Astronomy 161
Introduction to Solar System Astronomy
Prof. Paul Martini

Lecture 21: Matter

Key Ideas:

Atoms are composed of: a nucleus of protons and neutrons; orbiting electrons
Chemical Elements and Isotopes
Radioactivity
Four Fundamental Forces: Gravitational and Electromagnetic; Strong and Weak Nuclear Forces

Atoms

Ordinary matter is found primarily in the form of atoms
The word atom comes from the Greek word atomos, which means indivisible
Range of ordinary matter:: Free subatomic particles (protons and electrons); Single atoms (hydrogen, helium, gold, etc.); Simple molecules (O₂, H₂O, etc.); Macromolecules (DNA, complex polymers, oil, etc.); Macroscopic objects (diamonds, trees, people, planets, etc.)

The Gold Foil Experiment

Early models of the atom assumed atoms had uniform density
In 1909 Hans Geiger and Ernest Marsden shot charged particles (called alpha particles) through a thin sheet of gold foil
Surprisingly, about 98% went straight through, 2% were deflected, and 0.01% bounced straight back!
The Atom was nearly empty!

Atomic Structure

Nucleus of a heavy subatomic particles:: proton: positively charged; neutron: uncharged (neutral)
Electrons orbiting the nucleus: negatively charged particles; 1/1836th the mass of a proton
Atoms are mostly empty space: Only 1 part in 10¹⁵ of space is occupied; The rest of the volume is threaded by electromagnetic fields

Chemical Elements

Atoms are separated into distinct Elements by the number of protons in the nucleus
Atomic Number:: 1 proton: Hydrogen; 2 proton: Helium; 3 proton: Lithium
Number of electrons = Number of protons
All elements are Chemically Distinct
An atom is the smallest division of a chemical element

Top Ten Most Abundant Elements

Variation in Abundance

The average abundances of elements in the Universe need to be measured over a large Volume: For example, the entire Solar System (at least)
Local variations can be significant: Oceans mostly Hydrogen and Oxygen (in water); Earth's crust mostly Oxygen, Silicon, Aluminum, and Iron

Known Elements

117 elements are currently known:: 87 are metals; 11 are gasses; 2 occur as liquids (Bromine and Mercury); 26 are naturally radioactive; 24 are made only in particle accelerators
In addition, each element can have a number of different isotopes

Isotopes

A given element can have many Isotopes: Same number of protons; Different number of neutrons
Example:: ¹²C has 6 protons and 6 neutrons; ¹³C has 6 protons and 7 neutrons; ¹⁴C has 6 protons and 8 neutrons
These are chemically identical, but have different masses

Radioactivity

Discovered by Antoine Becquerel in 1896, who observed that Uranium would fog photographic plates
If a nucleus has too many or too few neutrons, it is unstable to radioactive decay
Examples:: ³H (1p + 2n) decays to ³He (2p + 1n) + 1 electron + 1 neutrino; ¹⁴C (6p + 8n) decays to ¹⁴N (7p + 7n) + 1 electron + 1 neutrino; This later reaction is the basis of radiocarbon dating
Free neutrons are also unstable: n decays to p + 1 electron + 1 neutrino

Radioactive Half-Life

Radioactive decay is a random process
The activity is measured by the Half-Life: Time for half of the atoms to decay; The more radioactive, the shorter the half-life
Examples:: ³H (1p + 2n) decays to ³He (2p + 1n) + 1 electron + 1 neutrino with a 12.26 year half-life; ¹⁴C (6p + 8n) decays to ¹⁴N (7p + 7n) + 1 electron + 1 neutrino with a 5730 year half-life; n decays to p + 1 electron + 1 neutrino with a 12 minute half-life

Uranium

²³⁸U has a half-life of 4.51 x 10⁹ years. The vast majority of natural Uranium is in this isotope
²³⁵U has a half-life of 7 x 10⁸ years. It is consequently very rare, but it produces lots of neutrons when it decays and can therefore sustain nuclear reactions (either for power or bombs)

Fundamental Forces of Nature

All interactions in nature are governed by four "fundamental" forces: Gravitational Force; Electromagnetic Force; Strong Nuclear Force; Weak Nuclear Force

Gravitational Force

Gravitation binds matter over long distances: Long-range attractive force; Weakest force of nature
Obeys the Inverse Square Law of distance:

Electromagnetic Force

Electromagnetic force acts between charged particles: Like charges repel each other; Opposite charges attract each other
Long-range, inverse-square law force:: Binds electrons and protons into atoms; Binds atoms together into molecules
Very strong:: 10³⁹ times stronger than Gravity

Strong and Weak Nuclear Forces

Short-range forces (<10^-15m) in atomic nuclei
Strong force:: Binds protons and neutrons into nuclei; Strongest force of nature
Weak force:: Responsible for radioactivity (what turns a neutron into a proton, electron, and neutrino during neutron decay); Second strongest force

The Interplay of Forces

Gravity rules on the largest scales:: Binds massive objects together; Controls orbital motions
Electromagnetism rules on atomic scales: Binds electrons to protons and atoms to atoms; Mediates chemical reactions
Strong and Weak Forces rule on nuclear scales: Binds protons to neutrons inside nuclei; Mediates radioactivity and nuclear reactions

See A Note about Graphics to learn why some of the graphics shown in the lectures are not reproduced with these notes.