1. Stars: Element Factories
Review of last class
Dimitri Mendeleev and the
Periodic Table
What is an isotope?
Internal structure of the Sun
Stars and nuclear fusion

2. 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

3. Avogadro’s Number Avogadro’s number: the number of atoms in 1 mole

4. 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 , 1879 and 1886)

5. Mendeleev’s Periodic Law
The properties of elements are periodic functions of their atomic weight Cs Rb K
Julius Meyer, 1870 Na

6. Mendeleev’s Periodic Law
The periodicity led him to organize the elements into the PERIODIC TABLE OF ELEMENTS

7. Sample calculation Atomic weight of copper (Cu): 63.55 g
A g sample of Cu (= 1 mole) contains  atoms.
A penny weighing 1 g contains  1023 63.55 = 9.48  1021 atoms (to 3 significant digits)

8. 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

9. What’s an Isotope???
Definition: An Isotope is one of two or more atoms having the same atomic number but different mass numbers.

10. 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 ~ g.

11. Composition of the universe
Helium
(7.8%)
Carbon (.03%)
and Oxygen (.06%)
Hydrogen
(92%)
All other elements (.11%)

12. How are the elements created?
The Sun

13. The sun’s size in perspective
To scale

14. 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

15. Solar Data
* Compared to energy consumption of U.S.A.

16. 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

17. 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

18. Evolution of a 15 solar-mass star
Source: In Quest of the Universe, Kuhn, 1998