General Astronomy

Atoms 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".

• 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

So what are these ‘atoms?’

By the 1750’s, Electricity had been discovered

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

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