•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