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

a nucleus of protons & neutrons
orbiting electrons

Chemical Elements and Isotopes

Radioactivity

Four Fundamental Forces:

Gravitational & Electromagnetic
Strong &Weak Nuclear Forces

Atoms

Ordinary matter is found primarily in the form of atoms.

Range of ordinary matter:

Fundamental Particles (quarks & leptons)
Subatomic Particles (protons, neutrons, & electrons)
Single Atoms (hydrogen, helium, gold, etc.)
Simple Molecules (O₂, H₂O)
Macromolecules (DNA, complex polymers)
Macroscopic Objects (rocks, people, planets, etc.)

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:

proton: positively charged
neutron: uncharged (neutral)

Cloud of Electrons orbiting the Nucleus:

negatively charged particles
masses are 1/1836^th the mass of a proton

Atoms are mostly empty space:

Only 1 part in 10¹⁵ of the volume is occupied.
Volume is threaded by electromagnetic fields.

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:

1 proton = Hydrogen
2 protons = Helium
3 protons = Lithium ... and so on

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:

87 are metals
11 are gasses
2 occur as liquids (Bromine & Mercury)
26 are radioactive
25 are made only in particle accelerators

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.

Isotopes

A given elements can have many Isotopes

Same number of protons.
Different number of neutrons.

Again, a simple cartoon:

[Click on image for full-size version (17Kb GIF)]

Example:

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.

Radioactivity

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

Examples:

    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:

Time for half of the radioactive isotopes in a sample to decay.
The more "radioactive" the isotope, the shorter the half-life.

Examples:

   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

Note:

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
Electromagnetic Force
Strong Nuclear Force
Weak Nuclear Force

Gravitational Force

Gravitation binds masses 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

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:

Binds quarks insde protons & neutrons.
Binds protons & neutrons into nuclei.
Strongest force of nature.

Weak Force:

Responsible for radioactivity (turns neutron into a proton, electron, and neutrino)
Second weakest force.

The Interplay of Forces

Gravity rules on the largest scales:

Binds massive objects together
Mediates orbital motions

Electromagnetism rules on atomic scales:

Binds electrons to protons, atoms to atoms
Mediates chemical reactions

Strong & Weak Forces rule on nuclear scales:

Binds protons to neutrons inside nuclei
Mediates radioactivity and nuclear reactions

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