Atomic Theory and Nuclear Atomic Theory and Nuclear Honors Chemistry 6.0.
Atomic Structure and Nuclear Stability
Transcript of Atomic Structure and Nuclear Stability
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.