Evolution of Atomic Models: From Ancient Philosophers to Quantum Mechanics
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Advanced Higher Physics
Introduction to Quantum Mechanics
History
Phenomena observed in early 20
th
century did
not follow ‘classical’ physical laws
New theories were developed to account for
these phenomena
Starting point taken as atomic structure
Atomic Models 1 - Ancients
‘Atom’ is derived from Greek ~
‘a’ meaning ‘not’ (like prefix un-)
‘tom’ meaning ‘cut’
Greek philosophers thought that atoms were
the smallest possible things, and therefore
indivisible – ‘unable to be cut’
This theory was widely accepted to be true
until the late 19
th
century
Atomic Models 2 - Thomson
1897 – Thomson’s
discovery of electron
leads to ‘Plum
Pudding’ model
Large positive mass
with randomly
arranged negative
charges
Atomic Models 3 - Rutherford
1909 -
Scattering
experiment
not
consistent with
Thomson model
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Atomic Models 3 (cont.)
Later work lead to
the discovery of
The proton
(Rutherford -1919)
The neutron
(Chadwick – 1932)
Atomic Models 4 - Bohr
Rutherford model still unable to explain
spectral lines associated with emission of light
from atoms
Atomic Models 4 (cont.)
Bohr’s model has electrons in orbit around a
central nucleus, but allows only certain orbits
for electrons.
For stable orbit, angular momentum must be a
multiple of
h / 2
π
A
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q
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d
n,
is order of electron
level
Atomic Models 4 (cont.)
Reasons for electron
stability related to De
Broglie wavelength
Treating the orbit of an electron as a continuous
wave, the path length (
2
π
r
) must be equal to a whole
number of wavelengths
i.e.
n
λ
= 2
π
r
Graphical Representation
Atomic Models 4 (cont.)
From De Broglie equation,
Combining with
,
Atomic Models 4 (cont.)
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/
2
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ħ
(
‘
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’
)
,
where
ħ
= h / 2
π
Scholar Bohr Hydrogen Atom demo
Energy Levels
For any quantum number,
n
, there exists a
single orbit with a specific angular momentum,
L = mvr
, and energy,
E
, which can be
calculated.
Each quantum number,
n
, relates to an
electron energy level,
E
n
,
in the atom.
When electrons move between energy levels
they either absorb energy (excite) or emit
energy (de-excite)
Spectral Lines 1
When an electron gains
energy, by absorbing a
photon, it rises to a higher
energy level (excitation)
When an electron loses
energy, by emitting a photon,
it falls to a lower energy level
(de-excitation)
Spectral Lines 2
Hydrogen has a number of groups or
series
of line spectra, each for transitions
to the same lower energy level.
Scholar Hydrogen emission demo
Tracing the evolution of atomic models from the ancient Greek philosophers' concept of indivisible atoms to the groundbreaking discoveries of electrons, protons, and neutrons. The journey through Thomson's Plum Pudding model, Rutherford's planetary model, Bohr's quantized model, and the introduction of quantum mechanics to explain atomic phenomena.
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Presentation Transcript
Advanced Higher Physics Introduction to Quantum Mechanics
History Phenomena observed in early 20thcentury did not follow classical physical laws New theories were developed to account for these phenomena Starting point taken as atomic structure
Atomic Models 1 - Ancients Atom is derived from Greek ~ a meaning not (like prefix un-) tom meaning cut Greek philosophers thought that atoms were the smallest possible things, and therefore indivisible unable to be cut This theory was widely accepted to be true until the late 19thcentury
Atomic Models 2 - Thomson 1897 Thomson s discovery of electron leads to Plum Pudding model Large positive mass with randomly arranged negative charges
Atomic Models 3 - Rutherford 1909 - Scattering experiment not consistent with Thomson model Rutherford postulated nucleus containing positive charges with electrons in orbits like planets
Atomic Models 3 (cont.) Later work lead to the discovery of The proton (Rutherford -1919) The neutron (Chadwick 1932)
Atomic Models 4 - Bohr Rutherford model still unable to explain spectral lines associated with emission of light from atoms
Atomic Models 4 (cont.) Bohr s model has electrons in orbit around a central nucleus, but allows only certain orbits for electrons. For stable orbit, angular momentum must be a multiple of h / 2 Angular momentum of electrons is quantised mvr = nh 2 n, is order of electron level
Atomic Models 4 (cont.) Reasons for electron stability related to De Broglie wavelength n = 6 Treating the orbit of an electron as a continuous wave, the path length (2 r) must be equal to a whole number of wavelengths i.e. n = 2 r Graphical Representation
Atomic Models 4 (cont.) h = From De Broglie equation, mv = 2 n nh r Combining with, = 2 r mv nh = mvr 2 nh = mvr 2
Atomic Models 4 (cont.) Angular momentum of electron in any orbit is always a multiple ofh / 2 This quantum of angular momentum is often expressed as ( h bar ), where = h / 2 Scholar Bohr Hydrogen Atom demo
Energy Levels For any quantum number, n, there exists a single orbit with a specific angular momentum, L = mvr, and energy, E, which can be calculated. Each quantum number, n, relates to an electron energy level, En, in the atom. When electrons move between energy levels they either absorb energy (excite) or emit energy (de-excite)
Spectral Lines 1 When an electron gains energy, by absorbing a photon, it rises to a higher energy level (excitation) When an electron loses energy, by emitting a photon, it falls to a lower energy level (de-excitation)
Spectral Lines 2 Hydrogen has a number of groups or series of line spectra, each for transitions to the same lower energy level. Scholar Hydrogen emission demo
