general astronomy atoms and molecules and ions, oh my!
TRANSCRIPT
The Atom – Structure and TheoryEarly History
The Greeks tried to explain chemical changes such as:
• Dying cloth and skins• Writing with charcoal• Finding medicines in plants• Observing rot and burning • Observing rusting, bleaching and silver
tarninishing
By 400 BCE, they had proposed that matter consisted of four elements: Fire, Water, Earth and Air
In approximately 450 BCE, Democritus coined the term atomos (Greek: aτομος), which means "uncuttable" or "the smallest indivisible particle of matter".
• Considered whether matter is continuous and therefore can be infinitely divisible into smaller pieces
• Whether it is composed of small indivisible particles
• In approximately 450 BCE, Democritus coined the term atomos (Greek: aτομος), which means "uncuttable" or "the smallest indivisible particle of matter."
The Atom – Structure and TheoryEarly History
mid 1800’s
• Groups of scientists were working on a variety of projects hoping to gain insight into the atom
• Beginning in around 1850 there were a significant number of discoveries that lead to a better understanding of the atom
Discovery of the Electron
J.J. Thomson • Constructed a CRT that had a fluorescent screen at the end• Was able to measure the effects of electric and magnetic
fields• Determined the charge to mass ratio of the “ray”• Concluded that the particles had a negative charge
• Bequerel was experimenting with fluorescence.
• Used pitchblende which contains uranium• Irradiated the rock• Cloudy day, found the same pattern in the
film and discovered radiation
Radioactivity
• Marie Sklodowska called the phenomena discovered by Becquerel - Radiation
• Madam Curie discovered a number of radioactive elements including thorium
• Died in 1934 from pernicious anemia
Describes the spontaneous decomposition of atoms into other elements with the loss of subatomic particles
Radioactivity
• Alpha Positively charged helium nucleus
• Beta Negatively charged; beam of electrons
• Gamma Possess characteristics similar to X-rays; more energetic than either gamma or alpha
Scientists knew that an atom was neutral
If an electron was negative, then what was positive?
Discovery of the Electron
One theory suggested that the positive and negative charges were placed randomly within the atom, This was called the ‘Plum Pudding Model’
Today we might call this the chocolate chip muffin model
Rutherford & the Plum Pudding Model
Ernest Rutherford set out to prove the Plum Pudding modelBy bombarding a piece of gold foil he felt he would see a uniform random pattern of scattering
Rutherford’s Results
• Most of the atom is empty space• Occasionally the alpha would come
close to a positive region in the atom• Rarely the alpha particle would be
deflected back to it’s origin• Continued work and discovered that
the positively charged hydrogen ion has the simplest nucleus consisting of a single + charge species, later called it a Proton
The Atom
• Still not complete• Mass of electrons + mass of protons did
not add up• Chadwick in 1932 discovered the
missing particle• Approximately the same mass as a
proton• Zero Charge• The Neutron
• The picture was finally complete• The atom was not ‘uncuttable’ , it
consisted of electrons, protons and neutrons
The Atom
Atomic particles
Particle
Charge
Mass (kg) Mass (MeV)
Location
Electron -1 9.1093897x10-
31
0.51099906 Orbiting/Cloud
Proton +1 1.672631x10-27 938.27231 In the nucleus
Neutron 0 1.6749286x10-
27
939.56563 In the nucleus
As you can see, a proton is about 1836 times more massive than an electron and very slightly less massive than a neutron
Others in the atomic zooFor introductory astronomy, we only need to deal with a few more of the many particles:Gamma particlesNeutrinosAnti-matter
Gamma particles (or gamma rays) are really high-energy light particles (photons)
Neutrinos are the ‘little neutral ones’ arising from nuclear processes in the stars
Anti-matter - for each particle of matter there is a corresponding anti-particle which has the same mass, but the opposite charge.
AtomsAn atom is composed of a nucleus (having protons and
neutrons) surrounded by a cloud of electrons.
Over 99.94% of an atom's mass is concentrated in the nucleus
For a neutral atom, the number of protons in the nucleus exactly matches the number of electrons surrounding it.
The simplest (somewhat incorrect, but useful) model is the Rutherford Atom which looks like solar system where the electrons orbit the nucleus.
HydrogenWe will discuss the problems with the Rutherford
model later on, but for now it is a useful way to visualize the atom.
The simplest atom is that of Hydrogen. It has a single proton for a nucleus and a single electron
Hydrogen AtomThis is designated as:
H11
Atomic Mass Number
Atomic Number
Symbol
(Count of protons OR electrons)
(Count of protons AND neutrons)
Helium
He42
4 mass units – 2 protons = 2 neutrons
Of course, the size of the nucleus is greatly exaggerated!
It’s closer to this:
Lithium
6
36 mass units – 3 protons = 3 neutrons
The rest of atoms in the Periodic Table of Elements is built up in the same way
Isotopes
Atoms may also occur where there are varying numbers of neutrons in the nucleus. These are known as isotopes of the atom.
H21 Deuterium
H31 Tritium
Molecules
Molecules are combinations of atoms. They can be as simple as a Hydrogen
molecule, H2 , where 2 Hydrogen atoms are bonded together.
There may be many atoms combined into a large molecule.
Most, in astronomy, are reasonably simple such as water (H2O) or Titanium Oxide (TiO) or formaldehyde (H2CO)
IonsUnder certain circumstances, the atom may lose one or more electrons thereby gaining a net positive charge. (It’s possible to get extra, gaining a net negative charge, but conditions are rarely good in astronomy for that to occur)
The degree of ionization gives the number of missing electrons (the net positive charge)State Astronomer’s
SymbolChemist’sSymbol
Neutral H I, He I, Fe I H, He, Fe
Single ionization
H II, He II, Fe II H+, He+, Fe+
Double ionization
He III, Fe III He + +, Fe+ +
For example, Fe XXVI is Iron with 25 electrons missing
Nuclear Reactions
HHeHeHe
HeHH
HHH
11
42
32
32
32
11
21
21
11
11
2 + ¾®¾+
+¾®¾+
++¾®¾+ +
g
nb
This is another way of describing the proton-proton reaction which powers the Sun
Getting the energyWe can get an idea of where the energy comes from using a 'Fermi Calculation†'
† From Enrico Fermi, famous for being able to make fast, back-of-the-envelope calculations to get approximate results
4 H He
1 H is 1.0080 amu 1 He is 4.003 amu
4 x 1.0080 - 4.003 = 0.029
Not much? Remember E = m c2 and c2 is pretty big!
What are the other particles?There were other particles produced during the reaction:
A ß-particle is the 'old' name for an electronWhich has a negative charge; therefore theß+ is a positive electron or positron
A γ-particle (Gamma) is a high energy photon
A is a neutrino (an 'electron' neutrino – more about that
later)
Just to be complete, the He nucleus consisting of 2 protons and 2 neutrons is also called an
a-particle