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•Review of last class
• Dimitri Mendeleev and the Periodic Table
•What is an isotope?
•Internal structure of the Sun
•Stars and nuclear fusion
Stars: Element Factories
What’s a Mole???
Mole (abbr. mol): An amount of an element (or compound) whose mass, expressed in grams, is numerically equal to its atomic (or molecular) weight.
Atomic Number (Z )
Atomic Symbol
Atomic Weight
Avogadro’s Number
• Avogadro’s number: the number of atoms in 1 mole
• 6.022 1023 mol-1
Dimitri Mendeleev
• At 33, Russian chemist Dimitri Mendeleev was appointed as chair of chemistry at the University of St. Petersburg.
• In 1871 he published a periodic table of the elements and predicted the properties of 3 new elements (Gallium, Scandium and Germanium, discovered in 1875, 1879 and 1886)
Mendeleev’s Periodic Law
• The properties of elements are periodic functions of their atomic weight
Julius Meyer, 1870KRb
Cs
Na
Mendeleev’s Periodic Law
• The periodicity led him to organize the elements into the PERIODIC TABLE OF ELEMENTS
Sample calculation
•Atomic weight of copper (Cu): 63.55 g
• A 63.55 g sample of Cu (= 1 mole)
contains 6.022 1023 atoms.
• A penny weighing 1 g contains 6.022 1023 63.55 = 9.48 1021 atoms (to 3 significant digits)
Atomic isotopes
• Mass number (A): number of protons + number of neutrons in an atom
•A is often indicated in the top left corner of the element symbol
16O, 18O
• Definition: An Isotope is one of two or more atoms having the same atomic number but different mass numbers.
Massnumber
(A)
Number ofprotons (Z)
Numberof
neutrons
Hydrogen (1H) 1 1 0Deuterium (2H) 2 1 1
Tritium (3H) 3 1 2
Uranium-235(235U)
235 92 143
Uranium-238(238U)
238 92 146
What’s an Isotope???
Molecules
• Definition: Molecules are two or more atoms joined together by chemical bonds.
• E.g., H2O
• The molecular weight is ~ the sum of the atomic weights.
• one mole of water weighs ~ 18.02 g.
Hydrogen(92%)
Helium(7.8%)
Carbon (.03%) and Oxygen (.06%)
All other elements (.11%)
Composition of the universe
How are the elements created?
The Sun
The sun’s size in perspective
To scale
Interior of the Sun
• The sun has multiple layers (like an onion)
• The sun’s energy source is generated by H fusion at the core
• Heat escapes the core by radiation
• In the outer part of the sun, heat is transferred by convection
Solar Data
Value Compared toEarth
Surface temp. 6000oC Hot!Diameter 1.392x106 km 109Density 1.41 g/ cm3 0.26Mass 1.989x1030 kg 330,000
Surface gravity 274.4 m/ s2 28Luminosity 3.9x1026 Watt 1.2x1014 *
Rotation periodEquator 25.4 days
40o latitude 28.0 days80o latitude 36.4 days
* Compared to energy consumption of U.S.A.
Nuclear fusion reactions
Definition: Nuclear fusion is the process of combining two (or more) atomic nuclei to create a new element.
• Requires extremely high temperature and pressure
• Energy release: E = mc2
Hydrogen fusion: the sun’s energy source (at the present)• All naturally occurring elements are created by fusion in the interiors of stars
•4 H nuclei are required to make 1 He nucleus
• Net mass reduction: 0.7%
• H -> He is a 3-step process
Evolution of a 15 solar-mass star
Elementfused
Fusionproduct
Time Temp. (K)
Hydrogen Helium 1010 years 4x106
Helium Carbon > 106 years 1x108
Carbon O, Ne, Mg 1000 years 6x108
Neon O, Mg A f ewyears
1x109
Oxygen Si, S 1 year 2x109
Silicon I ron A f ew days 3x109
Source: In Quest of the Universe, Kuhn, 1998