The History of AtomsThe History of AtomsNot the history of the atom itself, but the history of the idea of the

atom.

1. Democritus 460-370 B.C 1. Democritus 460-370 B.C Democritus (Greek Democritus (Greek

Philosopher 384-322 Philosopher 384-322 B.C.B.C.

Matter is made of Matter is made of Atomos Atomos

(He thought it (He thought it looked like this!)looked like this!)

Some weaknesses of Some weaknesses of Democritus's ideaDemocritus's idea

No empirical (experimental) No empirical (experimental) evidence, just a well thought out evidence, just a well thought out idea.idea.

““What holds the Atomos together?” What holds the Atomos together?” was one important question that was one important question that Democritus could not answer.Democritus could not answer.

2. Aristotle 384-322 B.C. 2. Aristotle 384-322 B.C.

Completely rejected Democritus’s ideaAristotle was so well regarded and influential that his rejection of the Atomos caused the idea of Atoms to be dismissed for nearly 2000 years .

3. John Dalton 1766-18443. John Dalton 1766-1844He revisited and revised Democritus’s ideas.He made careful observations on numerous chemical reactions that helped him refine his theories.

Dalton’s Atomic TheoryDalton’s Atomic Theory All atoms of an element are All atoms of an element are

identical having the same size, mass identical having the same size, mass and chemical properties. Atoms of and chemical properties. Atoms of an elements are different from all an elements are different from all other atoms of different elements.other atoms of different elements.

Dalton’s Atomic TheoryDalton’s Atomic Theory Atoms cannot be created, destroyed Atoms cannot be created, destroyed

or divided in to smaller particle.or divided in to smaller particle.

Dalton’s Atomic TheoryDalton’s Atomic Theory Atoms cannot be created, destroyed Atoms cannot be created, destroyed

or divided in to smaller particle.or divided in to smaller particle. Different atoms combine in simple Different atoms combine in simple

whole number ratios to form whole number ratios to form compounds.compounds.

Atoms are neither created nor Atoms are neither created nor destroyed in chemical reactions.destroyed in chemical reactions.CO2

Law of Conservation of MassLaw of Conservation of MassThe total mass of substances present The total mass of substances present at the end of a chemical process is at the end of a chemical process is the same as the mass of substances the same as the mass of substances present before the process took present before the process took place.place.

Is Dalton’s Theory Completely Is Dalton’s Theory Completely Accurate?Accurate?

It was a major step in the right direction.It was a major step in the right direction.

No, atoms of the same element can No, atoms of the same element can have slightly different masses (Isotopes).have slightly different masses (Isotopes).

No, atoms are divisible into neutrons No, atoms are divisible into neutrons protons and electrons.protons and electrons.

4. J. J. Thomson 4. J. J. Thomson

““Plum Pudding Plum Pudding Model”Model”

Around 1900

5. Rutherford’s Discovery of the 5. Rutherford’s Discovery of the NucleusNucleus

Ernest Rutherford Ernest Rutherford shot shot particles at particles at a thin sheet of gold a thin sheet of gold foil and observed foil and observed the pattern of the pattern of scatter of the scatter of the particles.particles.

Protons were discovered by Rutherford in 1919.

6. Niels Bohr “Planetary Atomic 6. Niels Bohr “Planetary Atomic Model” 1913Model” 1913

Electrons only occupy well-defined orbits with Electrons only occupy well-defined orbits with fixed energy levelsfixed energy levels

Discovered that electrons may absorb energy Discovered that electrons may absorb energy by jumping to higher energy levels and fall back by jumping to higher energy levels and fall back to the original level or orbit. This release of to the original level or orbit. This release of energy is called a energy is called a photonphoton..

7. Modern View of the 7. Modern View of the AtomAtom

Tiny nucleus surrounded by electron Tiny nucleus surrounded by electron “cloud”“cloud”

Nucleus accounts for all of the massNucleus accounts for all of the mass

Subatomic ParticlesSubatomic Particles

Protons (p+) - positive charge – Protons (p+) - positive charge – inside the nucleus – weight 1 amu inside the nucleus – weight 1 amu (atomic mass unit)(atomic mass unit)

Neutron (n0) – neutral charge – Neutron (n0) – neutral charge – inside the nucleus – weight 1amu inside the nucleus – weight 1amu

Electron (e-) - Negative Charge – in Electron (e-) - Negative Charge – in Electron Cloud – weight 1/1840 amuElectron Cloud – weight 1/1840 amu

Using the Periodic Table to Using the Periodic Table to Determine the Subatomic Determine the Subatomic

ParticlesParticlesEquals the Number of Protons (Also the Number of Electrons in a neutral atom)Equals the Number of Protons + Neutrons (since the nucleus is where all the weight is located)