Download Evolution of Atomic Models

Ch. 13 – Electrons in
Atoms
13.1 = Models of the atom
Down the wrong path…
•400 BC – Democritus proposed
that atoms make up substances
•Aristotle disagreed with him and
thought matter was uniform
throughout
•This was accepted for the next
2,000 years!
Evolution of Atomic Models
John Dalton (1808)- atom was a solid mass
(did not know about subatomic particles)
Billiard ball model
J.J. Thomson (1904)- discovered the
existence of electrons as part of an atom
(however, he did not identify their location).
Plum pudding model
Ernest Rutherford (1911)- discovered the
nucleus. Decided that the space surrounding
the nucleus contained electrons.
These models do not provide enough
information to explain all of the
properties of an element…
Many properties of atoms, ions,
and molecules are related to
specific arrangements of their
electrons
Examples: colors given off when
heated in flame, magnetic
properties, covalent and ionic
bonding, etc.
Niels Bohr (1913) – he proposed that
electrons travel in fixed orbits
Planetary model
If an excited electron drops from E3 to E2,
what is the wavelength that it gives off?
Planck’s constant = 6.63 x 10-34 Js
Evolution of Atomic
Models cont…
Erwin Schrodinger (1926)- used mathematical
equation to describe probable location and
energy of an electron (the quantum
mechanical model)
Electron cloud – region outside the nucleus
where the electron is most likely to be found
These models provide more
information based on an electron’s…
Energy level- region where
electron is most likely moving
The higher the energy level the
further it is from the nucleus
Low energy electrons = near the
nucleus!
•Quantum- amount of energy required to
move an electron from it’s present energy
level
-Energy is lost and gained
-Not always the same (depends on
energy level
-higher energy level = less distance
between energy levels thus less energy is
required to move from one level to the next
-lower energy levels have greater
distance between them = more energy to
transfer between levels)
The Quantum Mechanical Model
Does not define an exact path of an
electron but a probable location.
The region where electrons are
likely to be found are called, atomic
orbitals.
Atomic Orbitals
The Principal Quantum Number (n)represents the energy level (n=1,2,3,4,etc)
• The principal energy levels are assigned values
in order of increasing energy levels (ladder
example)
Within each principal energy level the electrons are
found in sublevels (L).
(number of sublevels = principal energy level)
Table 13.1
Atomic Orbitals
The sublevels are represented by the letters
s, p, d, and f.
Electrons in each of these sublevels travel in a
pattern that have a distinctive shape to that
sublevel.
• s- spherical shaped cloud
• p- dumbbell shaped cloud (contains node)
 Node- region close to nucleus where electron is
not likely to be found.
• d- cloverleaf shape (contains node)
• f- complex and harder to visualize
Letters denote atomic orbitals…
Electron
probability
clouds
Principal
energy
level
# of
sublevels
Distance of
electrons
from
nucleus
increases
with n
13.2 - Electron Arrangement in
Atoms
Electrons in an Energy level
The maximum # of electrons that can
occupy a principle energy levels is
represented by the formula,
2n²
-Note: n = principle quantum #
Electrons in a Sublevel
s: 2 electrons
p: 6 electrons
d: 10 electrons f: 14 electrons
Electron Arrangement in Atoms
-Electron ConfigurationsElectron Configurations- how the electrons are
arranged around the nucleus
Three rules tell you how to find the electron
configurations of atoms:
• 1. Aufbau Principle
• 2. Pauli Exclusion Principle
• 3. Hund’s Rule
Aufbau Principle- Electrons enter orbitals of
lowest energy first.
s sublevel is the lowest, then p, d, f, etc…
Hund’s Rule- When electrons occupy orbitals of
equal energy, one electron enters each orbital until all
the orbitals contain one electron with parallel spins.
Electron Arrangement in Atoms
-Pauli Exclusion PrinciplePauli Exclusion Principle- An atomic orbital
may describe at most two electrons
An s or p orbital may contain 1 or 2
electrons.
If two electrons are in the same orbital they
have opposite spins (clockwise or counterclockwise)
• The position of the electrons represent the
directions of the spin (
)
Periodic Table Arrangement
Writing Electron Configurations
There is a shorthand
Write the energy level and the symbol for every
sublevel occupied by an electron. Attach a
superscript to indicate the number of electrons
in that sublevel.
• Examples:
H = 1s¹ (1=energy level, s=symbol,
¹=superscript)
He = 1s²