notes: 5.1 revising the atomic model...bohr’s atomic model defects of bohr’s model: 1)...
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NOTES: 5.1 –
Revising the Atomic Model
Atomic Models ~ REVIEW!
● ATOM: the smallest particle that has the
properties of an element.
● From the early Greek concept of the atom to the
modern atomic theory, scientists have built on
and modified existing models of the atom.
● Atoms are composed of a positively charged nucleus surrounded by an electron cloud.
-NUCLEUS: (99% of atom’s mass) uncharged neutrons and positively charged protons.
-ELECTRON CLOUD: negatively charged electrons in constant motion creating a “cloud” like a fan.
Electron arrangements…
● the chemical properties of atoms, ions,
and molecules are related to the
arrangement of the electrons within them
● models of the atom will be reviewed with
an emphasis on the electrons
JOHN DALTON:
Dalton’s Atomic Theory:
-All elements are made of tiny atoms.
-Atoms cannot be subdivided.
-Atoms of the same element are exactly alike.
-Atoms of different elements can join to form molecules.
In 1808, this English
schoolteacher proposed his own
atomic theory that became widely
accepted.
● As it turns out, the atom can be divided into
subatomic particles.
● Thomson and Millikan are given credit for the first
discoveries relating to electrons.
● Rutherford discovered the positively charged
nucleus
3 types of particles that make up atoms:
Particles: Mass (amu): Charge:
PROTON 1.00728 + (pos.)
NEUTRON 1.00867 0 (neutral)
ELECTRON 0.00055 - (neg.)
J.J. Thomson (1856-1940)
● proposed the “plum-pudding” model:
-neg. charged electrons are dispersed
throughout a lump of pos. charged
material (like raisins stuck in dough)
How is the atom structured?
● each atom has a tiny, positively charged, massive (very dense) center called an ATOMIC NUCLEUS.
● The area around the nucleus is mostly empty space.
● How do we know?
Rutherford’s “Scattering”
Experiment:
-positively charged alpha particles (helium nuclei) were shot through a thin gold foil.
-most alpha particles passed through the foil, or were deflected through moderate angles.
-a few were reflected at extreme angles, or even came shooting right back to the source!
Rutherford’s Scattering Experiment
Rutherford’s Scattering Experiment
Rutherford’s Scattering Experiment
NIELS BOHR
● In 1913, this Danish scientist suggested
that electrons “orbit” the nucleus.
● In Bohr’s model, electrons are placed in
different energy levels based on their
distance from the nucleus. (lowest
energy level is closest to nucleus)
Summary of atomic structure:
Bohr’s Atomic Model
● What prevents an electron from falling into the
nucleus?
● The electrons in a particular path have a fixed
energy level
● The energy levels are like rungs of a ladder:
-lowest rung of ladder = lowest energy level
-one can climb up or down a ladder by going from rung
to rung = an electron can “jump” from one energy level
to another (by gaining or losing energy)
● The Bohr Atom was a “solar system” or
“planetary” model.
Bohr’s Atomic Model
● In the process of changing energy levels the electron
gains or loses energy by absorbing or emitting a
quantum of electromagnetic energy.
● the higher an electron is on the energy “ladder”, the
farther it is from the nucleus
● in the atom, the steps become closer together as an
electron climbs higher
● similarly, the higher energy level occupied by an
electron, the easier the electron escapes from the
atom…WHY?
Only certain orbits are allowed:
An electron
jumping down
gives off energy
An electron jumping up
absorbs energy
Only certain orbits are allowed:
Orbits – Energy Levels
n = 1
n = 2
n = 3
n = 4
n = 5
Orbits – Energy Levels
n = 1
n = 2
n = 3
n = 4
n = 5
Bohr’s Atomic Model
Defects of Bohr’s Model:
1) Contradicted known physics.
2) Couldn’t explain various intensities of the line
spectra.
3) Couldn’t explain WHY only certain orbits were
allowed.
4) Only worked for hydrogen.
In review…
• History of the Atomic Model VIDEO
• Bohr Model of the Atom VIDEO
Physicists were mystified, but intrigued by
Bohr’s theory of the atom.
Why are the energies of the hydrogen
electron quantized?
Why is the electron restricted to
orbiting at certain fixed distances?
Erwin Schrodinger (1887-1961)
tiV
xm
2
22
2
In 1926 Schrodinger
proposed an equation
describing the location
and energy of an electron
in a hydrogen atom.
This is very
difficult
stuff!
Solving the Schrodinger Equation gives
the probability of finding the electron at
a given place around the nucleus.
Quantum Mechanical Model:
The solution also gives rise to
energy level sub levels.
Quantum Mechanical Model:
● The quantum mechanical model of the atom
predicts energy levels for electrons; it is
concerned with the probability, or likelihood of
finding an electron in a certain location.
● In quantum theory, each electron is assigned
a set of 4 quantum numbers
**analogy: like the mailing address of an
electron!
● The first number is called the principal quantum
number (n). It describes the main energy level the
electron is on.
● The second quantum number (l) describes the sub
level, or orbital type, the electron is in. (s, p, d, f)
Quantum Mechanical Model:
Playing Chopin with Boxing Gloves
“Trying to capture the physicists’ precise
mathematical description of the quantum
world with our crude words and mental
images is like playing Chopin with a
boxing glove on one hand and a catcher’s
mitt on the other.”(1996). Johnson, George. On skinning Schrodinger’s Cat. New York
Times.
Electron Configurations
● Regions where electrons are likely to be
found are called orbitals. EACH ORBITAL
CAN HOLD UP TO 2 ELECTRONS!
S Orbitals
Energy level Sublevel Orbitals
n=1 1s 1 (1s)
n=2 2s 1 (2s)
2p 3 (2p)
n=3 3s 1 (3s)
3p 3 (3p)
3d 5 (3d)
Sublevel # Orbitals Max # elec.
s 1 2
p 3 6
d 5 10
f 7 14
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