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MODELS OF
THE ATOM &
QUANTUM
THEORY
NIELS BOHR (1913)
• Previous research had concluded that light has a dual
nature.
• It could act as both particles and waves
• Bohr proposed a model that explained why only certain
frequencies of light were emitted from hydrogen.
BOHR’S MODEL OF
THE ATOM
• Suggested that the electron in a hydrogen atom moves
around the nucleus in a circular path
• i.e. Like planets around a sun
•
Atoms can have multiple “orbits” that have a
definite, fixed amount of energy
•
The lowest allowable energy state of an
atom is the ground state
•
•
This is closest to the nucleus
The smaller the orbit, the lower the
energy and vise versa
Orbits
EXCITED ELECTRONS
• In the ground state, atoms do not radiate energy
• When energy is add from an outside source, electrons
move to a higher-energy orbit
• This movement raises the electron to an “excited” state
• Once excited, the electron can drop from the higherenergy orbit to a lower-energy orbit
• Results in the atom emitting a photon
LADDER ANALOGY
• Like the rungs of the
ladder on the right, the
energy levels in an atom
are not equally spaced
• The higher the energy level
occupied by an electron,
the less energy it takes to
move from the energy level
to the next highest energy
level
PROBLEM WITH
BOHR’S MODEL
• Bohr’s model had some limits
• It could not explain the spectra of light emitted for any
other element
• It did not fully account for the chemical behavior of atoms
ELECTRONS AS WAVES
• French scientist De Broglie pointed out the electron orbits
in Bohr’s model were similar to the behavior of waves
• Electrons, like waves, could be:
• Diffracted (bent)
•
Bending a wave as it passes by the edge of an object
• Interfere with each other
•
When waves overlap. Results in a reduction of energy in
some areas and an increase of energy in others.
UNCERTAINTY PRINCIPLE
• Werner Heisenberg – 1927
• Showed that it is impossible to take any measurement of
an object without disturbing the object!
• Detection of electrons
• Detected by their interaction with photons
•
Have about the same mass as an electron
• Found by “bumping” a photon into an electron
•
Doing so changes both wavelength of photon and the
position and velocity of the electron
• Heisenberg’s Uncertainty Principle:
• It is fundamentally impossible to know precisely both the
velocity and position of a particle at the same time.
SCHRÖDINGER'S
WAVE EQUATIONS
• Remember the problem with Bohr’s model?
• Only relevant to hydrogen…
• Erwin Schrodinger – 1926
• Developed an equation that treated electrons in atoms as
waves
• His new model not only could be applied to hydrogen, but
all the other elements as well!!! YAY
• This new model, in which electrons are treated as waves is
called:
•
The Quantum Mechanical Model of the Atom
• Other names: Wave Mechanical Model or Charge
Cloud Model
SCHRÖDINGER
SOUND FAMILIAR?
LOCATION OF ELECTRONS
• Electrons described as waves only have a certain
probability of being found within a particular volume of
space around the nucleus
• Schrödinger’s wave function predicts a 3D region around
the nucleus that described the electron’s probable
location
• Atomic Orbital:
• Description of the 3D region around the nucleus (looks like
a fuzzy cloud)
• Clouds show the region of probable electron locations
• Size and shape of cloud depends on the energy of the
electrons that occupy them
ACTIVITY
Probability of finding an electron around the nucleus activity
ELECTRONS AROUND
THE NUCLEUS
•
Orbitals can differ based on size and
• There are four types of orbitals
• S, P, D and F
•
Each orbital can hold two electrons
1s
•
An S orbital is shaped like a sphere
•
A 1s orbital is smaller than a 2s orbital
2s
ORBITAL SHAPE
ELECTRONS IN THE
ORBITALS
Orbital type
Number of
Types
Electrons that
fit into this
“shell”
s
1
2(1) = 2
p
3
2(3) = 6
d
5
2(5) = 10
f
7
2(7) = 14
• The right column tells us the total electrons that fit into all
types of that one orbital
RULES OF
ARRANGEMENT
• Three rules, or principles define how electrons can be
arranged in an atom’s orbitals
1. Aufbau Principle
2. Pauli Exclusion Principle
3. Hund’s Rule
AUFBAU PRINCIPLE
• States: each electron
occupies the lowest energy
orbital available
• First step in determining
the ground-state electron
configuration is to learn the
sequence of atomic orbitals
from lowest to highest
energy
• Each box in the Aufbau
diagram represents an
orbital
ELECTRON
CONFIGURATION
Based on Aufbau’s Principle, the order of the orbitals should
be:
1s – 2s – 2p – 3s – 3p – 4s – 3d – 4p – 5s – 4d – 5p – 6s…
All elements follow this rule with few exceptions
PAULI EXCLUSION
PRINCIPLE
•
Electrons in orbitals can be represented by arrows in boxes
•
Each electron has an associated spin
•
•
Arrow pointing up represents electron spinning in one
direction
• Arrow point down represents electron spinning in opposite
direction
• Empty box represents an unoccupied orbital
Pauli Exclusion Principle:
•
A maximum of two electrons can occupy a single atomic
orbital, but only if the electrons have opposite spins
= 0 e-
= 1 e-
= 2 e-
HUND’S RULE
• Remember that negatively charged electrons repel each
other
• Hund’s Rule:
• Single electrons with the same spin must occupy each
equal-energy orbital before additional electrons with
opposite spins can occupy the same orbital
ELECTRON
CONFIGURATION
•
The electron configuration on an atom is a shorthand method
of writing the location of electrons by sublevel
•
The first number describes the principle energy level
•
The letter represents the sublevel
•
The sublevel is followed by a superscript with the number of
electrons in the sublevel
•
If the 2p sublevel contains 2 electrons, it is written 2p2
WRITING ELECTRON
CONFIGURATIONS
First, determine how many electrons are in the atom
• Iron has 26 electrons
Arrange the energy sublevels to increasing energy
• 1s 2s 2p 3s 3p 4s 3d…
Fill each sublevel with electrons until you have used all the electrons
in the atom
• Fe: 1s2 2s2 2p6 3s2 3p6 4s2 3d6
The sum of the superscripts equals the atomic number of iron (26)
EXAMPLE
Phosphorus, an element used
in matches, has an atomic
number of 15. Write the
electron configuration of a
phosphorus atom.
Answer:
• 1s2 2s2 2p6 3s2 3p3
PRACTICE ELECTRON
CONFIGURATIONS
Write the complete electron configurations for:
•
•
•
•
Be
F
S
Ca
Things to double check:
1. All superscripts add up to total electrons
2. Orbital order follows the Aufbau Principle
ANSWERS
Be:
• 1s2 2s2
F:
• 1s2 2s2 2p5
S:
• 1s2 2s2 2p6 3s2 3p3
Ca:
• 1s2 2s2 2p6 3s2 3p6 4s2