2. Solid, homogeneous, indestructible and indivisible (400 BC)
The Atom: History Recap
Solid, homogeneous, indestructible and indivisible (400 BC)
Democritus: The indivisible atom
Matter is composed of empty space through which the atoms move
The differing properties of matter are due to the different sizes, shapes and movement of the atoms.
Changes in matter are due to changes in the arrangements of the atoms and not from changes in the atoms themselves.
(atomos = “uncuttable”)

3. Dalton’s atomic theory: particulate nature of matter
(1803)
Indivisible atom
All atoms of an element are identical (same size, mass and properties) and unique (not like any other element’s atoms)
Atoms combine in simple whole number ratios to form compounds.
Chemical reactions cause atoms to be separated, combined or rearranged. +
2 H2
1 O2
2 H2O

4. Plum-pudding model of the atom (1904)
J.J. Thomson: The ‘plum pudding model’
1897-Measures the mass to charge ratio of the electron and discovers that it is smaller than a hydrogen atom. The ‘indivisible’ atom is no longer indivisible.
1904-Proposes that the atom consisted of two parts…a large positively charged mass with electron ‘raisins’ mixed in with it.
Plum-pudding model of the atom (1904)
1909-Robert Millikan determines an electron’s charge (-1) and uses Thompson’s mass to charge ratio to calculate the mass of an electron:
9.1 x g

5. Ernest Rutherford: gold-foil experiment (1911)
Hypothesis: If Thomson’s model of the atom is valid, then a positive particle will be able to pass through an atom with little deflection.
Experiment: Bombard thin gold foil with alpha particles (nuclei of helium atoms) and measure the amount of deflection.

8. Ernest Rutherford: gold-foil experiment (1911)
Most of the atom consisted of empty space.
All of the positive charge and virtually all of the atom’s mass is contained in a dense region in the center called the nucleus.
The electrons move rapidly through empty space outside of the nucleus.
The nuclear atom
1920-Rutherford refines model: the nucleus contains positively charged particles called protons (+1 charge).
1932-Third subatomic particle, the neutron (0 charge) discovered by James Chadwick.

9. Niels Bohr: planetary model of the atom (1913)
Electrons travel along fixed orbits around the nucleus.
Each orbital has a specific energy value.
The energy of the orbits increases with orbital diameter.
Electrons only move to a higher energy orbital after absorbing a quanta (amount)
of energy equivalent to the difference in energy values of the two orbitals.
When electrons ‘fall’ back down into a lower energy orbital, they release the quanta of energy.
Bohr planetary model

11. Schrödinger: Electron cloud model (1926)
In the quantum mechanical model of atomic structure, electrons do not travel along fixed paths like planets around the sun.
Instead, electrons can be found most frequently in certain areas around the nucleus.

13. y x z
Quantum mechanical model: 90% probability of finding electron within 3D orbital with a maximum energy defined by the principal quantum number, n. (n = the row number on the periodic table)

14. n = 3 9 sublevels 18 electrons3p
6 e-
3d orbitals
10 e-

15. Homework tonight: Writing Assignment #2
Write at least one paragraph to answer the following question:
How does the development of the atomic model (Democritus to Schrödinger) exemplify application of the scientific method?
DemocritusDalton ThomsonRutherfordBohrSchrodinger
400 BC