Physical Structure


Brother Gregory speaks to his class,

The subject for today is the basic, subcellular physical structure of all living things. You must read amd follow this lesson, answer the questions as you go, then complete the required research investigation(s) as instructed.

"Let us begin .......

"Although many people have contributed to our understanding of the atom and atomic structure, it was the Danish physicist, Neils Bohr who first showed us what interior of these fundamental particles could possibly look like. He gave us the 'modern' picture of the atom.

"It was a very strange picture indeed. Atoms, it appeared, consisted of even smaller particles of mass called protons, neutrons and electrons, which existed in a quantum world where nothing was as it seemed. Most of the mass of an atom was at it's center, where the protons and neutrons were located. But the electrons were a different story. The were found in orbitals and electron shells somewhere in the space around the atomic center - but no one was sure where.

As you proceed through this lesson, test yourself as you go by answering some of these "true/false" questions.

"Electrons were the first sub-atomic particles to be discovered. The English physicist J. J. Thomson discovered that "cathode rays" were actually streams of electrons, that could be ripped away from their places in the atom and sent flying across vacuums. Using his results as a guide he then took the very bold step of naming these strange new 'rays' as a fundamental part of the atom.

"Thompson proposed that all atoms looked like chocolate chip cookies, with the main part of the cookie a difuse positively charged cloud in which the negatively charged electrons were held like chocolate chips.

"Thompson's picture of the atoms did not last long, however. It was Ernest Rutherford who used streams of the much heavier and positively charged alpha particles to show that the atom was not at all like a chocolate chip cookie at all.

Rutherford's work led to the discovery of another fundamental sub-atomic particle - the proton - which was very dense and held a single positive charge.

When his co-worker, Hans Geiger, fired alpha particles at a thin piece of gold foil everyone was surprised when most of these massive particles passed straight though the gold metal.
It appeared that most of the atom was empty space with a tiny, but very dense atomic center , where the protons were located.

"This picture of the "nuclear atom" was completed with the discovery of the neutrons, the last of the sub-atomic particles to be discovered.


Matter Waves and Quanta - the strange world of light and electrons.

"But finding the atomic center of the atoms was the easy part. The next question was 'Where were the electrons?' Answering this question is not easy. To help us, we have to think of bees ...

"All physical objects, such as bees, have a mass and behave as if they were small physical 'bullets'. But, as Louis Victor de Broglie also showed, they also behave as if they were waves. Difficult as this is to understand, all objects have particle-wave duality, and can exist simultaneously in both states - particles and waves!

"These 'matter waves' don't seriously affect the properties of large objects, like baseballs, but this duality does need to be considered very seriously when looking at the properties of very, very small objects such as electrons and light.

Light shows 'particle-wave duality', the 'colors' of light are a property of light as a wave, and the energy carried by light comes in lumpy quantities called quanta - a property of light as a particle or 'photon'.

"Between them, Max Plank and Albert Einstein, explained most of the properties of light using the idea that this form or electromagnetic radiation consisted of quanta of energy (the 'photons') and at the same time a wave (the 'colors').

"Electrons, strange as it may seem, also have 'particle-wave duality', and the properties of electrons in atoms can only be accounted for by considering them as particle and waves at the same time!

"It was using light that the nature of electrons was first investigated. When light hits electrons all kinds of things happen including knocking electrons away from atoms, or moving the electrons around within the atom itself.

"Slowly, evidence began to accumulate about the location of electrons within the atom. It appeared that classical models in which electrons orbited, like planets, around a dense atomic center, (as originally proposed originally by Rutherford), would not work as the electrons would never be stable within such an atomic structure.


The Quantum Atom - energy comes in lumps

"A new picture was needed, and it was Niels Bohr who supplied it. He used the idea that energy came in lumps - the 'quanta' found by Plank and others - to explain how atoms could be stable only if their electrons were located at certain fixed positions. This was the first time any one had proposed a quantum atom, in which the behavior of electrons could only be explained if they absorbed and gave off energy in 'lumps'.

"But the wave properties of electrons could not be ignored. Electrons had mass, and therefore momentum, but, as Heisenberg showed, if these electrons were 'hit' by a photon of energy, (so you could 'see' it and find its location) you could not be certain what would happen afterwards. Thus the position and momentum of an electron could not both be known at the same time. This is the Heisenberg Uncertainty Principle.

"Consequently, since you can never know exactly where an electron is at any instant, you can only speak of the 'probability' of finding an electron in any general area. Thus, instead of thinking of electrons as 'planets orbiting the atomic center', we need a new image in which electrons don't exist as tiny planets but as diffuse waves of probability.

"It is also because of the wave-properties of electrons that they are restricted to certain volumes of space - the orbitals. Once again, thinking of bees helps us understand why this is so.

"When electrons take up quanta of energy from photons of light, they have to respond. An electron will only absorb a quantum of energy if that allows it to move from one permitted electron shell or orbital to another. This means that only those wavelengths of light that are carrying quanta of energy that enables this move will be absorbed or used by electrons.

"Such properties give rise to phenomena such as lines of light in spectra.


Joining Forces - bringing and holding atoms together

Listen at the door...
... to four friends talking about the discovery of atoms and molecules ...
... and some of their properties ...

"... how do we measure and known about the unseen world of atoms...?"

"... how do we know what happens whenatoms and molecules react with one another ...?"

"... how do we symbolically represent atoms on paper when we want to talk about them ...?"

"... how do we know what is in an organic molecule such as sugar ...?"

"... how do we know the shape and structure of organic molecules ...?"

"... how do we make molecules to order when we need them ...?"

"... how do we change the structure of molecules to change their properties ...?"

"Atoms are at their most stable when their outermost electron shell, or energy level, is either completely full, or completely empty of electrons. Atoms of certain elements, therefore, easily give up an electron or accept an additional electron. This changes the amount of positive or negative charge on the atom, and it is now called an 'ion'.

"Another way atoms can completely fill an outermost electron energy level is by sharing. When two atoms share electrons they form a covalent bond between them, which holds the atoms together in a very stable relationship. Atoms held together in this way are 'molecules'.

"Water is a very special molecule in the study of life and living things. The unequal sharing of electrons in a water molecule means that water has a number of unique properties all of which are critical in understanding why living organisms behave the way they do.


The Macromolecules of Life - biopolymers and their properties

"But living organisms contain more than simple molecules such as water. Most of the important functions of living cells are provided by giant, huge, macromolecules, all of which are polymers of one kind or another.

"For example, lipids and polysaccharides are both examples of homo-polymers; those polymers made of 'all of the same' monomer.

"On the other hand, heteropolymers are much more complex, because their polymers contain a variety of different monomers. Proteins are a very diverse group of macromolecules that can carry out a huge range of different functions.

"The polynucleotides are also hetero-polymers. These macromolecules have important roles in both the storing of biological information and also in transmitting this information from one generation to the next.

"But that is an different story ..."

Research investigation
check your schedule to see if this is required
How many carbon atoms?
Research Investigation
Concept questions
for the lesson

check your schedule to see if this is required
Physical Structure Set
Concept Questions and personal question page
Required Readings
for the lesson
Key Concepts
Part One -|- Part Two -|- Part Three
e-textbook: the main concepts

Table of Contents -|- A bit of history

the parts -|- building an atom -|- electrons

ions -|- covalent bonds -|- water

polymers -|- lipids and polysaccharides

proteins -|- polynucleotides

e-textbook: atomic structure

Table of Contents -|- electrons

protons -|- atomic center -|- neutrons

matter waves -|- light and quanta -|- photoelectric

nuclear atom -|- quantum atom

orbitals -|- electron energy

How we know:
read about how atoms, molecules and their properties were first discovered

measuring and knowning the unseen -|- getting a reaction

letter perfect -|- coming to a conclusion

putting molecules in shape -|- making molecules to order

useful additions

Suggested supplementary
reading for this lesson.
Meet Brother Gregory
Read - Chapter the Seventh

Science@a Distance
© 2003, Professor John Blamire