The Atom: History Recap

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.

Solid, homogeneous, indestructible and

indivisible (400 BC)

• Changes in matter are due to changes in the arrangements of the atoms and not from changes in the atoms themselves.

(atomos = “uncuttable”)

Dalton’s atomic theory: particulate nature of matter

• 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

+

(1803)

• 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.

J.J. Thomson: The ‘plum pudding model’

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 10-28 g

Ernest Rutherford: gold-foil experiment (1911)

Experiment: Bombard thin gold foil with alpha particles (nuclei of helium atoms) and measure the amount of deflection.

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.

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.

Niels Bohr: planetary model of the atom (1913)

Bohr planetary model

• 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.

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.

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)

3d orbitals

10 e-

n = 3 9 sublevels 18 electrons n = 3 9 sublevels 18 electrons

3s2 e-

3p6 e-

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?

DemocritusDalton ThomsonRutherfordBohrSchrodinger400 BC 1803 1904 1911 1913 1926

Homework tonight: Writing Assignment #2