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Saturn from Cassini Astronomy 161:
An Introduction to Solar System Astronomy
Prof. Richard Pogge, MTWThF 9:30

Lecture 23: Worlds Within: Atoms

Key Ideas:

Atoms are composed of Chemical Elements and Isotopes


Four Fundamental Forces:


Ordinary matter is found primarily in the form of atoms.

Range of ordinary matter:

On the Earth, matter is rarely in the form of fundamental or subatomic particles, but most often in the form of simple atoms and larger.

Atomic Structure

Nucleus of heavy subatomic particles:

Cloud of Electrons orbiting the Nucleus:

Atoms are mostly empty space:

A simple, yet familiar cartoon:
Simple Atoms (Schematic)
Real atoms don't look like this, but it can help you visualize the locations and numbers of the different components (nucleus and electrons).

Chemical Elements

Distinguish atoms into Elements by the total number of protons in the nucleus.

Atomic Number:

Number of electrons = Number of protons

All elements are Chemically Distinct

Top Ten Most Abundant Elements

10) Sulfur
9) Magnesium
8) Iron
7) Silicon
6) Nitrogen
5) Neon
4) Carbon
3) Oxygen
2) Helium
1) Hydrogen

Known Elements

118 elements are currently known:

In addition, each element can have a number of isotopes.

The newest possible member of the table of the elements, with atomic number 118, was jointly announced by Russian and American scientists who briefly made this unstable atom by colliding atoms of cesium (20 protons) and Californium (98 protons) in a particle accelerator.


A given elements can have many Isotopes Again, a simple cartoon:
isotope schematic [Click on image for full-size version (17Kb GIF)]


12C has 6 protons and 6 neutrons
13C has 6 protons and 7 neutrons
14C has 6 protons and 8 neutrons

All isotopes of a given element are chemically identical, but they have different masses.


If a nucleus has too many or too few neutrons, it becomes unstable to radioactive decay


    3H (1p+2n) -> 3He (2p+1n) + e- + neutrino

    14C (6p+8n) -> 14N (7p+7n) + e- + neutrino 
                      (basis of radioactive carbon dating)

Free neutrons are also unstable:

    n -> p + e- + neutrino

All of the above are examples of "beta decay", in which one of the neutrons turns into a proton, shedding its excess mass by spitting out an electron and a neutrino.

Radioactive Half-Life

Radioactive decay is a random process.

The activity is measured in terms of the Half-Life of the isotope:


   3H -> 3He + e- + neutrino : half-life = 12.26 years

   14C -> 14N + e- + neutrino : half-life = 5730 years
   n -> p + e- + neutino : half-life = 12 minutes
Radioactivity is an example of a random process, and the half-life of an element is a strictly statistical measurement of its radioactivity. This makes it somewhat unfamiliar to most people at first encounter.

An every-day analogy that will help you to better understand both random processes and half-lives is to consider popping popcorn. You never pop popcorn one kernel at a time, you always make a batch (i.e., you work with a "sample" made up of many kernels). Similarly, popcorn popping is a random process like radioactive decay: you don't know when any particular kernel will pop. Some will pop right away, and some never seem to pop.

A way to measure the "activity" of the popcorn is to measure the time it takes for half of the kernels to pop on average. We would call this time the "half-life" of the popcorn. Very poppy popcorn that pops up quickly has a short half-life, while popcorn that takes a long time to pop has a long half-life. Regular sweet corn is "inactive", in the sense that no matter how long you wait, it never pops.

Fundamental Forces of Nature

All interactions in nature are governed by four "fundamental" forces:

Gravitational Force

Gravitation binds masses over long distances.

Electromagnetic Force

Electromagnetic force acts between charged particles

Long-range, inverse-square law force:

The Electromagnetic force is very strong: approximately 1039 times stronger than Gravity.

Strong & Weak Nuclear Forces

Short-range forces (<10-15m) in atomic nuclei

Strong Force:

Weak Force:

The Interplay of Forces

Gravity rules on the largest scales:

Electromagnetism rules on atomic scales:

Strong & Weak Forces rule on nuclear scales:

Return to [ Unit 4 Index | Astronomy 161 Main Page ]
Updated: 2006 October 23
Copyright Richard W. Pogge, All Rights Reserved.