Transcript The Development of the Atomic Theory

The Development of the Atomic
Theory
Dalton’s Atomic Theory
founder of the atomic theory
atoms in Greek means indivisible, indestructible
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All matter is composed of atoms.
Atoms cannot be divided.
Atoms cannot be created or destroyed (LCM)
Atoms of the same element are identical in
mass, size & other properties.
5. Properties of one element differ from other
elements.
6. Atoms combine in small, whole-number ratios
forming compounds.
James Chadwick
• Discovered a neutral particle in the nucleus
• Called it a neutron
Eugen Goldstein
• Discovered the proton
J.J. Thomson
• Discovered the electron (negatively charged
particle)
• Suggested atom consisted of positive &
negative charges
• Came up with “raison bun” or “plum pudding”
model of the atom
• Suggested atom consisted of a positive charge
cloud with electrons dispersed through it
Ernest Rutherford
• Suggested that instead of a positive charge
cloud, a nucleus existed in the atom where the
positive charges existed
• The nucleus is very small & very positively
charged
• Electron cloud surrounds the nucleus
• These ideas came from the GOLD FOIL
experiment
GOLD FOIL EXPERIMENT
• If Thomson was right, the alpha particles
would pass through the positive charge cloud
without being deflected
• But, Rutherford found that some alpha
particles were deflected and some came right
back
• So, the positive charge is small in size (since
most alpha particles passed straight through
• And, when the alpha particles came close to
this positive charge, they were deflected a
great deal, so the positive charge is very dense
Rutherford’s Model of the Atom
protons
neutrons
The Bohr Atom (Niels Bohr)
• Proposed that electrons in atoms were
restricted to certain energy levels
• When electrons moved from one energy level
to another, they either absorbed or gave off a
specific amount of energy (“what goes up
must come down”)
• Continuous spectrum (ROYGBIV)
• Using spectroscopy, the pattern of coloured
lines separated by black spaces is called a line
spectrum
• Each element has its own unique line
spectrum
Energy Levels
n=7
n=1
• The atom is in its ground state when the
electrons in the atom are in the lowest possible
energy level.
• The atom is in its excited state if an electron
moves to a higher energy level by absorbing
energy.
• The maximum number of electrons in any given
energy level is 2n2, where n is the number of
energy level.
• Hence, we get the “Bohr-Rutherford “ Diagrams.
Bohr Atom Video
Limitations to Bohr’s Theory
• Bohr’s theory is only able to explain the line
spectrum of atomic hydrogen (1 p+ & 1 e-);
however, it cannot explain the observed line
spectra of other elements.
• Quantum mechanics or wave mechanics is the
theory used to explain the line spectra of
elements other than hydrogen.
• Nevertheless, Bohr’s theory is still the basic
concept of energy levels in atoms.
Today’s theory of the atom
• The atom consists of a nucleus surrounded by
regions of space where electrons are likely to
be found.
• These regions are referred to as orbitals.