General Astronomy Atoms and Molecules and Ions, Oh My!

General AstronomyAtoms and Molecules and Ions, Oh My!

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".

The Atom – Structure and TheoryEarly History • 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."

So what are these ‘atoms?’ By the 1750’s, Electricity had been discovered

So what are these ‘atoms?’ Positive and negative charges

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 • 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 J.J. Thomson

Radioactivity 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 • 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

Radioactivity Describes the spontaneous decomposition of atoms into other elements with the loss of subatomic particles • 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

Discovery of the Electron Scientists knew that an atom was neutral If an electron was negative, then what was positive? 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 model By 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 Atom • The picture was finally complete • The atom was not ‘uncuttable’ , it consisted of electrons, protons and neutrons

Atomic particles 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 zoo For introductory astronomy, we only need to deal with a few more of the many particles: Gamma particles Neutrinos Anti-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.

Atoms An 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.

Hydrogen We 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 Atom This is designated as: Atomic Mass Number Symbol (Count of protons AND neutrons) Atomic Number (Count of protons OR electrons)

Helium 4 mass units – 2 protons = 2 neutrons Of course, the size of the nucleus is greatly exaggerated! It’s closer to this:

Lithium 6 6 mass units – 3 protons = 3 neutrons 3 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. Deuterium 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)

Ions Under 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) For example, Fe XXVI is Iron with 25 electrons missing

+ + ¾ ¾ ® + b + n 1 1 2 H H H 1 1 1 + ¾ ¾ ® + g 2 1 3 H H He 1 1 2 + ¾ ¾ ® 1 + 2 H 3 3 4 He He He 2 2 2 1 Nuclear Reactions This is another way of describing the proton-proton reaction which powers the Sun

Getting the energy We can get an idea of where the energy comes from using a 'Fermi Calculation†' 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! † From Enrico Fermi, famous for being able to make fast, back-of-the-envelope calculations to get approximate results

What are the other particles? There were other particles produced during the reaction: A ß-particle is the 'old' name for an electron Which has a negative charge; therefore the ß+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

General Astronomy Atoms and Molecules and Ions, Oh My!