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But one doesn't what to draw to hard a line.
In many early mythic conceptions, the earliest gods are rather peronalityless and resemble elementary or primeval forces.
This is true of Greek mythology where the earliest god is Chaos is formless or void state from which everything originated---as recounted in Hesiod's (circa late 8th century BCE) poem Theogony.
Caption: "So-called Seneca. Ancient Roman bronze now at the National Archaeological Museum of Naples, Italy." The bust was originally thought to be of Seneca (c. 4 BCE--65 CE). But nowadays the Pseudo-Seneca is considered to possibly be an imagined portrait of Hesiod (circa late 8th century BCE).
Hesiod is the author of the long poems Theogony and Works and Days.
Hesiod as revealed in Works and Days---which not be by Hesiod, but by another Greek of the same name---is a rather gloomy, pessimistic farmer-poet---maybe a lot like Robert Frost (1874--1963)---the bust artist may have been trying to capture that character.
Credit: Ancient Roman sculptor. Image by Massimo Finizio.
Linked source: Wikipedia image http://en.wikipedia.org/wiki/File:Seneca.JPG.
Permission: Licensed under the Creative Commons Attribution ShareAlike 2.5.
Gaia was somehow produced by Chaos and Gaia (Mother Earth) gave birth parthenogenetically to Uranus (Father Sky), and the story goes onto more generations of more personalized gods.
It seems that Hesiod and other mythological thinkers may have already had a rudimentary idea that the universe could NOT have started with quirky personalities.
Nevertheless, a thoroughgoing attempt to study nature from general principles---natural philosphy---starts with the Presocratics.
Caption: "Graphical relationship among the various pre-socratic philosophers and thinkers; red arrows indicate a relationship of opposition."
A flow-chart of the Presocratics.
Credit: User:Tresoldi AKA Tiago Tresoldi.
Image linked to Wikipedia.
Permission: Licensed under the Creative Commons Attribution ShareAlike 2.5.
One must emphasize, the Presocratics did very little experimentation of detailed observation. Their explanations of natural phenomena in terms of their various sets of general principles were vague and handwaving.
The theories of the Presocratics can be described a rational mythmaking.
Still their theories are often fascinating laid a foundation for later science.
The idea of atoms began with the atomism of the shadowy Leucippus (ca. 1st half of 5th century BCE and his much better known student Democritus (ca. 460--ca. 370 BCE)
Caption: "Democritus (ca. 460--ca. 370 BCE) meditating on the seat of the soul (1868) by Leon-Alexandre Delhomme (1841--1893 or 1895)."
Democritus was foremost exponent of atomism among the ancient Greeks
Credit: User:Jean-Louis Lascoux.
Image linked to Wikipedia.
Permission: Use under GNU Free Documentation License.
Ancient atomism is very far from out modern concepts.
The atomists were thinking of minute, absolutely uncutable hard bodies. The word atomos means uncutable. They though dust motes may have been visible atoms.
How the atoms interacted to make the atomists ever came up with the idea of atoms. But perhaps it wasn't so hard. From a simpleminded perspective, one might say there are only two choices: matter is infinitely divisible or it's not. In the later case, some kind of atoms becomes a plausible idea.
The atomists and their followers came up with many ingenious ways to explain in vague ways the universe in terms of atoms.
But they never had solid evidence.
Also atomism fell into disfavor in Greco-Roman antiquity because it was incorporated into Epicureanism which was considered atheistic---actually the Epicureans believed the gods exist, but that they didn't do anything in the human world.
The idea of atoms didn't disappear and in the Scientific Revolution of the 17th century was picked up by, among others, Rene Descartes (1596--1600) and Newton (1643--1727).
But no solid evidence for them was yet known.
Solid evidence appeared with John Dalton (1766--1844) who was able to explain chemistry in terms of the interactions of atoms.
To understand the basics of Dalton's we consider the following example using modern values.
2.0158 grams of hydrogen gas + 15.999 grams of oxygen gas always react together to give 18.015 grams of liquid water.Now hydrogen gas, oxygen gas, and liquid water all seem to be homogeneous substances.
No matter how small a sample you take of any (as long as it is macroscopic), the sample's chemical properties are the same.
So let's posit hydrogen consists of atoms of mass 1.0079 atomic mass units (AMUs) and oxygen of atoms of mass 15.999 AMUs.
An atom's mass in AMUs is its atomic mass.
The modern definition of atomic mass unit is the 1/12 of mass of an unperturbed Carbon-12.
The number of AMUs in 1 gram is:
1 gram ---------------------- = Avogadro's number 1 AMU in grams =6.02214179(30)*10**23 = approx 6*10**23 = approx 10**24 = 1 moleDividing any mass of a substance sample by the substance's atomic mass and Avogadro's number gives the number of particles in the sample in moles.
Such a molecule of water would have mass 18.015 AMUs.
In stead of using masses for the reactants, we can use moles (i.e., counts of particles).
In that case 2 moles of hydrogen reacted with 1 mole of oxygen always give 1 moel of
2 moles hydrogen gas + 1 mole of oxygen gas always react together to give 1 mole of liquid water.The assigned atomic mass water.
If you reverse, the above discussion you see how the atomic hypothesis plus atomic masses and Avogadro's number can be used explain the amounts of substances needed in chemical reactions.
Dalton did not know the modern atomic masses and Avogadro's number, but he could infer relative atomic masses and and use those to explain the relative amounts of substances in chemical reactions.
Caption: `A scan of the first page of John Dalton's (1766--1844) "A New System of Chemical Philosophy", published in 1808. Please do not "update" the list with modern spellings. This is a historic list and the old spellings are intentional. Yes, it's "carbone", not "carbon".'
The relative atomic masses of the atoms, but not molecules shown in the figure.
1. Hydrogen, its relative weight 1 2. Azote 5 3. Carbone or charcoal 5 4. Oxygen 7 5. Phosphorous 9 6. Sulphur 13 7. Magnesia 20 8. Lime 23 9. Soda 28 10. Potash 42 11. Strontites 46 12. Barytes 68 13. Iron 38 14. Zinc 56 15. Copper 56 16. Lead 95 17. Silver 100 18. Platina 100 19. Gold 140 20. Mercury 167
Credit: John Dalton (1766--1844), User:haade.
Permission: Public domain at least in USA.
Image linked to Wikipedia.
Caption: "Reproduced from the book of Jean Baptiste Perrin (1870--1942), `Les Atomes', three tracings of the motion of colloidal particles of radius 0.53 µm, as seen under the microscope, are displayed. Successive positions every 30 seconds are joined by straight line segments (the mesh size is 3.2 µm)."
Because, the observations are 30 seconds apart, the motion between the observations is NOT seen in detail and is only crudely approximated by the straight line segments.
The colloidal particles (which are macroscopic) are driven in a random walk by random collisions with molecules.
The collodial particle motion is called Brownian motion after the effective discoverer of the phenomenon Robert Brown (1773--1858) in 1827.
Credit: Jean Baptiste Perrin (1870--1942), User:MiraiWarren.
Permission: Public domain at least in USA.
Image linked to Wikipedia.
Caption: "Beam of electrons moving in a circle in a magnetic field (cyclotron motion). Lighting is caused by exitation of atoms of gas in a bulb."
Electrons were definitively discovered as new particles in 1896 by J. J. Thomson (1856--1940 and colleagues.
They could be produced in beams from hot metal surfaces and directed by electric fields and magnetic fields.
Electrons had negative charge and were ubiquitous in all materials.
This suggested electrons were constituents of atoms and that atoms needed to have positive constituents as well in order to be mainly neutral overall.
The fact that atoms had constituents meant they could not be the simple, uncutable entities the ancient atomists hat thought.
Credit: User:Sfu AKA Marcin Bialek.
Permission: Use under GNU Free Documentation License.
Image linked to Wikipedia.
Caption: A of an atom changing it's energy state and emitting a photon.
Atoms don't really look like this.
This is Bohr atom, a model invented by Niels Bohr (1885--1962) in 1913.
The Bohr atom correctly incorporated the recently discovered atomic nucleus.
But essentially, it is a wrong model. It is still used for illustrative purposes.
The center of the ``orbits'' is the atomic nucleus.
There is only one electron shown and it can be in a discrete set of ``orbits'' ONLY.
These ``orbits'' are the quantized energy states or levels of the atom.
When the electron changes energy state a photon is emitted or absorbed.
The emitted or absorbed photon maintains the conservation of energy.
An absorption process requires an incident photon.
An emission process can happen spontaneously or can be stimulated to happen by an incident photon.
But there is NO spontaneous emission from the lowest energy level---this the ground level in quantum mechanics jargon.
Since the energy levels are quantized, only certain energies of photons are allowed.
This means only certain frequencies and wavelengths are allowed.
The energy levels are pictured as circular orbits, but nowadays we know that this is not really true.
The energy levels must be represented by distributions of positions since the electrons are in a continuum superposition positions.
But early model of the atom, the Bohr atom, did posit circular orbits.
Credit: User:JabberWok.
Image linked to Wikipedia.
Permission: Use under GNU Free Documentation License.
Caption: Cartoon of gas molecules motion at finite temperature.
Credit: User:Greg L.
Image linked to Wikipedia.
Permission: Use under GNU Free Documentation License
A Helium-4 (He-4) atom and nucleus.
The size scale of the atom is about an angstrom (10**(-10) m).
The grey shading represents the electron distribution.
There are only two electrons, but they exist in superposition of locations.
Quantum mechanical particles are fractionally in a continuum of places at once. This is one of the things that makes quantum mechanics so tricky.
The scaled up region represents the atomic nucleus which consists of two protons and two neutrons bound together by the strong nuclear force.
The positively charged protons repel each other by the electromagnetic force, but the strong nuclear force overcomes this.
The proton and two neutron actually exist in a superposition of locations too---the illustration is only meant to be symbolical.
The size scale of the nucleus is about a fermi (10**(-15) m).
So the nucleus is about 10**5 times smaller than the atom, but it is about 4000 times more massive.
The negatively charged electrons are bound to the nucleus by attraction to the positively charged protons.
Credit: Yzmo.
Image linked to Wikipedia.
Permission: Use under GNU Free Documentation License.