopes: chlorine-35 and chlorine-37.

Isotope    

Mass  (amu)    

Natural  Abundance  (%)    

Chlorine-­‐35    

34.969  amu    

75.53%    

Chorine-­‐37    

36.966  amu    

24.47    

How to calculate average atomic mass (continued)

To calculate atomic mass (weighted average) of an element- multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. For chlorine (34.969 amu x 0.7553) = 26.4120857 amu + (36.966 amu x 0.2447) = 9.0455802 amu 26.4120857 amu + 9.0455802 amu = 35.4576659 26.4120857 amu + 9.0455802 amu = 35.4576659 or 35.457 amu (mass displayed on periodic table)

Review

Calculate the atomic mass of silicon, which has three isotopes.

Mass  number   Exact  weight  (amu)   Percent  Abundance  

28   27.976927   92.23  

29   28.976495   4.67  

30   29.973770   3.10  

Ions

•  Atoms  are  electrically  neutral  because  they  have  the  same  number  of  protons  and  electrons.  

•  Ion  is  an  atom  or  group  of  atoms  that  has  a  posi9ve  or  nega9ve  charge.  –  Ions  are  formed  when  electrons  are  transferred  between  atoms.      

•  CaCon-­‐  is  an  ion  with  a  posi9ve  charge.  It  is  formed  when  an  atom  loses  it  electron.  –  The  value  of  the  posi9ve  charge  =  number  of  electrons  lost.  –  Metals  tend  to  lose  electrons  and  form  ca9ons.  Ex:  Na1+  

•  Anion  is  an  ion  with  a  nega9ve  charge.    It  is  formed  when  an  atom  gains  electrons.  –  The  value  of  the  nega9ve  charge  =  number  of  electrons  gained.  –   Nonmetals  tend  to  gain  electrons  and  form  anions  .  Ex:  S2-­‐.  

 Charge  of  ion  =  number  of  protons  –  number  of  electrons  

 

Discovery of Radioactivity (1896)

Henri Becquerel, Marie Curie, and Pierre Curie

1.  Henri Becquerel notices uranium salts were able to fog photographic plates with absence of sunlight.

2.  Marie and Pierre Curie showed the rays emitted by uranium atom caused the fogging of the plates.

3.  Radioactivity- particles which emit radiation spontaneously. 1.  Nuclear reactions –the nuclei of unstable isotopes,

radioisotopes, gain stability by undergoing changes. 2.  The changes are accompanied by the emission

(discharge) of large amounts of energy. 3.  Changes in pressure, temperature, or the presence of a

catalysts has no effect on nuclear reactions.

Changes in the Nucleus

Chemical reactions atoms interact through their electrons only.

Nuclear reactions are not the same. These reactions occur when the nucleus changes.

* When the nucleus changes alpha, beta, and gamma radiation are emitted.

* Alpha & beta particles are emitted from the nucleus.

What causes Nuclear Stability?

• When more than one proton is present in the nucleus a repulsion force is present. • The neutrons in the nucleus keep the nucleus intact by acting as a glue (nuclear force). • As the # of protons increase eventually the # of neutrons are unable to override the force of repulsion and the nucleus becomes unstable. • Elements with atomic # of 84 and higher are radioactive. • As the nucleus becomes unstable radiation is emitted.

Three types of radiation

h"p://www.crea,on-­‐science-­‐prophecy.com/intro-­‐atomic.htm  

When  radia9on  passes  through  electrically  charged  plates  it  splits  to  produce:  •  Alpha  parCcles  -­‐posi9ve  charged  •  Gamma  parCcles  -­‐no  charge  •  Beta  parCcles  -­‐nega9ve  charge  

Penetrating Power

www.deq.state.id.us/.../images/alpha_beta.jpg

Half-life

                               #  half  lives    =  Cme  elapsed  

                                                                                               t  ½    

MEMORIZE  

•  Half-­‐life  is  the  amount  of  9me  it  takes  for  one-­‐half  of  the  nuclei  of  radioisotope  sample  to  decay  to  products.  –     Afer  each  half-­‐life,  half  of  the  exis9ng  radioac9ve  atoms  have  

decayed  into  atoms  of  a  new  element.  –  Half-­‐life  can  be  as  short  as  a  frac9on  of  a  second  or  as  long  as  a  

billion  years.